Class T_A_55_i. COPYRIGHT DEPOSm CHEMISTRY OF FOOD AND NUTRITION THE MACMILLAN COMPANY NEW YORK ■ BOSTON • CHICAGO ■ DALLAS ATLANTA • SAN FRANCISCO MACMILLAN & CO., Limited LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, Ltd. TORONTO CHEMISTRY OF FOOD AND NUTRITION BY HENRY C. SHERMAN, Ph.D. PROFESSOR IN COLUMBIA UNIVERSITY SECOND EDITION REWRITTEN AND ENLARGED Ncto lark THE MACMILLAN COMPANY 1918 Ali rights reserved (\\'6 Copyright, xgii and igiS, By the MACMILLAN COMPANY. Set up and electrotyped. Published February, 1911. Second Edition, Rewritten and Enlarged, March, 1918 MAR 21 1918 Nortoool! lPrfB8 J. S. Cushinf? Co. — Berwick & Smith Co. Norwood, Mass., U.S.A. ©CI.A494166 .. CeHioOe + 6 O2 Glucose is the most familiar representative of a group of simple sugars (monosaccharides or monosaccharoses) which are in composition direct polymers of formaldehyde (CH2O) and which are classified, according to the number of carbon atoms in the monosaccharide molecule, as trioses, pentoses, hexoses, etc. Classification Definitions of the term " simple sugar " vary somewhat, de- pending chiefly upon the views of different authors as to how simple a compound may properly be called a sugar. According to Browne, a simple sugar or monosaccharide may be defined as an aldehyde alcohol or ketone alcohol of the ali- phatic series, the molecule of which contains one carbonyl and one or more alcohol groups, one of the latter being always adjacent to the carbonyl group. All simple sugars contain, therefore, HC-OH I c=o * For concise discussion of the sj-nthesis of carbohydrates in plants the reader may be referred to Armstrong's The Simple Carbohydrates and the Glueosides, pages g2-g6; Browne's Handbook of Sugar Analysis, pages 532-534; and Mathews' Physiological Chemistry, pages 44-49. A somewhat fuller account will be found in Jost's Pflanzenphysiologie and Euler's P_flanzcnehcmif, and a very detailed treatment of the subject in Czapek's Biochemie der Pjlanzen. For discussion from a more physiological standpoint, see Pfeffer's Plant Physiology and summary of recent work by Jorgenson and Stiles in The New Phytologisl. CARBOHYDRATES 3 as a characteristic group upon the presence of which the chief chemical properties of the sugars depend. The simplest possible sugar according to this definition is glycolaldehyde, CH2OH. — CHO, which (in analogy with the nomenclature of the familiar sugars) may also be called gly co- lose. The structural formulae of glucose and fructose, the most familiar representatives of the aldehyde-alcohol (aldose) and ketone-alcohol (ketose) sugars, respectively, are as follows : Glucose Fructose CH2OH 1 CH.2OH HOCH HOCH HOCH HOCH HCOH 1 HCOH HOCH C=0 HC=0 CHoOH Since glucose gives aldehyde reactions but not so readily as the above structural formula would lead one to expect, it is believed that in ordinary solutions of glucose the substance exists partly in the condition indicated by the aldehyde formula and partly in a tautomeric form represented by the lactone or " oxygen bridge " formula. Following are the aldehyde and lactone formulae written without reference to the spatial relationships of the hydrogen and hydroxyl groups : Aldehyde form : CHoOH— CHOH— CHOH— CHOH— CHOH— CHO Lactone form : 0— - CH.OH— CHOH— CH— CHOH— CHOH— CHOH 4 CHEMISTRY OF FOOD AND NUTRITION The name monosaccharide (" single sugar ") implies that the monosaccharide molecule contains only one sugar radicle — that it cannot be spHt by hydrolysis into sugars of lower molecular weight. A substance Hke cane sugar which on hydrolysis spUts to two molecules of simple sugar is called a disaccharide or disaccha- rose (" double sugar ")• Trisaccharides and tetrasaccharides are also known. Substances which like starch are of high mo- lecular weight and on complete hydrolysis yield many molecules of simple sugar are called polysaccharides * or polysaccharoses. The term " carbohydrates " covers all the simple sugars and all substances which can be converted into simple sugars by hy- drolysis. The term " glucosides " is applied to substances which consist of combinations of carbohydrate radicles with radicles of other kinds and which therefore >'ield on hydrolysis both a simple sugar and one or more products of other than carbohy- drate nature. CLASSIFICATION OF CARBOHYDRATES f MONOSACCHARIDES (Monosaccharoses) Dioses (C2II4O2) — Glycolose. Trioses (CsHfiOs). Aldoses — Glycerose. Ketose — Dioxyacetone. Tetroses (C4H8O4). Aldoses — Erythrose,^ Threose.' Ketose — Erythrulose.- Pentoses (CsHjoOs). Aldoses — Arabinose,2 Xylose,^ Ribose,- Lyxose.' Ketoses — Araboketose^ Xyloketose (ketoxylose).* [Methyl pentoses (CeHioOj) — Rhamnose,- Fucose -]. * Some writers use the term polysaccharides to include all carbohydrates other than monosaccharides. Mathews applies it to all carbohydrates more complex than the disaccharides. t Names of a few of the most important carbohydrates are printed in small capitals. Separate mention of the d, I, and dl forms of the various sugars is omitted, since in the study of food and nutrition we are practically concerned only with that one of the three forms which is found in or derived from natural products. CARBOHYDRATES S Hexoses (C6H12O6). Aldoses — Glucose,! ^Mannose,^ Galactose,^ Gulose,'' Idose,' Talose,' Allose,' Altrose.' Ketoses — Fructose," Sorbose,^ Tagatose.^ Heptoses (CtHhO;). Aldose — Mannoheptose.i Ketose — Sedoheptose.^ Note. — No attempt is here made to summarize the occurrence of any but the tetroses, pentoses, hexoses and heptoses. Glycolose and the trioses if formed in nature are probably too reactive to accumulate sufficiently for identification. DISACCHARIDES (Disaccharoses). Dihexoses (Hexobioses) — (C12H22O11). A nhydride of glucose + fructose — Sucrose. Anhydrides of glucose + galactose — Lactose, Melibiose. Anhydrides of glucose -{- glucose — Maltose, Isomaltose, Trehalose, Turanose. TRISACCHARIDES (Trisaccharoses) . Trihexoses (C1SH32O16). Anhydride of glucose + galactose -{-fructose — Raffinose. Anhydride of glucose + glucose + glucose — Melezitose. A nhydride of fructose + fructose + fructose — Secalose. TETRA SACCHARIDES (Tetrasaccharoses) . Tetrahexoses (C24H42O21). Anhydrides of 2 galactose -(- glucose -\- fructose — Stachyose, Lupeose. POLYSACCHARIDES (Polysaccharoses) . Pentosans (chief constituents of gums and mucilages). Anhydrides of xylose — Xylans. Anhydrides of arabinose — Arabans. Hexosans. Anhydrides of glucose — Starch, Cellulose, Glycogen, Dextrin (and other "dextrans"). Anhydrides of mannose — Mannans. Anhydrides of galactose — Galactans (pectins). Anhydrides of fructose — Inulin (and other "levulans"). ^Occurs free in nature. ^ Not yet found free in nature (or only in small amounts) but obtained by hy- drolysis or fermentation of natural product. ' Known only (with certainty) as a laboratory product. 6 CHEMISTRY OF FCJOD AND NUTRITION PROPERTIES OF THE CHIEF CARBOHYDR.\TES OF FOOD Monosaccharides The monosaccharides are all soluble, crystallizable, diffusible substances, unaffected by digestive enzymes, and if not attacked by bacteria in the digestive tract, they are absorbed and enter the blood current unchanged. All of the three hexoses described below are susceptible to alcoholic fermentation, and are utilized for the production of glycogen in the animal body and the maintenance of the normal glucose content of the blood. A few of the leading facts regarding the occurrence in food and the nutritive relations of indixddual monosaccharides are given below. Glucose ((/.glucose, dextrose, grape sugar, starch sugar, diabetic sugar) is widely distributed in nature, occurring in the blood of all animals in small quantity (usually about o.i per cent) and more abundantly in fruits and plant juices, where it is usually associated with fructose and sucrose. It is especially abundant in grapes, of which it often constitutes 20 per cent or more of the weight of the fresh fruit and considerably more than half of the solid matter. Sweet corn, onions, and unripe potatoes are among the common vegetables containing con- siderable amounts of glucose. Glucose is also obtained from many other carbohydrates by hydrolysis either by acids or by enzymes, and thus becomes the principal form in which the carbohydrate of the food enters into the animal economy. In the healthy animal body the glucose of the blood is constantly being burned and replaced. In dia- betes the body loses to a greater or less degree the power to burn glucose, which then accumulates in excessive amount in the blood, from which it escapes through the kidneys. A tem- porary and usually unimportant loss of glucose in the urine may occur as the result of feeding large quantities at a time. This condition is known as alimentary glycosuria. Ordinarily CARBOHYDRATES 7 any surplus of glucose absorbed from the digestive tract is con- verted into glycogen which, as described beyond, is readily reconvertible into glucose. Thus, while other carbohydrates occur in food in greater quantity, glucose occupies a very prom- inent place, partly because it is more widely distributed than any other carbohydrate, being a normal constituent of both plants and animals, and partly because it is the form in which most of the carbohydrate material of the food comes ultimately into the service of the body tissues (Chapter V). It is esti- mated that over half the energy manifested in the human body is derived from the oxidation of glucose. It is not to be inferred from the foregoing statement that the body obtains the energy of the glucose by oxidizing it di- rectly as such. The aldehydic properties of glucose make it susceptible to direct oxidation ; but, as the elaborate researches of Nef have shown, the glucose molecule in alkaline solution breaks up to form simpler substances of 2, 3, and 4 carbon atoms which are more readily oxidizable than glucose itself. There is strong evidence (Chapter V) that in the body tissues glucose is broken into 3-carbon molecules, which latter readily undergo oxidation. Fructose (J. fructose, fruit sugar, levulose) occurs with more or less glucose in plant juices, in fruits, and especially in honey, of which it constitutes about one half the sohd matter. It results in equal quantity with glucose from the hydrolysis of cane sugar and in smaller proportion from some other less common sugars. Fructose may occur in normal blood, but probably only in insignificant amounts. It serves, Hke glu- cose, for the production of glycogen ; and the fructose which enters the body either through being eaten as such or as the result of the digestion of cane sugar is mainly changed to gly- cogen on reaching the liver, so that it does not enter largely into the blood of the general circulation. Glucose and fructose are partially convertible, either one into the other, under the 8 CHEMISTRY OF FOOD AND NUTRITION influence of very dilute alkalies. It is not surprising, there- fore, that fructose should be converted in the liver into glycogen, which on hydrolysis yields glucose. Galactose is not found free in nature, but results from the hydrolysis of milk sugar, either by acids or by digestive enzymes, and appears to have the same power as glucose and fructose to promote the formation of glycogen in the animal body. Anhy- drides of galactose, known as galactans, occur quite widely distributed in plants ; and galactosides, which are compounds containing galactose in chemical combination with radicles of other than carbohydrate nature, are found in the animal body, notably as constituents of the brain and nerve tissues. Disaccharides The three disaccharides here considered are di-hexoses or hexo-bioses of the formula C12H22O11, and are crystallizable and diffusible. Sucrose crystallizes anhydrous ; maltose and lactose, each with one molecule of water, which can be removed by drying at temperatures of 100° and 130° respectively. They are soluble in water; less soluble in alcohol. Lactose is much less soluble than sucrose and maltose. These disac- charides are important constituents of food and are changed to monosaccharides during the process of digestion. Sucrose (saccharose, cane sugar) is widely distributed in the vegetable kingdom, being found in considerable quantity, generally mixed with glucose and fructose, in the fruits and juices of many plants. The commercially important sources of sucrose are the sugar beet, the sugar and sorghum canes, the sugar palm, and the sugar maple ; but many of the common fruits and vegetables contain notable amounts. For example, sucrose is said to constitute at least half the solid matter of pineapples and of some roots such as carrots. On hydrolysis each molecule of sucrose yields one molecule CARBOHYDRATES 9 each of glucose and fructose. These sugars all rotate the plane of vibration of polarized light, sucrose and glucose to the right ( + ), and fructose to the left (-). The terms "dextrose" and " levulose," synonyms for glucose and fructose respectively, arose from this behavior of the sugars in rotating the plane of polarized light to the right and left. Since at ordinary tem- peratures the fructose rotates more strongly to the left than the glucose does to the right, the result of the hydrolysis of sucrose is to change the sign of rotation from + to — . For this reason the hydrolysis of cane sugar is often called " inversion," and the resulting mixture of equal parts glucose and fructose is known as " invert sugar." Sucrose is very easily hydrolyzed either by acid or by the sucrase (" invertase " or " inverting " enzyme) of yeast or of intestinal juice. So far as known neither the sahva nor the gastric juice contains any enzyme capable of hydrolyzing cane sugar, and the slight amount of hydrolysis which takes place in the stomach is beUeved to be due simply to the presence of hydrochloric acid. Under normal conditions the sucrose of the food passes mainly into the intestine unchanged and is there spHt by the sucrase of the intestinal juice, and the result- ing glucose and fructose are absorbed into the portal blood. When large amounts of sucrose are fed, some absorption takes place in the stomach; but the unchanged sucrose thus ab- sorbed appears to be largely, if not wholly, lost through the kidneys, as it is when injected directly into the blood current. Sugar eaten in concentrated form or in considerable quantities at a time is apt to cause irritation of the stomach either directly, or as the result of undergoing an acid fermentation, or in both of these ways. According to Herter sucrose and glucose are more likely to ferment in the stomach than is lactose. In cases where fermentation does not occur and the sucrose itself has no irritating effect, it may be especially useful as a rapidly avail- able foodstuff. However, it is not known that sucrose has any lO CHEMISTRY OF FOOD AND NUTRITION advantage over maltose and lactose in this respect, and the latter are less apt to irritate the stomach and cause indigestion. Lactose (milk sugar) occurs in the milk of all mammals, constituting usually from 6 to 7 per cent of the fresh secretion in human milk and 4.5 to 5 per cent in the milk of cows and goats. At the time of parturition, or if the milk is not with- drawn from the udder, some lactose may occur in the urine. If in such a case the mammary glands are removed, the percentage of glucose in the blood increases, and glucose (but no lactose) may appear in the urine (Abderhalden). These observations indicate that lactose is formed in the mammary gland and prob- ably from the glucose brought by the blood. Lactose is less sweet and much less soluble than sucrose, dis- solving only to the extent of about i part in 6 parts of water. When hydrolyzed either by heating with acids or by an enzyme, such as emulsin or the lactase of the intestinal juice, each molecule of lactose yields one molecule of glucose and one of galactose. In normal digestion, probably none of the lactose eaten is absorbed as such, for lactose injected into the blood is eliminated quickly and almost completely through the kidneys, whereas large amounts of lactose can be taken by the mouth without any such loss. As already noted, Herter found lactose to be less subject to fermentation in the stomach than is sucrose. Also, because of the much lower solubility, there is less danger of direct irritation of the stomach membrane by lactose than by sucrose. Recently Mathews has suggested that the occurrence in milk of lactose, a sugar having the galactose radicle, may be of special significance as a source of material for the synthesis of the galactosides of the brain and nerve tissues of the rapidly growing young mammal. Maltose (malt sugar) is formed from starch by the action of diastatic enzymes (amylases) and is therefore an important constituent of germinating cereals, malt, and malt products. It is also formed as an intermediate product when starch is CARBOHYDRATES II hydrolyzed by boiling with dilute mineral acids, as in the manufacture of commercial glucose. In animal digestion maltose is formed by the action of the ptyalin of the saliva or the amylopsin of the pancreatic juice upon starch or dextrin. The maltose-spHtting enzyme of the intestinal juice readily hydrolyzes maltose to glucose. Maltose is also readily and completely hydrolyzed by boiling with dilute mineral acids. In either case each molecule of maltose yields two molecules of glucose. While it is probable that little if any maltose is absorbed as such from the digestive tract under ordinary conditions, it is possible that such absorption may occur and that maltose as such may play a part in the normal carbohydrate metaboHsm ; for when injected into the blood it appears to be utilized to better advantage than either sucrose or lactose, and it may be obtained from glycogen by the action of diastatic enzymes in much the same way as from starch and dextrin. Polysaccharides The polysaccharides are all colloids insoluble in alcohol. Some " dissolve " in water in the sense that they form colloidal dispersions which will pass through filter paper ; some swell and become gelatinous ; some are unchanged. The members of greatest importance in nutrition are starch and glycogen, the typical reserve carbohydrates of plants and animals respectively. Pentosans, (C5H804)i, occur in the greatest variety of plants and in various parts of the plant organism. As a rule, how- ever, they are abundant only in the fibrous tissues and gummy exudations and not in the starchy and succulent parts which are more commonly used for human food. Moreover experi- ments have not yet succeeded in demonstrating in man or other mammals any enzyme capable of digesting the pentosans (Swartz). It is therefore believed that, notwithstanding their 12 CHEMISTRY OF FOOD AND NUTRITION wide distribution in plants, the pentosans can play only a very small, if appreciable, part in the nutrition of man. Starch, (CeHioOs)!, is the form in which most plants store the greatest part of their carbohydrates, and is of great im- portance as a constituent of many food materials and as the source of dextrin, maltose, commercial glucose, and many fer- mentation products. Starch is found stored in the seeds, roots, tubers, bulbs, and sometimes in the stems and leaves of plants. It constitutes one half to three fourths of the solid matter of the ordinary cereal grains and at least three fourths of the solids of mature potatoes. Unripe apples and bananas contain much starch which is to a large extent changed into sugars as these fruits ripen, while, on the other hand, young tender corn (maize) kernels and peas contain sugar which is transformed into starch as these seeds mature. Unchanged starch occurs in distinct granules, and those formed in different plants vary in size and structure,* so that in most cases the source of a starch which has not been altered by heat, reagents, or ferments can be determined by microscopi- cal examination. Starch granules are scarcely affected by cold water ; on warming they absorb water and swell. Finally the starch passes into a condition of colloidal dispersion or semi- solution, " starch paste." Starch which has been heated in water (either admixed or naturally present with the starch as in a potato) until the granules are ruptured and the material more or less dispersed is very much more rapidly hydrolyzed by digestive ferments than is raw starch. To colloids such as starch, the usual methods of determining molecular weight are not applicable. It is certain, however, from the chemical complexity of some of the dextrins which * A very detailed study of the starch granules of different species of plants has been made by Reichert and published by the Carnegie Institution of Washington. (Sec references at end of chapter.) CARBOHYDRATES 1 3 result from hydrolysis of starch, that the molecular weight of starch must be very high and its chemical constitution very complex. Probably the value of x in the formula (CeHioOs)! is very large, perhaps in the neighborhood of 200, corresponding to a molecular weight of about 32,000. For a full discussion of the more important facts bearing on the chemical constitution of starch, see the paper by Thomas cited in the list of refer- ences at the end of the chapter. Starch either in the soUd or in the " soluble " (dispersed) form is colored intensely blue when treated with iodine. This well-known reaction is deHcate and distinctive, but is now be- lieved to be due to colloidal adsorption rather than to the for- mation of a definite chemical compound. The term " starch," as we ordinarily use it, probably covers at least two substances. The more abundant of these, a-amylose (also called " amylopectin "), forms on heating in water a viscous opalescent paste, gives a somewhat purplish blue color with iodine, is evidently of great molecular complexity, and has recently been found to contain a small amount of phos- phorus * as an essential constituent. The less abundant com- ponent of starch, yS-amylose (also called " amylose "), forms when heated in water a clear, limpid solution which gives a pure blue color with iodine. The starch-digesting enzymes hydrolyze both a-amylose and )3-amylose, but not always with equal faciHty.f Starch on hydrolysis by means of acid gives first mixtures of dextrin and maltose, and finally glucose only as an end-product. The most satisfactory hydrolysis of starch to glucose is ac- complished by boiling or heating in a boiling water bath with hydrochloric acid of a concentration of about 2.5 per cent. When brought in contact with saliva, starch is hydrolyzed by * In the case of potato starch about 0.06 per cent. See papers by Northrup and Nelson and by Thomas referred to at the end of the chapter. t See paper by Sherman and Baker referred to at the end of the chapter. 14 CHEMISTRY OF FOOD AND NUTRITION the ptyalin, with the formation of dextrin and maltose. A similar hydrolysis is affected by " amylopsin," the starch- splitting enzyme of the pancreatic juice, preferably known as pancreatic amylase (see terminology of enzymes, Chapter IV). " Soluble starch," largely used for laboratory experiments, is usually prepared by soaking raw starch in cold hydrochloric acid (about 7 per cent HCl) for several days, and then washing with cold water. Dextrins, (CeHioOs)^ or (CgHio05)x-H20, are formed from starch by the action of enzymes, acids, or heat. Small amounts of dextrin are found in normal, and larger amounts in germi- nating, cereals. Malt diastase, acting for some time upon starch in fairly concentrated solution, yields usually about one part of dextrin to four of maltose. During acid hydrolysis, dextrin is formed as an intermediate product between soluble starch and maltose. Commercial dextrin, the principal constituent of " British gum," is obtained by heating starch, either alone or with a small amount of dilute acid. The dextrins are much more soluble than the starches; and dextrin molecules while doubtless very large and complex are probably not over one fifth the size of starch molecules. The digestion of dextrin has already been mentioned in connection with that of starch, both saHva and pancreatic juice forming dextrin during the digestion of starch and acting upon it with the production of maltose. Complete hydrolysis of dextrin, as by boihng with acid, yields glucose as the sole product. Glycogen, (CcHioCOj;, plays much the same role in animals which starch plays in plants, and is sometimes called " animal starch." Glycogen also takes the place of starch as reserve carbohydrate in fungi and other forms of plant life not pro- \nded with the chlorophyll apparatus. It is a white, amor- phous powder, odorless and tasteless, which swells up and ap- CARBOHYDRATES 15 parently dissolves in cold water to an opalescent colloidal dis- persion which is not cleared by repeated filtration, but loses its opalescence on addition of a very small amount of potassium hydroxide or acetic acid. Water solutions (dispersions) of glycogen are readily precipitated by alcohol. When treated with iodine they react yellow-brown, red-brown, or deep red. Hydrolysis of glycogen yields glucose only, as end-product. Glycogen occurs in the lower as well as the higher animals, and in all parts of the body, but is especially abundant in the liver. The amount of glycogen in the liver depends to a great extent upon the condition of nutrition of the animal. In the average of seven experiments by Schondorff in which dogs were fed for the production of as much glycogen as possible, 38 per cent of that found was in the liver, 44 per cent in the muscles, 9 per cent in the bones, and the remaining 9 per cent in the other tissues of the body. But the distribution of glycogen in the body as shown by these experiments was quite variable, even among animals of the same species which had been fed in the same way. It is well known, too, that some species store glycogen in their muscles to a greater extent than others, attempts even having been made to distinguish analytically between horseflesh and beef by the difference in their glycogen content. The storage of glycogen in the body is promoted by rest as well as by Hberal feeding, and stored glycogen is used up rapidly during active muscular work. Cellulose, (CeHioOs)!, the chief constituent of wood and of the walls of plant cells generally, is an anhydride of glucose and can be made to yield the latter when hydrolyzed by suit- able treatment with strong acid. Typical cellulose of mature fiber (such as cotton, linen, or wood fiber) is, however, quite resistant to the action of dilute acids or of ordinary enzymes and passes through the digestive tract for the most part unchanged. The toughness of the cellulose differs with the stage of growth or maturity, and some of the less resistant forms of cellulose, 1 6 CHEMISTRY OF FOOD AND NUTRITION such as that of tender whito. cabbage, may disappear from the digestive tract in appreciable amounts. Experiments to de- termine whether the cellulose thus disappearing is digested to sugar and absorbed or merely decomposed by bacteria in the digestive tract have not given conclusive results. According to Swartz : " In any event, the quantities of cellulose which the alimentary tract of man is capable of absorbing are, apparently, too small for it to play a role of any importance in the diet of a normal individual." The cellulose in the food may, how- ever, serve a very useful purpose in giving bulk to the food residues and thus facilitating their passage along the digestive tract. Hemicelluloses is a term somewhat loosely applied to poly- saccharides, usually occurring as constituents of cell walls in plants, which are not digested by the starch-spHtting enzymes but are usually much more readily hydrolyzed by acid than is cellulose. In many plant tissues the hemicellulose consists chiefly of pentosans ; in other cases it is largely mannan or ga- lactan. Mannans, (CeHioOs)!, anhydrides of mannose, are widely distributed in the vegetable kingdom and, as Swartz points out, show great differences in solubility, ranging from the readily soluble mucilaginous forms found in certain legumes to the horny matter of such seeds as the date, a form of mannan which was long confused with true cellulose. The experiments of Swartz upon the mannan of salep showed it to disappear com- pletely in its passage through the human digestive tract, al- though tests with individual digestive enzymes gave negative results. In what way and to what extent the mannan thus dis- appearing from the digestive tract becomes available in nu- trition is still a subject of investigation. Galactans, (CeHioOj)!, anhydrides of galactose, are widely distributed in plants. They occur in the seeds of legumes and to a slight extent in the cereals also, in by-products of beet CARBOHYDRATES 1 7 sugar manufacture and abundantly in several of the algae and lichens, including Chinese moss, agar-agar, and Irish moss. The pectins are said to consist largely of galactans, apparently either in combination or admixture with pentosans and perhaps other complexes as well. The galactans differ in their solu- bihties and apparent digestibility when eaten by man or other animals, but on the whole do not appear to be of much nutri- tive value. Those of agar-agar and Irish moss, which are most used as food, are not digested. Levulans is the term under which a number of polysaccharides of the composition (CeHioOb)^, and yielding fructose (levulose) on hydrolysis have been described. The most important of these, at least so far as is at present known, is inulin, a white, powdery substance occurring in the tubers of the Jerusalem artichoke and to a less extent in the bulbs of onions and garlic as well as in various parts of plants not commonly used for food. By the action of acids inulin is very readily hydrolyzed to levulose, but the digestive juices do not seem to contain enzymes capable of hydrolyzing inulin and it appears to be of practically no importance as human food. REFERENCES Abderhalden. Physiologische Chemie (3. Aufl.). Abderhalden. Biochemisches Handlexicon. , Abderhalden. Handbuch der Biochemischen Arbeitsmethoden. Armstrong. The Simple Carbohydrates and the Glucosides. Armstrong. Article on Carbohydrates in Thorpe's Dictionary of Applied Chemistry (Revised Edition). Browne. Handbook of Sugar Analysis. Cohen. Organic Chemistry. CZ.A.PEK. Biochemie der Pflanzen. LippiiANN. Chemie der Zuckerarten. Mathews. Physiological Chemistry. Nef. (Behavior of the sugars toward alkalies and oxidizing agents.) Leibig's Annalen der Chemie, Vol. 357, page 214; Vol. 376, page i; Vol. 403, page 204. c l8 CHEMISTRY OF FOOD AND NUTRITION Northrop and Nelson. The Phosphorus Content of Starch. Journal of I he American Chcmkal Society, Vol. 38, page 472 (igi6). Reichert. The DilTerentiation and Specificity of the Starches in Relation to Genera and Species. Carnegie Institution of Washington, Publica- tion No. 173. ScHRYVER A.NTJ Hayxes. Pectin Substances of Plants. Biochemical Journal, Vol. 10, page 539 (1916). Sherman and Baker. Experiments upon Starch as Substrate for Enzyme Action. Journal of the American Chemical Society, \o\. 38, page 1885 (1916). Svvartz. Nutrition Investigations on the Carbohydrates of Lichens, Algae and Related Substances. Transactions of the Connecticut Academy of Sciences, Vol. 16, pages 247-382 (1909). Thomas. The Phosphorus Content of Starch. Biochemical Bulletin, Vol. 3, page 403 (191 4). Thomas. The Chemical Constitution of Starch. Biochemical Bulletin, Vol. 4, page 379 (1915)- TOLLEN'S. Kurzes Handbuch der Kuhlcnhydrate. CHAPTER II FATS AND LIPOIDS Almost as widely distributed in nature as the carbohydrates, and constituting a much more concentrated form of fuel to supply energy in nutrition, are the fats. Fats are glyceryl esters of fatty acids, and since glycerol is a triatomic alcohol and the fatty acids are monatomic, a normal glyceride is a triglyceride and on hydrolysis yields three molecules of fatty acid and one molecule of glycerol. Thus, for example : C3H5(Cl8H3502)3 + 3 HoO ^ C3H5(OH)3 + 3 C18H36O2. Stearin Glycerol Stearic acid (glyceryl tristearate) When the splitting of the fat is brought about by means of an alkali instead of water, the corresponding products are glycerol and three molecules of the alkali salt of the fatty acid. Since alkali salts of the fatty acids are commonly known as soaps, this reaction is usually called saponification of the fat. The fats are therefore a definite group of chemical compounds, and the term appHes equally to the solid and the Hquid members of this group. As a matter of convenience, however, the liquid fats are often called " fatty oils." The fatty oils are also sometimes called " fixed oils," since a spot made by drop- ping a fatty oil on paper cannot be removed by drying (as can a volatile oil), nor by washing with water (as can glycerin). Another property which helps to characterize the fats is that glycerol, or the glyceryl radicle of a fat, when heated to a high temperature (300° C. or over), decomposes with production of acrolein, an aldehyde of characteristic odor and very irritating to the mucous membranes. Doubtless also fatty acid radicles 19 20 CHEMISTRY OF FOOD AND NUTRITION may sometimes conlributc to the production of irritating fumes when fat is overheated. The fats, including fatty oils, are lighter than water, their specific gravities ranging between 0.90 and 0.97. They are poor conductors of heat and therefore tend to conserve the heat of the body, while they show on oxidation a much higher fuel value than any of the other foodstuffs. All of the fats are practically insoluble in water, and all ex- cept those of the castor oil group are sparingly soluble in cold alcohol, but dissolve readily in petroleum ether and mix in all proportions with light petroleum oils. Light petroleum dis- tillate (" petroleum ether ") is often used as a solvent for fat. All of the fats are readily soluble in ether, carbon bisulphide, chloroform, carbon tetrachloride, and benzene. Since neither carbohydrates, proteins, nor ash constituents are soluble in ether (or the other " fat solvents "), it follows that the fat of a food may be readily separated from the other chief components by drying the food and extracting the dry material with pure ether. After the fat has been completely dissolved away from the other foodstuffs, it can be recovered from the solvent by evaporating the latter at a relatively low temperature. This is the method commonly used to estimate the percentages of fat in foods and to obtain small portions of fat for examination. It must be noted, however, that the fat thus obtained is not always pure in the sense of consisting entirely of substances meeting the definition of fat as given above. Obviously, such an extract will contain, along with the fat, any other ether-sol- uble substances which were present in the food, and may con- tain substances which, while not appreciably soluble in ether alone, are dissolved by the mixture of ether and fat. It is there- fore somewhat more accurate to speak of the material extracted by ether as " ether extract " rather than as " fat," and it will be found so designated in some statements of analytical results. In most human foods — at least those which are important as FATS AND LIPOIDS 21 sources of fat — the constituents of the ether extract other than true fat are for the most part fat-Hke substances and we shall therefore be sufl5ciently accurate in most cases if we desig- nate the material extracted by ether by the simple term " fat," remembering, however, that we may thus include along with the glycerides (and any free fatty acids which may be present) small amounts oi fat-like substances or lipoids, and of fat-soluble or other ether-soluble matter. The food fats of commerce have been separated from the materials in which they naturally occurred not by solvents as above described but by mechanical means such as churning (butter) or pressing (olive or cottonseed oil) but even in this case the naturally occurring fat-soluble substances will still remain dissolved in the separated fat. Recent investigations indicate that these fat-like and fat-soluble substances, although occurring only in small quantities, may have very important functions in nutrition. We shall have occasion to study them in that connection later. The actual glycerides of any common natural fat, with the exception of butter, would if obtained absolutely pure be color- less, tasteless, and odorless. The colors, tastes, and odors of fats are therefore ordinarily due to substances present in small amount which might be removed by refining processes. All of the quantitative differences among the fats are to be accounted for by the kinds and the amounts of the fatty acids which enter into the composition of the glycerides. Fatty Acids The greater number of the fatty acids belong to a few homol- ogous series. The series to which stearic acid belongs may be represented by the general formula, CnHonOo, and is made up of homologues of acetic acid. The principal members of physi- ological importance are as follows: 2 2 CHEMISTRY OF FOOD AND NUTRITION Acids of the Series C„H2„02 Butyric acid (C^HsOi) occurs as glyceride to the extent of about 5 to 6 per cent in butter and in very small quantities in a few other fats. Caproic acid (C6H12O2) is obtained from goat and cow butter and coconut fat. Caprylic acid (C8H16O2) is obtained from coconut oil, butter, and human fat. Capric acid (C10H00O2) is obtained from coconut oil, butter, and the fat of the spice bush. Laurie acid (C12H24O2) occurs abundantly as glyceride in the fat of the seeds of the spice bush, and in smaller propor- tions in butter, coconut fat, palm oil, and some other vege- table oils. Myristic acid (C14H28O2) is obtained from nutmeg butter, coconut oil, butter, lard, and many other fats, as well as from spermaceti and wool wax. Palmitic acid (C16H32O2) occurs abundantly in a great va- riety of fats, both animal and vegetable, including many fatty oils, and also in several waxes, including spermaceti and beeswax. Stearic acid (C18H36O2) is found in most fats, occurring more abundantly in the soHd fats and especially in those ha\ang high melting points. Butyric acid is a mobile liquid, mixing in all proportions with water, alcohol, and ether, boihng without decomposition, and readily volatile with steam. With increasing molecular weight the acids of this series regularly show increasing boiling or melting points, decreasing solubiHty, and loss of volatihty with steam. For ordinary temperatures the dividing line between liquids and solids falls at about capric acid. Stearic acid is a crystalline soHd, insoluble in water, and only moderately soluble in alcohol and ether. FATS AND LIPOIDS 23 Acids of the Series C„H2n-202 These are unsaturated compounds. Each molecule contains one ethylene linkage or " double bond," and can take up by addition two atoms of halogen to form a saturated compound.* These unsaturated acids have, as a rule, much lower melting points than the saturated acids containing the same number of carbon atoms. The glycerides show correspondingly lower melting points than those of the saturated fatty acids and are therefore found more largely in the soft fats and the fatty oils. Such soft fats or fatty oils can be hardened to any desired con- sistency (up to that of stearin) by hydrogenation, which changes the unsaturated fatty acid radicles into the corresponding members of the saturated series. In recent years this process has been exploited commercially and large quantities of refined cottonseed oil are now hydrogenated to the consistency of lard and sold under trade names as lard substitutes. Other oils are also hardened by hydrogenation. Phycetoleic acid (Ci6H3o02) is obtained from seal oil and sperm oil ; an isomeric acid, hypogaic, occurs in peanut oil. Oleic acid (C18H34O2) occurs as glyceride in nearly all fats and fatty oils and is much the most important member of the series. Many of the typical oils of both animal and vegetable origin, such as lard oil and oHve oil, consist mainly of olein. Erucic acid (C22H42O2) is obtained from rape seed and mustard seed oils, and is not found in animal fats except when oils which contain this acid have been fed to the animal. The gradual change in physical properties with increasing molecular weight which is noticeable in the stearic acid series is not apparent in this series, probably because the known acids * The relative number of double bonds is measured analytically by determining the percentage of iodine which the fat or fatty acid will absorb. Thus pure oleic acid (mol. wt. 282) absorbs 2 atoms of iodine, giving an "iodine number" of go; pure Unoleic acid would absorb 4 atoms of iodine to the molecule, giving an "iodine number" about twice as great. 24 CHEMISTRY OF FOOD AND NUTRITION of the series differ as regards the position of the double bond and are therefore not strictly homologous. Other Unsaturated Fatty Acids Acids of the series C„H2„_402, C„H2„-602, and CnH2„_802 have been found to occur as glycerides in some of the fats. Linoleic acid, C18H32O2, and linolenic acid, C18H30O2, are the best known of these acids. They are found abundantly in linseed oil and in others of the so-called " drying oils," which on account of the affinity for oxygen of their highly unsaturated glycerides are gradually oxidized to solids on exposure to the air. Fatty acids having the same number of double bonds, but not the same property of oxidizing to hard, solid films are found in fatty oils of animal origin, especially those obtained from marine animals and from fishes. Since the acids of this series have still lower melting points than the corresponding acids of the oleic series, and since the physical properties of the glyc- erides follow those of the fatty acids which they contain, a fat containing an acid isomeric with linoleic or linolenic acid will be more fluid at any given temperature than one containing oleic acid in the same proportion. Hence, it is apparent that glycerides of the highly unsaturated and more fluid acids are physiologically adapted to the cold-blooded animals, and it is found that they are especially abundant in fish fat ; the acids of the series CnH2n_802 have been obtained as yet only from fish oils. " Simple " and " Mixed " Triglycerides Triglycerides in which the three fatty acid radicles are of the same kind are known as simple triglycerides. Tristearin, triolein, tripalmitin, etc., are examples of simple triglycerides. A mixed triglyceride is one in which the three fatty acid radicles are not all of the same kind. For example, distearo-olein (having two radicles of stearic and one of oleic acid), dioleo-palmitin (having FATS AND LIPOIDS 25 two of oleic and one of palmitic), or stearo-oleo-palmitin (hav- ing one radicle each of stearic, oleic, and palmitic acids), is each a mixed triglyceride. It is evident from the chemical structure of glycerol that there can be only one simple triglyceride of any given fatty acid but that with two fatty acid radicles alike (Ro) and one dif- ferent (Ri) the triglyceride may have either of the following forms : H H I I HC-OR2 HC-ORi I I HC-ORi HC-OR2 I I HC-OR2 HC-OR2 I I H H That is, the two radicles of the same kind may be on the ter- minal carbons or may be adjacent. It will be noted that the two substances here represented have exactly the same compo- sition, but different constitution. If now the triglyceride contains one each of three different acid radicles (Ri, R2, R3) there are plainly three possible forms : H H H HC-ORi 1 HC-OR2 HC-ORi 1 HC-OR2 1 HC-ORi 1 1 HC-OR3 1 HC-OR3 1 HC-OR3 1 HC-OR2 1 1 H 1 H 1 H Each of these three substances has exactly the same com- position, though the constitution is different for each. 26 CHEMISTRY OF FOOD AND NUTRITION It should he noted that these five formula; represent types of structure and that the actual number of triglycerides possible from three fatty acid radicles is greater since we may have sub- stances corresponding to either of the first two in which R3 replaces either Ri or R2 ; and it is plain that as the number of fatty acids is increased beyond three the number of possible mixed triglycerides increases very rapidly so that with the large number of fatty acids which are now known to be of fairly com- mon occurrence in fats the possible number of mixed triglycer- ides must be almost unlimited. The simple triglycerides cor- responding to the common fatty acids are all known, but naturally not all of the practically innumerable possible mixed triglycerides have been separated or prepared. Berthelot in 1869 suggested that fats probably contain mixed glycerides and in 1889 Blyth and Robertson reported a palmito-stearo-olein in butter, but it is only since Kreis and Hafner (1903) described the preparation of palmito-distearin from beef tallow and Bomer (1909) separated stearo-dipalmitin from mutton tallow and palmito-distearin from lard that the widespread occurrence of mixed glycerides in the familiar fats has been generally accepted. Among the other mixed glycerides reported as having been isolated from natural fats are : Myristo-pahnito-olein in cacao butter (Klimont, 1902), dipalmito-olein and stcaro-pahnito-ohin in tallow (Hansen, 1902), distearo-olein in cacao butter (Fritzweiler, 1903) and in Borneo tallow (Klimont, 1905), stearo-diolein in human fat (Partheil and Ferie, 1903). The fact long known to analysts that fats too nearly identical in composition to be distinguished by chemical analysis may still show differences in crystalline structure under the microscope is now explained as due to the presence of different mixed tri- glycerides containing the same fatty acid radicles. Thus beef fat rendered at such a temperature as to contain the glycer- FATS AND LIPOIDS 27 ides of stearic, palmitic, and oleic acids in practically the same proportions as in lard still differs so constantly from lard in its microscopic appearance as to indicate the presence of dis- tinct chemical substances and has now been shown to contain different mixed triglycerides. The fact that tributyrin has an intensely bitter taste makes it seem probable that none of this substance occurs in butter but rather that the butyric acid in butter fat is in the form of mixed glycerides. Probably mixed glycerides are as abundant as simple glycerides in natural fats. Formation and Composition of Natural Fats Fats are formed both in plants and in animals. The con- ditions which determine fat formation, and the character of the fat formed in different species and under different conditions, are better known than the chemical steps involved in the process. It is hardly necessary to mention the fact that the true fats are composed of the same three chemical elements of which the carbohydrates are composed (carbon, hydrogen, and oxygen) and that since the fats contain less oxygen and more carbon and hydrogen than the carbohydrates, they con- stitute a more concentrated form of fuel or a more compact and hghter medium for the storage of energy for future use. The question therefore presents itself whether either the plant or the animal organism (or both) has the power to change car- bohydrate material into fat. Formation of Fat from Carbohydrate In plants there are many indications of the formation of fat from carbohydrate, as when decrease of starch and increase of fat go on simultaneously in a ripening seed, or when sugars are found to be constantly brought to a tissue in which fat is form- ing and there disappear as the formation of fat progresses. It 28 CHEMISTRY OF FOOD AND NUTRITION is probably because no one has doubted the formation of fat from carbohydrate in plants that the process has not been more rigorously investigated. In animals it is certain that fat may be formed from carbo- hydrate. From the standpoint of our present knowledge it would seem that the readiness with which farm animals are fattened on essentially carbohydrate food should have been sufficient to convince early observers ; but this evidence appears to have been overlooked formerly because of the idea, for a long time prevalent, that simpler substances are built up into more complex compounds only in the plant, and not in the ani- mal organism. In recent years it has become necessary to aban- don this latter assumption completely, and there is now abun- dant evidence that the animal body synthesizes fat from carbohydrate. The most obvious method of demonstrating the conversion of carbohydrate into fat is that followed by Lawes and Gil- bert. Several pigs of the same Htter and of similar size were selected; some were killed and analyzed as " controls," while the others were fed on known rations and later weighed, killed, and analyzed to determine the kinds and amounts of material stored in the body. In several cases the amounts of fat stored during such feeding trials were found to have been much larger than could be accounted for by all of the fat and protein fed, so that at least a part, and in some cases the greater part, of the body fat must have been formed from the carbohydrate of the food. Many similar experiments have been made, and the transformation of carbohydrate into fat has been demonstrated by this method in carnivorous as well as herbivorous animals. It has also been shown that carbohydrates contribute to the production of milk fat. Jordan and Jenter kept a milch cow for fifty-nine days upon food from which nearly all of the fat had been extracted. During this period about twice as much milk fat was produced as could be accounted for by the total FATS AND LIPOIDS 29 fat and protein of the food, and in addition the cow gained in weight and her appearance showed that she had more body fat at the end than at the beginning of the experiment. Instead of determining directly the fat formed in the animal fed on carbohydrate, the production of fat from carbohydrate may be demonstrated by keeping the animal experimented upon in a respiration chamber so arranged that the total carbon given off from the body may be determined and compared with the total carbon of the food. If in such a case the body is found to store more carbon than it could store as carbohydrate or pro- tein, it is safe to infer that at least the excess of stored carbon is held in the form of fat. Many such experiments upon dogs, geese, and swine have shown storage of carbon very much greater than could be accounted for on any other assumption than that a part of the carbon of the carbohydrates eaten re- mained in the body in the form of fat. Further evidence of the transformation of carbohydrate into fat in the animal body is obtained from the " respiratory quotient." The discussion of the quotient and the significance of the information which it furnishes, as also the study of the chemical steps through which the transformation of carbohydrate into fat may take place, will be taken up in connection with the general study of the fate of the foodstuffs in metabolism (Chap- ter V). Composition and Properties of Animal Fat Just as we found that the character of the fat of the cold- blooded animals is adapted to the maintenance of a fluid or plastic consistency at the low temperature to which it is ex- posed, so to a less degree the character of the fat of warm- blooded animals appears to vary with its position in the body and with the temperature to which the body is subjected during the time that the fat is in process of formation. Thus Hen- riques and Hansen concluded from experiments with pigs that 30 CHEMISTRY OF FOOD AND NUTRITION the thick layer of subcutaneous fat on the back, where it was not thoroughly warmed by the blood and therefore had an aver- age temperature considerably below that of the interior of the body, was richer in unsaturated compounds (olein, etc.) and had a lower melting point than the fat of the body as a whole ; while the fat from animals which had been grown in a warm room, or which had been heavily jacketed so that the skin was not exposed to cold air, contained near the skin fat of more nearly the same composition as in the interior of the body. Moulton and Trowbridge have observed that the fat in beef animals becomes richer in olein and therefore softer with age, with fatness, and with nearness to the surface of the body. Usually, however, the nature of the fat found in the body is more or less characteristic of each species or group of closely related species. Herbivora contain as a rule harder fats than carnivora, land animals have harder fats than marine animals, and all warm-blooded animals have fats which are decidedly harder than those found in fishes. The fats of different mam- mals were investigated by Schulze and Reineke, whose results * showed little variation from an average of carbon, 76.5 per cent ; hydrogen, 12 per cent; oxygen, 11.5 per cent, as may be seen from the following : Kind or Fat Carbon Hydrogen Oxygen Human fat f Beef fat Mutton fat Pork fat 76.62 76.50 76.61 76.54 11.94 II. 91 12.03 11.94 11.44 11-59 11.36 11.52 The foregoing statements refer to the fat of the adipose tis- sues. In the fat extracted from the Hver, kidney, and heart, * Armsby's Principles of Animal Nutrition, page 6i. t Benedict and Osterberg {American Journal of Physiology, Vol. 4, page 6g) found in 8 samples of human fat an average of 76.08 per cent carbon and n.78 per cent hydrogen. FATS AND LIPOIDS 3 1 Hartley* finds fatty acids of the series C„H2„_402, C„H2„_602, and possibly C„H2„_802. A possible explanation of this difference between the fat of the adipose tissues and of the actively functioning organs is to be found in the greater reactivity of the unsaturated acid radi- cles. The saturated fatty acid radicles are relatively stable and inert ; and when the glycerides of such acids are deposited in the inactive adipose tissues, the fats may remain unaltered for a long time and accumulate in considerable quantities. The unsaturated fatty acid radicles are less stable and more readily acted upon and broken up. This is consistent with the fact that we find them more abundantly in fats of the organs in which metaboHsm is more active and has led to the view that the desaturation of fatty acid radicles by the active organs of the body may be an important preHminary to the metabolism of the fat. On the other hand, the formation of unsaturated fatty acid radicles such as oleic and linoleic does not, according to our present knowledge, seem essential to the " /8-oxidation theory " which is now generally held as most probably repre- senting the main course of fatty acid metaboHsm (Chapter V). It is therefore entirely possible that the highly unsaturated fatty acids found, for example, in the liver, may be present as constituents of the protoplasm of these cells, essential to the properties which enable them to carry out some of their func- tions but not necessarily connected with the metabolism of fat itself. Butter fat differs from body fat in containing fatty acids of lower molecular weight (particularly butyric acid, which is fairly characteristic of butter), and so shows a higher percent- age of oxygen and lower percentages of carbon and hydrogen. The most abundant acids of butter fat are, however, palmitic, oleic, and myristic, and the ultimate composition is not very greatly different from that of body fats. A sample of butter * Journal of Physiology, Vol. 36, page 17. 32 CHEMISTRY OF FOOD AND NUTRITION fat analyzed by Browne* showed 75.17 per cent carbon, 11.72 per cent hydrogen, and 13. 11 per cent oxygen. Storage of Food Fat in the Body In discussing the formation of body fat from carbohydrate it was shown that often the greater part of the fat stored is manu- factured in the body from carbohydrate. So striking were the results of some of the experiments demonstrating the synthesis of fat from carbohydrate, that physiologists came to question for a time whether any of the fat deposited in the tissues comes from the fat of the food. Abundant evidence that food fats may be directly deposited in the body has been obtained by feeding characteristic fats to dogs and showing that these fats can afterwards be recognized in the tissues of the animals. Experiments of this kind have been made, using Unseed oil, rapeseed oil, or mutton tallow, any of which is easily distinguish- able by its chemical and physical properties from the fat nor- mally found in the body of the dog. For example, Munk starved a dog for 19 days, and then for 14 days fed a mixture of the fatty acids obtained from mutton tallow, as a consequence of which about one half of the weight lost by fasting was regained. The dog was then killed and yielded on dissection iioo grams of fat melting at 40°, which is about the melting point of mutton tallow, whereas normal dog fat melts at about 20°. In another experiment by Munk rape oil was fed and the fat obtained from the dog was found to contain 82.4 per cent of oleic and erucic acids and 12.3 per cent of soHd acids, whereas normal dog fat had only 65.8 per cent oleic, no erucic, and 28.8 per cent of solid fatty acids. The occurrence in the body fat of properties usually char- acteristic of some particular fat which has been fed is now very well known and is recognized in estabUshing standards of purity * Journal oj the American Chemical Society, Vol. 21, page 823 (1899). FATS AND LIPOIDS 33 for fats of animal origin. Thus, the lard obtained from swine which have been fed cottonseed meal shows the characteristic color reactions of cottonseed oil, and more elaborate tests must be made in order to determine whether cottonseed fat has actually been mixed with the lard. European food officials sought to establish an easy method of distinguishing between butter and its substitutes by requiring manufacturers of any butter substitute to use a certain pro- portion of sesame oil in the preparation, sesame oil having a characteristic color reaction which can be very easily demon- strated without the use of laboratory faciUties. It was found, however, that the same sesame oil reaction might be exhibited by a perfectly pure butter fat from cows which had been fed upon sesame meal. Evidence of the formation of body fat from food fat has also been obtained by experiments upon the total amount of fat formed in the body when the amount and composition of the food eaten was accurately known. Hoffmann starved a dog until its weight had decreased from 26 to 16 kilograms, so that it must have been almost devoid of fat. He then fed small amounts of meat and large amounts of fat for five days, after which the dog was killed and analyzed. The body contained 1353 grams of fat, of which not over 131 grams could have come from proteins, and only a few grams at most from the small amount of carbohydrates in the meat fed, so that about nine tenths of the fat which the animal had laid on must have come from the fat of the food. Thus there is abundant experimental evidence that both the carbohydrate and the fat of the food may serve as sources of body fat. In a later chapter it will be shown that protein also may contribute to the production of fat in the body. A question naturally arises as to how, if proteins, fats, and carbohydrates of food may all contribute to the production of body fat, the nature of the fat can still be to any significant de- D 34 CHEMISTRY OF FOOD AND NUTRITION gree characteristic of the species in which it is found. A partial explanation appears to be furnished by the recent work of Bloor, who finds that when the fat of the food has been spUt to glycerol and fatty acids in the course of digestion and these digestion products are taken up and resynthesized to fat in the intestinal wall, there may go into the resynthesized fat not only the fatty acid radicles of the food fat but also fatty acid radicles formed in the body. These latter, entering into the consti- tution of the absorbed fat, tend to give it some of the proper- ties characteristic of the species while at the same time some of the characteristics of the food fat may be retained. Thus when a dog is fattened by feeding mutton tallow which contains more stearin and less olein than ordinary dog fat the organism may, if the fattening is gradual, furnish enough oleic acid radicles to bring the resynthesized fat to the consistency ordinarily found in dog fat, or if the fattening is more rapid the oleic acid radicles may not be supplied at a sufficiently rapid rate to yield this result and the dog will then lay on fat of a character somewhere between that of mutton tallow and ordinary dog fat, the in- fluence of the food fat upon the character of the stored fat being more pronounced the more rapidly the fattening is carried out. It will be noted that, even if the fatty acid radicles synthesized in the body are built into the absorbed fat to such an extent as to bring its consistency and other physical properties to what is characteristic for the species, yet such body fat may still con- tain some radicles of fatty acids characteristic of the experi- mental food and not ordinarily found in the fat of the animal, as in the case of erucic acid in the experiment cited above (page 32). Fats and Lipoids as Body Constituents From what has been stated above, fat is seen to be a form of reserve fuel to which any of the organic foodstuffs may contribute (see also the discussion of fate of the foodstuffs in FATS AND LIPOIDS 35 Chapter V). It is as reserve fuel that the large deposits of body fat are chiefly significant, but it should not be forgotten that even this " depot fat " may function as a protection to the body from mechanical injury and too rapid a loss of heat when exposed to cold, and as a packing and support to the visceral organs, particularly the kidneys. In recent years it has come to be rec- ognized that modified fats and fat-hke substances (Hpoids) are essential constituents of body tissues. Thus cell membranes are not simply walls of protein matter but probably are composed of both proteins and lipoids of different kinds and in varying pro- portions, and protoplasm is to be thought of as an emulsion of proteins and lipoids rather than as a jelly of proteins alone. Taylor, writing in 191 2, says: * " Fat plays two roles within the body. Fat represents the ultimate form of the storage of fuel, and the depot fats are quite the most inert and dead of any of the body structures. On the other hand, fats joined with protein and in complex combinations of still unknown composition, represent the most essential structures in cellular protoplasm, cell membranes, and in the central nervous system. The subjects of fat in its cellulometaboHc relations and fat in the energy metabolism are almost as distinct as though different substances were under consideration. Our information on the two subjects is not equal ; we know much concerning fat as fuel; we know little concerning fat in cellular structure." Mathews, in 1915,! writes: "It will be recalled that all living matter contains a larger or smaller amount of organic substances which are soluble in alcohol, ether, and other fat solvents. These substances help to give to protoplasm its properties of containing large amounts of water but not dis- solving ; and also the power of taking up readily and in large amounts chloroform, ether, and other substances soluble in fats but not readily soluble in water. They are among the funda- mental and ever-present constituents of living matter." * Digestion and Metabolism, page 342. f Physiological Chemistry, page 61. 36 CHEMISTRY OF FOOD AND NUTRITION Following the suggestion of Gies,* Mathews includes all such substances under the group name of lipins (from the Greek, lipos, fat) which is thus made to cover both the true fats and all fat-like or lipoid substances. According to Mathews' classi- fication based on that proposed by Gies, the term " lipins " covers : " Alcohol-ether soluble constituents of protoplasm hav- ing a greasy feel and insoluble in water." These are divided into nine groups as follows : 1. Fats and fatty acids, the term "fat" being here confined to those neutral glycerides which are solid at 20° C. 2. Fatty oils (liquid at 20°C.) including (i) drying oils such as linseed oil, (2) semidrying oils such as cottonseed oil, (3) non-drying oils such as olive oil. 3. Essential oils. Volatile, generally odoriferous, oil substances of varied chemical nature. 4. Waxes. Esters of fatty acids with monatomic alcohols of high molec- ular weight such as the sterols. 5. Sterols. Alcohols, generally of terpene group, soluble at ordinary temperatures. Cholesterol, phytosterol, etc. 6. Phospholipins. Phosphatids. Fatty substances, yielding on hydroly- sis phosphoric acid and fatty acids (as well as glycerol). Lecithin, cephalin. 7. GlycoHpins. Fatty substances free from phosphorus, yielding on hydrolysis fatty acids and a carbohydrate. Cerebron, phrenosin. 8. Sulpholipins. Fatty substances, yielding on hydrolysis fatty acids and sulphuric acid. Sulphatide of brain. 9. Aminolipins. Fatty substances, free from phosphorus, which contain amino nitrogen. Mathews remarks : " While the group of lipins contains such widely different chemical substances as the aromatic essential oils, like clove oil, the true neutral fats, like mutton tallow, the sterols, which are aromatic alcohols, and the phosphatids, or phospholipins, which contain large amounts of phosphoric acid, the members of the group all possess two or three proper- ties by virtue of which they are called lipins. These properties are their greasy or fat-Hke feel, their solubility in chloroform and fat * A more elaborate classification of the lipins is suggested by Gies and, Rosen- bloom in the article cited at the end of this chapter. FATS AND LIPOIDS 37 solvents, and their insolubility in water. They constitute, then, a very heterogeneous group, chemically and physiologically." We have therefore, in the large heterogeneous group of sub- stances called lipins: (i) true fatty substances — fats, fatty oils, fatty acids, (2) fat-Hke or lipoid substances — some of these latter (Hke lecithin and other phospholipins) being closely related to the fats both chemically and biologically, others (like the sterols) showing little direct chemical relation to the fats but apparently bearing significant biological relationships, while still others (like certain of the essential oils) appear to bear httle relationship to the fats and to be classified as lipins merely because of their physical properties. If the term " Hpins " is to be so broadly used, it may be convenient to apply the term " lipoid " to substances other than fats or fatty acids but which are related to them chemically or biologically. Prominent among the lipoids (or fat-like substances other than true fats) are the sterols (solid alcohols) and the phospho- lipins or phosphatids. The latter are substances which contain a substituted phosphoric acid radicle in place of one or more of the fatty acid radicles of a fat. Sterols occur, at least in small amounts, in all natural fats. The best-known sterols are cholesterol (C27H44O) and phytosterol (C27H46O). Cholesterol occurs in animal fats, and phytosterol (or the closely related sitosterol) in those of vegetable origin. One method of determining whether vegetable fat is present in butter or lard is to examine for the presence of phytosterol, since phytosterol is not, like the substances to which the color reactions of cottonseed and sesame oils are due, carried over from the fat of the food to that of the animal body. Although its functions are not yet clearly defined, cholesterol appears to be a substance of much physiological significance. The name indicates " bile-solid-alcohol," as it was earhest and best known as a prominent constituent of gall stones. Its deposition in the form of gall stones is attributed to the presence 38 CHEMISTRY OF FOOD AND NUTRITION of an insufficient amount of bile salts to keep the cholesterol of the bile in solution. It may also be deposited in the walls of the arteries. As a constituent of the blood cholesterol acts to protect the red blood cells against the action of hemolytic sub- stances, which unless neutrahzed by cholesterol would tend to cause anemia through excessive destruction of red corpuscles. According to Mathews, cholesterol is one of the most abundant lipins of the brain and occurs in nearly all Uving tissues; as a constituent of waxes and the sebum of the skin it protects the dermal structures ; it, or its degradation products, aids the other lipins in giving to cells their power of holding large quantities of water without dissolving or losing their peculiar semifluid characters ; it is believed to be the mother substance from which the bile acids are derived and so plays an important part in the intestinal digestion and absorption of fat ; and, on the other hand, cholesterol itself appears to check the action of fat- splitting enzymes in the body and thus to function as a regu- lator in the metabolism of the cell lipins. Phospholipins or phosphatids are also widely distributed in liv- ing cells and doubtless essential to their structure and functions. Of the phospholipins or phosphatids the best-known are the lecithins, which are abundant in egg yolk and occur also in significant quantities in brain and nerve tissue, blood, lymph, milk, many seeds, and other plant and animal tissues. The structure of lecithin has usually been represented by the formula H HC— OR HC— OR HC— O— PO(OH)— O— C2H4— N(CH3)0H. I H in which R stands for a fatty acid radicle. FATS AND LIPOIDS 39 On hydrolysis such a compound would yield glycerol, fatty acids, phosphoric acid, and the nitrogenous base choline (tri- methyl oxyethyl ammonium hydroxide). If one of the radicles be that of oleic and the other that of palmitic acid the hydrolysis may be represented thus : C42H84NPO9 + 4 HoO^ C3H8O3 + C,8H3402 + C,6H3202 Glycerol Oleic Palmitic acid acid + H3PO4 + C5H15NO2 Phosphor- Choline ic acid Recent investigations throw doubt upon the view that the nitrogen of typical lecithin is present only as choline groups. Taylor defines the simpler phosphatids as " lipoids in which two molecules of a higher fatty acid are combined with glycerol- phosphoric acid, to which is bound an amino body." A phosphatid which, like the above, contains one atom of nitrogen and one of phosphorus to the molecule is classified as a monamino-monophospholipin or monamino-monophosphatid. Monamino-diphospholipins, diamino-monophospholipins and triamino-monophospholipins have also been described. The fat of the active tissues of the body, as distinguished from that of the adipose tissue, seems to consist largely of phospholipins. Thus MacLean and Williams found 84 per cent of the total ether extract of pigs' liver to consist of phospholipins. Bang holds that it is " no mere coincidence that the most highly organized cells are always richest in lipoids." Other lipoids may also prove to be of much importance in nutrition. Butter fat and some other natural fats show nutri- tive functions which cannot be attributed to their glycerides alone and appear to be due to other substances soluble in fats and perhaps of the nature of lipoids. Such as yet unidentified fat-soluble substance appears to be absolutely essential to a fully complete diet since several investigators (Stepp, McCollum and Davis, Osborne and Mendel) have found it impossible to 40 CHEMISTRY OF FOOD AND NUTRITION raise young animals to full maturity on rations apparently ade- quate otherwise but lacking in this " lipoid " of " fat-soluble " factor. These experiments will be cited more fully in con- nection with the discussion of the specific relations of food to growth (Chapter XIII). REFERENCES Abderhalden. Lehrbuch der Physiologischc Chemie. Abderhalden. Biochemisches Handlexicon. Abderhalden. Handbuch der Biochemischen Arbeitsmethoden. Bang. Chemie und Biochemie der Lipoide. Bloor. Absorption and Metabolism of Fat. Journal of Biological Chem- istry, Vol. II, page 429; Vol. 15, page 105; Vol. 16, page 517; Vol. 17, page 377; Vol. 19, page i; Vol. 22, page 133; Vol. 23, page 317; Vol. 24, pages 227, 447 (1912-16). Browne. The Chemistry of Butter Fat. Journal of the American Chemical Society, Vol. 21, pages 632, 823, 975 (1899). GiES AND Rosenbloom. Classification of the Lipins. Biochemical Bulletin, Vol. I, page 51 (1912). Glikin. Chemie der Fette, Lipoide und Wachsarten. Hammarsten. Textbook of Physiological Chemistry. Hartley. On the Fat of the Liver, Kidney and Heart. Journal of Physi- ology, Vol. 38, page 353 (1909)- Henriques and Hansen. Influence of Food Fat and Other Conditions upon Body Fat. Skandinavisches Archiv Physiologic, Vol. 11, page 151 (1901). Jordan and Jenter. The Source of Milk Fat. New York Agricultural Experiment Station (Geneva, N. Y.). Bull. 132 (1897). Leathes. The Fats. Lewkowitsch. Oils, Fats and Waxes. MacLean and Williams. Nature of the Fat of the Tissues and Organs. Biochemical Journal, Vol. 4, page 455 (1909). McClendon. Formation of Fats from Proteins in Eggs of Fish and Am- phibians. Journal of Biological Chemistry, Vol. 21, page 269 (1915). Mathews. Physiological Chemistry. Mendel and Daniels. Behavior of Fat-Soluble Dyes and Stained Fat in the Animal Organism. Journal of Biological Chemistry, Vol. 13, page 71 (1913)- MouLTON and Trowbridge. Composition of the Fat of Beef Animals FATS AND LIPOIDS 4 1 on Different Planes of Nutrition. Journal of Iiiduslrial and Engineering Chemistry, Vol. i, page 761 (1909). Richardson. Influence of Food and other Conditions on the Chemical Characteristics of Lard. Journal of the American Chemical Society, Vol. 26, page 372 (1904). Smedley. Formation of Fat from Carbohydrate. Biochemical Journal, Vol. 7, page 364 (1913). Taylor. Digestion and Metabolism. Ulzer and Klimont. Allgemeine und Physiologische Chemie der Fette. CHAPTER III PROTEINS Carbohydrates and fats are the chief sources of energy for the activities of the body, but not the chief constituents of which the active tissues are composed. Muscle tissue, for instance, is almost devoid of carbohydrate and often contains very little fat. The chief organic constituents of the muscles, and of the protoplasm of plant and animal cells generally, are substances which contain nitrogen and sulphur in addition to carbon, hy- drogen, and oxygen. Mulder, in 1838, described a nitrogenous material which he believed to be the fundamental constituent of tissue substances and gave it the name protein, derived from a Greek verb meaning " to take the first place." While Mul- der's chemical work did not prove to be of permanent value, the term which he introduced has been retained, and in the plural form, proteins, is now used as a group name to cover a large number of different but related nitrogenous organic compounds which are so prominent among the constituents of the tissues and of food that they may still be accorded some degree of pre- eminence in a study of the chemistry of food and nutrition. Proteins are essential constituents of both plant and animal cells. There is no known life without them. Plants build their own proteins from inorganic materials obtained from the soil and air. Animals form the proteins characteristic of their own tissues, but in general they cannot build them up from simple inorganic substances such as suffice for the plants, and must depend upon the digestion products obtained from the proteins of their food. Since animals must have proteins for the con- struction and repair or maintenance of their tissues, and since, 42 PROTEINS 43 broadly speaking, they cannot make their proteins except from the cleavage products of other proteins, it follows that proteins are necessary ingredients of the food of all animals. Chemical Nature and Physical Properties of Proteins in General Generally speaking, the proteins of dififerent kinds of tissue, and even of the corresponding tissues of different species, are not identical substances. The total number of different proteins occurring in nature must therefore be very great. Of these, some fifty or sixty have been sufficiently isolated and studied to warrant description as chemical individuals. All of these have proven to be very complex substances and in no case has the chemical structure of a natural protein been fully deter- mined. It has, however, been shown that the typical proteins are essentially anhydrides of the following amino acids : AMINO ACIDS OF COMMON PROTEINS Monaminomonocarboxylic acids Glycine, amino-acetic acid, CH2(NH2) • COOH. Alanine, a-amino-propionic acid, CH3CH(NH2) ■ COOH. Valine, a-amino-isovaleric acid, (CH3)2CH • CH(NH2) • COOH. Leucine, a-amino-isocaproic acid (a-amino-isobutyl-acetic acid), (CH3)2CH • CH2 ■ CH(NH2) • COOH. Phenylalanine, phenyl-a-amino-propionic acid, C6H5CH2 • CH(NH2) • COOH. Tyrosine, oxyphenyl a-amino propionic acid, C6H4(OH) ■ CH2 • CH(NH2) ■ COOH. Serine, a-amino-)8-hydroxy-propionic acid, CH2(0H) : CH(NH2) • COOH. Cystine (dicysteine), or di-(a-amino-/8-thio-lactic acid), S-CH2-CH(NH2) • COOH. I S-CH2-CH(NH2) • COOH. 44 CHEMISTRY OF FOOD AND NUTRITION Monaminodicarboxylic acids Aspartic acid, amino-succinic acid, COOH • CH2 • CH(NH2) • COOH. Glutamic (glutaminic) acid, amino-glutaric acid, COOH • CH2 • CH2 • CH(NH2) • COOH. Diaminomonocarboxylic acids Ornithine, a, 8, diamino-valeric acid, CH2(NH2) • CH2 ■ CH2 CH(NH2) COOH. Arginine, 8-guanidino-a-amino-valeric acid, NH (H2N)C- NH • CH2 • CH2 • CH2 • CH(NH2) • COOH. Lysine, a, c, diamino-n-caproic acid, CH2(NH2) • CH2 • CH2 • CH2 • CH(NH2) • COOH. Heterocyclic Amino Acids : Histidine, a-amino-/3-imidazol propionic acid, HC=C ■ CH2 • CH(NH2) • COOH. I I HN N CH Proline, pyrrolidin-carboxylic acid, H2C— CH2 I I H2C CH • COOH \/ NH Trj^tophane, a-amino-/?-indol-propionic acid, HC^ "^C C • CH2 ■ CH • (NH2) • COOH. II CH / QW ^NH HC^ yCv PROTEINS 45 It will be noted that these constituents of the protein molecule differ much in structure among themselves. They are, how- ever, alla-amino acids, i.e., the amino group (or one of them if there be more than one) is attached to the carbon atom adjacent to the carboxyl. In view of the wide occurrence of the alanine radicle in proteins and the frequency with which we shall have occasion to discuss the behavior of alanine (as a typical amino acid) in metabolism, it may be of interest to point out that several of the amino acids, even including some of unique constitution, may be regarded as derived from alanine by the substitution of a simple or complex radicle for one of the hydrogens on the /3 carbon of alanine. Thus by the substitution of an — OH or — SH group one obtains serine or cysteine respectively ; by substituting the phenyl or oxyphenyl group, there results phenylalanine or tyrosine ; by the imidazole (C3H3N2), histidine ; by the indol (CsHeN) radicle, tryptophane. CH3 CH2OH CH2SH CHNH2 CHNH2 CHNH2 COOH COOH COOH Alanine Serine Cysteine Cri2C6H6 CH2C6H5OH CH2C3H3N2 CHzCsHfiN CHNH2 1 CHNH2 1 CHNH2 1 CHNH2 1 COOH COOH COOH 1 COOH Phenylalanine TjTosine Histidine Tryptophane The Hnkage of the amino acid radicles in the protein molecule is chiefly through the carboxyl group of one amino acid reacting with the amino group of another. Thus two molecules of glycine combined by eHmination of one molecule of water yield glycyl-glycine, CH2NH2-CO CH2NH-COOH. which is the simplest of an immense group of anhydrides of amino acids, all of which are called " peptids." Dipeptids 46 CHEMISTRY OF FOOD AND NUTRITION contain two amino acid radicles, tripeptids contain three, etc. Fischer, by uniting 7 to 19 amino-acid radicles, has pro- duced synthetic polypeptids which in some of their properties resemble the peptones, the simplest substances usually regarded as true proteins. Peptones are formed in nature by the diges- tive hydrolysis of ordinary proteins, whose structure is doubt- less considerably more complex. A certain analogy between carbohydrates and proteins may be noted. As starch on hydrolysis yields the polysaccharide dextrins, the disaccharide maltose, and finally as end product the monosaccharide glucose, so the native protein is hydrolyzed through peptones, polypeptids, and di- or tri-peptids, to amino acids. Thus the amino acid bears the same general relation to the protein which glucose bears to starch ; and just as the molec- ular weight of starch is very high and a single starch molecule yields a large (unknown) number of monosaccharide molecules, so the molecular weight of the protein is very high and the pro- tein molecule yields a large (unknown) number of amino acid molecules. There is, however, this important difference : the molecules of monosaccharide resulting from complete hydroly- sis of starch are all alike (glucose), whereas the complete hy- drolysis of any typical protein yields several of the above-men- tioned amino acids, in the case of most proteins from twelve to twenty. In view of the marked differences in structure existing among these amino acids it becomes important to know the relative proportions in which the various amino acid radicles exist in the different proteins. This is studied by hydrolyzing the pro- tein and separating and recovering as completely as possible the amino acids resulting from the hydrolysis. Since the recovery of the amino acids cannot be accomplished without loss, the results obtained are not strictly quantitative and our knowledge of the radicles which make up the protein molecule remains incomplete. It is believed by the investigators who have given PROTEINS 47 most attention to the question that the failure of the recovered amino acids to show a summation of one hundred per cent is more probably due to unavoidable losses in estimating the known amino acids than to the presence of other amino acids not yet identified. The accompanying table shows the percentages of amino acids obtained from four proteins occurring in different food materials. Percentages of AmNo Acids from Four Different Proteins * Casein (from Milk) Gelatin Gliadin (from Wheat) Zein (from Maize) Glj'cine Alanine Valine Leucine Proline Aspartic acid .... Glutamic acid .... Phem'lalanine .... Tyrosine Serine O.xyprolinc Histidine Arginine Lysine Tryptophane .... Cystine Ammonia o.oo 1.50 7.20 9-35 6.70 ■1-39 15-55 3.20 4-50 •50 •23 2.50 3.81 7.61 1-5 .06 1.61 16.5 0.6 I. 9.2 10.4 1.2 16.8 I. 0. •4 3.0 •4 9-3 6. 0.0 •4 0.00 2.00 3-34 6.62 13.22 0.58 43.66 2.35 1.50 •13 1.84 3-i6 0.92 I.O •45 5.22 0.00 13-39 1.88 19-55 9.04 1. 71 26.17 6.55 3-55 1.02 .82 1-55 0.00 0.00 3-64 Summation 67.21 76.21 85.67 88.87 From the data given in the table it will be seen that the pro- portions in which a given amino acid radicle occurs in various * In general each figure given in the table is the highest of the results reported In recent investigations. This is deemed more accurate than to give average results, because of the unavoidable losses referred to above. The data given for casein, gliadin, and zein are taken chiefly from the work of Osborne and his associates; those for gelatin chiefly from that of Skraup and Behler. 48 CHEMISTRY OF FOOD AND NUTRITION proteins may be quite different. The four proteins here shown yield from o.o to 16.5 per cent of glycine; from 0.6 to 9.8 per cent of alanine; from i.o to 7.2 per cent of valine, from 6.6 to 19.6 per cent of leucine. Of lysine, zein yields none, gliadin about I per cent, gelatin 6 per cent, and casein about 8 per cent. Of tryptophane, zein and gelatin yield none, gliadin about i per cent, casein about 1.5 per cent. For more detailed comparisons of the percentages of amino acids in different proteins and also in the flesh of four widely separated species, the more extended table further on in this chapter may be consulted. Whether it be essential that the proteins of the food shall furnish all the amino acids which the body proteins contain will naturally depend upon whether the body is able to make individual amino acids or not. Experi- mental evidence has shown that the animal body can make glycine readily, so that the absence of glycine radicles in the food proteins does not detract from their nutritive value. On the other hand the animal body does not seem able to make tryp- tophane, and as this is an essential constituent of body tissue the food proiein must always furnish tryptophane if it is to meet the needs of animal nutrition. Feeding experiments have also shown that the rate of growth of young animals may be largely influenced by the lysine content of the proteins fed ; food pro- teins in which lysine is lacking or inadequate may suffice for the maintenance of full grown animals but fail to support normal growth in the young of the same species. Such facts as these make it important that we study the proteins not only as a group but also individually and that we learn as much as possible about the kinds and amounts of amino acid radicles in the individual proteins. The ultimate composition of the proteins shows a general similarity throughout the group. All contain carbon, hydro- gen, oxygen, nitrogen, and sulphur; some also phosphorus or iron. PROTEINS 49 Composition of Some Typical Proteins according to Osborne Carbon Hydro- gen Nttro- GEN Oxygen Sulphur Iron Phos- phorus CENT PER CENT PER CENT PER CENT PER CENT CENT PER CENT Egg-albumin 52.75 7.10 15.51 23.024 1.616 Lact-albumin . 52.19 7.18 15.77 23.13 1-73 Leucosin . . . 53-02 6.84 16.80 22.06 1.28 Serum-globulin 52.71 7.01 15.85 23.32 I. II Myosin . . . 52.82 7. II 16.67 22.03 1.27 Edestin . 51.50 7.02 18.69 21.91 0.88 Legumin . 51.72 6.95 18.04 22.905 0.385 Casein 53.13 7.06 15.78 22.37 0.80 — 0.86 Ovo-vitellin 51.56 7.12 16.23 23.242 1.028 — 0.82 Gliadin . 52.72 6.86 17.66 21.733 1.027 Zein . . 55.23 7.26 16.13 20.78 0.60 Oxyhemoglobin 54.64 7.09 17.38 20.165 0.39 0.335 It will be seen that all these typical proteins contain 51 to 55 per cent carbon, about 7 per cent hydrogen, 20 to 23 per cent oxygen, 15.5 to 18.7 per cent nitrogen, 0.3 to 2.0 per cent sulphur. Other typical proteins thus far studied have shown ultimate composition within these same limits. Similarity of elementary composition is entirely consistent with the belief that there may be an enormous number of chemical individuals among the proteins of nature. Fischer has recently illustrated the vast number of isomers which may exist among polj^jeptids and proteins by pointing out that a synthetic 19-peptid obtained by linking 15 glycine and 4 leucine molecules is only one of 3876 possible isomers, without considering the tautomerism of the pep- tid linking. When more than two kinds of amino acids are involved, the possible number of isomers increases very rapidly. If a protein be imagined made up of 30 molecules of 18 different amino acids, one taken twice, one 3 times, another 3, one 4, one 5 times, and 13 taken once each, there would be 10-^ isomers even if there were no tautomerism of the peptid group and if the linking took place only in the simple way as with monamino-mono- carboxylic acids. It is easy to see that when one considers not only isomerism but the vast number of compounds of slightly different composition which can be obtained E 50 CHEMISTRY OF FOOD AND NUTRITION by varying the kinds and proportions of the amino acid radicals in the protein molecule, the possible number of different proteins of very similar elementary composition is practically unlimited. Probable molecular weights. — From the results of ultimate analysis an appro.ximate indication of the minimum molecular weight may often be obtained by a very simple calculation. Thus, oxyhemoglobin contains only 0.335 per cent of iron, and since there must be at least one iron atom in the molecule, it is obvious from a simple proportion making use of the atomic weight of iron, 0.335 : 56 : : 100 : x, that the molecular weight of hemoglobin must be in the neigh- borhood of 16,800 or a multiple of this. To take an example from the simple proteins, zein contains 0.60 per cent of sulphur, of which one third is much more readily spHt off than the other two thirds, from which it appears that the molecule contains three, or a multiple of three, sulphur atoms. Then by the proportion, 0.60: (32 X 3) :: 100: X, it is found that about 16,000 or a multiple thereof is the probable molecular weight of zein. Estimates of the same order of magnitude are obtained if we base our calculations on the proximate rather than the ulti- mate analyses of the purified protein preparations. Osborne, Van Slyke, Leavenworth, and Vinograd have recently concluded from a very searching investigation that the lysine content of gliadin must lie between 0.64 and 1.20 per cent. Since the molecular weight of lysine is 146 it follows that the correspond- ing minimum estimate of the molecular weight of ghadin must fall between 12,000 and 23,000. The experimental facts do not permit the assumption of any lower molecular weight but are not inconsistent with the view that the true molecular weight may be some multiple of this. PROTEINS 51 Physical properties. — In only a few cases have proteins been obtained in crystalHne form. Generally speaking the proteins may be regarded as typically colloidal substances. This does not preclude the beUef that in the tissues and fluids of the body the proteins may exist largely in combination with elec- trolytes. In view of the fact that the behavior of proteins in the tissues is largely dependent upon their colloidal character it is of interest to bear in mind the very high molecular weights of the proteins as mentioned in the last paragraph. Discus- sions of colloids commonly emphasize the fact that the smallest particles demonstrable under the ultramicroscope must still be of quite a different order of magnitude from that calculated for ordinary molecules. In such a case as that of starch or a typical protein, however, the probable molecular weight is so enormous as to make it a debatable question whether the in- dividual molecules may not constitute colloidal particles when dispersed in water (Bayliss). The proteins are insoluble in all of the solvents for fats (ether, acetone, chloroform, carbon disulphid, carbon tetrachlo- rid, benzene, and petroleum distillate). They differ in their solubilities in water, salt solutions, and alcohol, and these dif- ferences play a considerable part in the present schemes of classification. Classification. — There was formerly considerable confusion in the classification and terminology of the proteins and some differences of usage will still be met in the literature. At pres- ent, however, the majority of writers follow the recommenda- tions made by a joint committee of the American Physiological Society and the American Society of Biological Chemists in December, 1907. The full text of these recommendations will be found in the appendix. The following is an outUne of the classification thus recommended ; to which have been added examples covering most of the food proteins thus far described as chemical individuals. 52 CHEMISTRY OF FOOD AND NUTRITION I. Simple Proteins. Protein substances which yield only amino acids or their derivatives on hydrolysis, (a) Albumins. Simpleproteinssolubleinpurewater andcoag- ulable by heat. Examples: egg albumin, lactalbumin (milk), serum albumin (blood), leucosin (wheat), legumelin (peas). (b) Globulins. Simple proteins insoluble in pure water, but soluble in neutral salt solutions. Examples: muscle globulin, serum globulin (blood), edestin (wheat, hemp seed, and other seeds), phaseolin (beans), legumin (beans and peas), vignin (cow peas), tuberin (potato), amandin (almonds), excelsin (Brazil nuts), arachin and conarachin (peanuts). (c) Glutelins. Simple proteins insoluble in all neutral solvents, but readily soluble in very dilute acids and alkaHes. The best- known and most important member of this group is the glu- tenin of wheat. {d) Alcohol soluble proteins. Simple proteins soluble in relatively strong alcohol (70-80 per cent) but insoluble in water, absolute alcohol, and other neutral solvents. Examples: glia- din (wheat), zein (corn), hordein (barley), kafirin (kafir corn). {e) Albuminoids. These are the simple proteins character- istic of the skeletal structures of animals (for which reason they are also called scleroproteins) and also of the external pro- tective tissues, such as the skin, hair, etc. None of these pro- teins is used for food in the natural state, but collagen when boiled with water yields gelatin. (/) Hi stones. Soluble in water, and insoluble in very dilute ammonia, and in the absence of ammonium salts insoluble even in an excess of ammonia ; yield precipitates with solutions of other proteins and a coagulum on heating which is easily soluble in very dilute acids. On hydrolysis they yield several amino acids, among which the basic ones predominate. The only members of this group which have any considerable im- portance as food are the thymus histone and the globin derived from the hemoglobin of the blood. PROTEINS 53 (g) Protamins. These are simpler substances than the preceding groups, are soluble in water, not coagulable by heat, possess strong basic properties, and on hydrolysis yield a few amino acids among which the basic amino acids greatly pre- dominate. They are of no importance as food. II. Conjugated Proteins. Substances which contain the protein molecule united to some other molecule or molecules otherwise than as a salt. (a) Nucleo proteins. Compounds of one or more protein molecules with nucleic acid. Examples of the nucleic acids thus found united with proteins are thymo-nucleic acid (thy- mus gland), tritico-nucleic acid (wheat germ). ih) Glycoproteins. Compounds of the protein molecule with a substance or substances containing a carbohydrate group other than a nucleic acid. Example : mucins. (c) Phospho proteins. Compounds in which the phosphorus is in organic union with the protein molecule otherwise than in a nucleic acid or lecithin. Examples: caseinogen (milk), ovo\dtellin (egg yolk). {d) Hemoglobins. Compounds of the protein molecule with hematin or some similar substance. Example: hemoglobin of blood. (The redness of meat is due chiefly to the hemoglobin of the blood which the meat still retains.) (e) Lecitho proteins. Compounds of the protein molecule with lecithins or related substances. III. Derived Proteins. I. Primary protein derivatives. Derivatives of the protein molecule apparently formed through hydrolytic changes which involve only slight alterations. (a) Proteans. Insoluble products which apparently result from the incipient action of water, very dilute acids, or enzymes. Examples: casein (curdled milk), fibrin (coagulated blood). {b) Metaproteins. Products of the further action of acids and alkalies whereby the molecule is sufficiently altered to form 54 CHEMISTRY OF FOOD AND NUTRITION proteins soluble in very \ycak acids and alkalies, V:)ut insoluble in neutral solvents. This group includes the substances which have been called " acid proteins," " acid albumins," " syntonin," "alkali proteins," " alkaU albumins," and *' albuminates." (c) Ccagulated proteins. Insoluble products which result from (i) the action of heat on protein solutions, or (2) the action of alcohol on the protein. Example : cooked egg albumin, or egg albumin precipitated by means of alcohol. 2. Secondary protein derivatives. Products of the further hydrolytic cleavage of the protein molecule. (a) Proteoses. Soluble in water, not coagulable by heat, precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. The products commercially known as " peptones " consist largely of proteoses. {b) Peptones. Soluble in water, not coagulable by heat, and not precipitated by saturating their solutions with ammonium sulphate or zinc sulphate. These represent a further stage of cleavage than the proteoses. (c) Peptids. Definitely characterized combinations of two or more amino acids. An anhydride of two amino acid radicles is called a " di-peptid " ; one having three amino acid radicles, a " tri-peptid " ; etc. Peptids result from the further hydro- lytic cleavage of the peptones. As was mentioned above, many peptids have also been made in the laboratory by the linking together of amino acids. Substances simpler than the peptones but containing several amino acid radicles are often called " polypeptids." Properties of Some Individual Proteins Albumins and globulins are very often associated, as, for example, in blood serum and in the cell substance. As a rule the albumins are the more abundant in animal fluids, while PROTEINS 55 the globulins predominate over albumins in animal tissues and in plants. There appears to be no sharp dividing line between the albumins and the globulins. While the globulins are in- soluble in pure water, a water extract of animal tissue (muscle, for example) will contain, in addition to albumin, a considerable amount of globulin carried into solution by the salts present in the tissue, and if the salts are removed as completely as possible by dialysis, some of the globuhn still remains in solution ; sepa- rations based upon saturation with neutral salts are also apt to be unsatisfactory (Howell). Notwithstanding these diflficulties, a considerable number of individual albumins and globuHns have been isolated, purified, and analyzed. In ultimate composition they show a general similarity except that the albumins are richer in sulphur than the globulins. Several members of each group have also been studied to determine the kinds and amounts of amino acid radicles which they contain, with the results shown in the table on pages 60 and 61. It is of interest to compare the amino acid make-up of typical proteins with their adequacy in nutrition. A few studies of tliis sort, notably those of Kauffmann with gelatin and Willcock and Hopkins with zein, had been made some years earUer, but much the greater part of our knowledge in this field is due to the recent investigations of Osborne and Mendel (191 1 et seq.). Rats have been chiefly used as the experimental animal. Egg albumin, perhaps the most familiar of all proteins and the one most often chosen to illustrate, in the laboratory, the properties of proteins in general, will be seen to yield no glycine but to furnish all the other usual amino acids in quite appreciable proportions. The feeding experiments show that with a diet adequate as regards all other factors animals may be maintained in normal nutrition and young animals may make normal growth with egg albumin as the sole protein food. 56 CHEMISTRY OF FOOD AND NUTRITION Lactalbumin shows this same property in even greater degree. It appears to be the most efficient in supporting growth of all the proteins which have been studied, and this is believed to be due primarily to its high lysine content (see table beyond). Legumelin and leucosin have not yet been studied in feeding experiments of this kind, nor have such experiments been made with amandin or vicilin. Only preliminary feeding experiments not entirely success- ful as regards growth have been reported for legumin, phaseolin, and vignin; but each of the other three vegetable globulins shown — edestin, excelsin, and glycinin — has been found to suffice for maintenance and normal growth when fed as the sole protein in a diet adequate in other respects.* In fact Osborne and Mendel have kept one family of rats through three generations with edestin as a sole protein food. Glutelins and the alcohol-soluble proteins (prolamins) are im- portant as constituents of the cereal grains. The best-known examples of the respective groups are glutenin and gliadin of wheat flour. These proteins resemble each other in ultimate composition, but differ not only in solubilities, but also in their cleavage products. They are much the most important of the proteins of the wheat kernel, the gliadin making up about 50 per cent and the glutenin about 40 per cent of the total protein present. The gliadin and glutenin together constitute the glu- ten of wheat flour. Glutenin (wheat glutelin) and maize gliitelin have each been shown capable, in the rat-feeding experiments cited above, of meeting the requirements not only of maintenance but also of normal growth when fed as the sole protein food in diets adequate in other respects. Gliadin, hordein, and the prolamin of rye, when fed singly in the same manner, are found capable of maintaining grown rats * Factors necessary to make a diet adequate will be discussed in Chapters XII and XIII, where experiments upon growth will be considered in greater detail. PROTEINS 57 but not of supporting normal growth. Zein, fed alone in similar experiments, did not suffice either for maintenance or for growth. Osborne and Mendel concluded from these experiments that the failure even to maintain the grown animals was due to the absence of tryptophane ; while the failure of the rats to grow on gliadin, hordein, or rye prolamin was due to the fact that these proteins either lack lysine or contain it in insufficient quan- tity. This interpretation was confirmed by later experiments in which they found that adding tryptophane to the zein food made it adequate for maintenance and adding lysine to the gliadin food made it adequate to support growth. Gelatin, the only member of the albuminoids (scleroproteins) which is of practical importance as food, has long been known to be unable to support protein metabolism when fed as the sole protein food. This inadequacy now appears to be due to the absence of tryptophane and tyrosine and perhaps in part also to the fact that some of the other amino acids, cystine and histidine, are furnished by gelatin in only very small proportion. As early as 1905 Kauffmann tried the experiment of living upon a diet in which gelatin was the sole protein, but was supplemented by additions of tyrosine, tryptophane, and cystine. So far as could be determined by a short experiment the addition of these amino acids seemed to make good the deficiencies of the gelatin. Nucleoproteins are the characteristic proteins of cell nuclei, and are therefore especially abundant in the highly nucleated cells of the glandular organs, such as the thymus, the pancreas, and the liver. They are compounds of simple proteins with nucleic acid or nuclein. The chemical nature of the latter and their behavior in metabolism will be considered in Chapter V. Phospho proteins occur especially in milk and eggs, which ob- viously function in nature to provide the material for growth and development of new animal tissue. The phosphorus, while prob- ably present in the form of a more or less modified phosphoric 58 CHEMISTRY OF FOOD AND NUTRITION acid radicle, appears to be more closely bound in these than in the nucleoproteins. Casein of milk and the vitellin of egg yolk (ovo-vitellin) are the most prominent members of the group. These are sometimes classed with simple proteins under the name nucleo-albumins. Phosphoprotein preparations show on analysis small amounts of iron, which has usually been neglected as an impurity but which is not improbably an essential con- stituent. Casein and ovo-vitellin fed singly as the sole protein of the ration in the experiments by Osborne and Mendel described Fig. I. — Showing typical curves of growth of rats on diets otherwise similar and adequate but containing in each case only a single protein, casein, gUadin, or zein. Courtesy of Dr. L. B. Mendel and the Journal of the American Medical Association. above have each been found capable of supporting both main- tenance and normal growth, as their amino acid make-up and their place in nature would lead us to expect. The curves in Fig. I illustrate the rapid growth on casein as compared with the very slow growth on gliadin and the loss of weight when zein was the sole protein food. The rations were alike except PROTEINS 59 for the nature of the protein fed ; the percentage of protein in the ration was the same in each case. It will be seen that the rat receiving casein grew over 200 grams in 140 days while the one fed with gUadin grew only 20 grams during the same period. The third rat, which had been growing rapidly on mixed food, began at once to lose weight when put on a ration of which zein was the sole protein. Hemoglobins^ consisting of combinations of simple proteins with coloring matter, serve as carriers of oxj^gen from the air to the tissues. On boiling or heating with acids or alkahes they are spht into their constituent parts : for example, ordinary hemoglobin \aelds about 4 per cent of hematin, C32H32N4Fe04, and a residue of globin which was formerly considered a globulin but is now assigned to the histone group. Proteoses and peptones are products derived from other pro- teins by digestion or by simple hydrolysis. They are soluble in water and not coagulated by boihng their aqueous solutions. No sharp line can be draw^n either between proteoses and pep- tones, or between peptones and the simpler nitrogen compounds which result from prolonged digestion. As the terms are gen- erally used, peptones may be considered as the products of diges- tion or hydrolysis which still show the usual color reactions of proteins and are precipitated by strong alcohol ; but are not precipitated by saturation of their solutions with zinc or ammo- nium sulphate, as is the case with proteoses. Proteoses (albu- moses) are intermediate products between metaprotein and pep- tones. In addition to the protein reactions shown by peptones, the proteoses are precipitated from aqueous solutions at ordinary temperatures by adding acetic acid and potassium ferrocyanide, or by saturating the solution \vith zinc or ammonium sulphate. The term " peptone " was formerly apphed to all digestion products not coagulated by boiling, and is still popularly used in the same sense, the best commercial " peptones " consisting largely of proteoses. 6o CHEMISTRY OF FOOD AND NUTRITION a j^ 8 t~. Tf N r^ vO r^ 00 00 10 c tN to VO 1 1 111 0> fO 00 P) rj C> 00 p< c) «^ d d d t^ ir> ro t-i Tt ro •o ^ ^ tN VO ,(_» 1 Is Si o o "^ 00 M 00 ■+ M I^ c ro ■* lO " rooq ro to ro ON "^ ■* q t^ d d o d\ ro vO r^ M 00 c VO ro O .S >.a t> o Tj-CO 00 00 Tl" M (^ ro t^ VO t^ d od ro m' ro 0^ VO cs M ro CM 4 •? J2 w 1-1 VO o c _c — o ro w "O ^0 VO Tj- ■* VO c IsS" vO rO Lo 1^ 10 00 1-1 10 ^ 00 tN d cj M 00 ro ro ro tN vd M c 00 00 T3-C u 00 O tN 10 q w ro VO f^ M vO M M ^ tN IN tN tN CO O 4 ro m' d M ■^ 00 Tj- M (N ri- W^'" " M M 1m a. 00 ;! 1? M o o 10 ro tS M Q. VO •9« 't- lO CO ^ CO '^ VO ro Tj- VO ro 00 C M r^ o S c> Tj- W ro 00 ro^ ror^Ot^OOM^ Tt tN s-l • * * * t/} d 4 d M ro ro ro to tN tN to 4> W d i-i VO J^ Q. i.sl o o) O (O o> MO VO ro r^ C vO to lO '?' ^. vO I^ 10^ M ■* tN 0^
  • d »'^ , ^2-3 3 a; Lh 'r-" h-1 Ph H V3 5"a3Mwit/)Ob >^ B .^i«— >^ >>"-; i- X i-i 3 o <> U < < J Ko, H < PROTEINS 6i ->-> o. 8 00 10 t>. 00 00 r^ N 00 a 00 ^ ^_^ 1 1 "^ °^ =^^ Tj- 00 CO r^ M to 1 1 00 Tj- W CO '* t^ to cs ts to ■3 n 0\ rO ^ o> CO CO ■^ to to t^ c CO to ^ 1 t^ ro ° "?n.. J^ W CO -"i- to M CO n 1 d d CO Cl w r^ 0) CO -* NO .«_) !^ 0 tJ- Tf to t^ 00 in ;= M M OV 0) CO CN) CS t^ Tt IH rf a; CO Oi 0--" M a to O a C -yi^ n >o 0000 On to M M CO M M Cl< S S:^ LO c^ rrj (^ u~> m CO to 00 NO to »^ to NO 0) o n " P'c 1-1 t^ 0\ CO rj- M to CO r^ tN| NO M " NO si a Hi LO tN M d d 1 o to M M CO M " "^ kS -i- M a SC' 't c^ to CO to 00 NO NO 0 •H -^ CO ■^ 1-1 M Tt ^ On — a. ' 1 « . . . -0:2 ■ • • • jSHcaiU <0<^ffiCu,OH 3 ♦3 >- cS en vO -7; !" 6^ 3?£ ^ 3 2 ■" (J 3 — ' -" 1) 1 §■§ o _^ Z XI E 62 CHEMISTRY OF FOOD AND NUTRITION Relation between Chemical Constitution of the Proteins and Their Food Value Several facts bearing upon the relation between the feeding values of individual proteins and their amino acid make-up have been cited in the preceding pages. The subject is of great im- portance and is now under active investigation. Since the experimental facts are still being determined, any attempt to generalize broadly at this point would be premature. A few important conclusions may, however, be deduced from the facts already given. Glycine, although an essential constituent of body tissue, need not be furnished by the food, for several proteins which do not yield glycine on hydrolysis have been shown to be adequate when fed as sole protein of an experimental ration. It appears therefore that supplies of glycine fully adequate to meet all normal needs may be formed within the body itself. Tryptophane, on the other hand, apparently must always be supplied to the animal body ; food furnishing no tryptophane has always proven inadequate even for maintenance of full- grown animals. Apparently the animal body is unable to make tryptophane (or at least to make it at the rate required for normal metaboHsm) and proteins lacking the tryptophane radicle must be regarded as always inadequate as a sole protein food. Lysine, again, is especially important in connection with growth. Proteins which yield little, if any, lysine (and which are otherwise adequate in their amino acid make-up) appear to suffice as the sole protein food in the maintenance of full- grown animals (rats) but not to support a normal growth of the young. As regards the influence of the presence or absence of glycine, lysine, and tryptophane radicles in the protein molecule, it seems possible to correlate the chemical structure and the nutritive value of the proteins quite definitely. In estabhsh- PROTEINS 63 ing this correlation, Osborne and Mendel have made one of the most important advances in the entire development of the chemistry of food and nutrition. That the inadequacy of zein for maintenance is essentially due to the lack of tryptophane, they demonstrated by feeding a ration with zein as sole protein but with tryptophane added. This mixture permitted maintenance without growth (rat 1892, middle portion of Fig. 2). Then by the addition of lysine to the zein and tryptophane diet they induced normal growth as shown by the continuation of the weight curve of rat 1892 at urves of Growth =>" Zein + Amino Acids iphane Fig. 2. — Showing the effect of adding tryptophane or tryptophane and lysine to a diet containing zein as the sole protein (compare Fig. i, page 58). Courtesy of Dr. L. B. Mendel and the Journal of the American Medical Association. the right of Fig. 2. In another case (rat 1773, at the left of Fig. 2) a rat which was rapidly losing weight on the zein diet was restored to a condition of normal growth by the addition of tryptophane and lysine to the food. As Mendel expresses it : " If we analyze the situation as revealed in the charts of some actual experiments, it becomes apparent that both lysine and tryptophane are unquestionably necessary as constructive units in growth. The decline brought about by the zein food can be stopped by the addition of trypto- phane, as such, to the diet. This results in maintenance ; but no growth ensues until lysine also is added." Osborne and Mendel also showed that the addition of lysine to the gliadin ration made it adequate to support normal 64 CHEMISTRY OF FOOD AND NUTRITION growth. They have also shown that retardation of growth may sometimes be due to restricted intake of some amino acid other than lysine. In the experiments above described the rations always con- tained a liberal amount (usually i8 per cent) of protein. If, on the other hand, the percentage of protein in the food be 020 280 240 .S 120 .90 40 1 / / ^3-.; . /^...^ Case in T v^yo / / / Y r / ¥ V / 1 1 M / { j/ 1 1 f\ 1 J 1 1 1 ^ 1/ 1 ] y^ 40 Days < ■> Fig. 3. — Showing that the insufficiency of a low-casein diet was essentially due to its relative deficiency in cystine. Courtesy of Dr. L. B. Mendel and the Journal of the American Medical Association. sufficiently reduced, the growth may be retarded even though the protein be of a kind which is entirely adequate when liberally fed. Thus on a ration containing g per cent of casein the rats grew only about half as rapidly as when they received 18 per cent ; * and in this case the limiting factor was not lysine but * On account of the very different rates of growth, not to mention other differ- ences between the species, one must not attempt to apply the quantitive data of the rat-feeding experiments directly to the problem of protein requirement in man. PROTEINS 65 cystine, for the addition of cystine to the low-casein diet in- duced a normal rate of growth which was immediately checked when the cystine was v/ithdrawn and resumed when the cystine was again added to the ration (Fig. 3). In all of -the experiments cited thus far each ration contained only a single isolated protein. This is the ideal condition for the experimental comparison of individual proteins, but is quite different from ordinary or "practical" conditions, since our common protein foods all contain mixtures of proteins, so that even if only a single article of food were consumed the diet would still furnish more than one protein at a time. By feed- ing definite mixtures of pure proteins Osborne and Mendel have beautifully demonstrated the way in which proteins supple- ment each other in nutrition. Thus zein alone is, as we have seen, always inadequate as a sole protein food ; lactalbumin is adequate when fed in sufficient quantity but when constituting only 4.5 per cent of the food mixture of rats it supports only slow growth; but a food mixture containing 4.5 per cent of lactalbumin and 13.5 per cent of zein supports growth at a fully normal rate (Fig. 4). This shows that a relatively small amount of lactalbumin (one fourth of the protein fed) sufficed to furnish the amino acid groups which the zein lacked. It shows also that zein, which when fed as a sole protein is insuf- ficient even for maintenance, is able as a constituent of a proper food mixture to take part in supplying the materials for growth, to such an extent as to more than double the growth-rate. Thus zein, although inadequate for either maintenance or growth when isolated and fed alone, may nevertheless take an important part in both maintenance and growth when fed as a part of a proper mixed diet. Moreover it may not even be necessary to resort to a mixture of food materials in order to make good the deficiencies of the individual incomplete protein. Corn fmaize) itself, along with zein, contains an almost equal amount of another protein, maize glutelin, which Osborne and Mendel 66 CHEMISTRY OF FOOD AND NUTRITION have shown to be capable of supporting a normal rate of growth — not to mention the proteins in the embryo of the maize kernel which appear to have a still higher nutritive efficiency (Hart and Humphrey; McCollum and Davis). Thus it is plain that the mixtures of proteins contained in different articles of food as wc cat them do not differ in such a 240 / ^ ^, / / V ¥/ "jj -^ f . ■ao A.^ ^ jo^ \y^ 40 Days c > •5 120 i~ -c: •^ 30 Fig. 4. — Showing the efficiency of lactalbumin as a supplement to zein, and also that zein may take an important part in growth although zein alone is inadequate either for growth or maintenance. Courtesy of Dr. L. B. Mendel and the Journal of the American Medical Association. striking way as do the individual proteins when isolated and fed singly ; but neither is it true that the proteins of different articles of food are equivalent for all practical purposes. Hart, McCollum, and their associates have shown that the natural protein mi.xture of milk is more efficient than an equal weight of the mixed proteins of wheat or corn (maize) both for the sup- port 01 growth in young animals (pigs) and as food for the pro- PROTEINS 67 duction of milk in dairy cattle. While it is always possible that in comparisons between natural food materials the results may be influenced by differences in the unknown food constituents which may be present, yet in the cases here cited it is probable that the differing efficiencies ascribed to milk and grain proteins are mainly due to the same differences of chemical constitution ("amino acid make-up") to which are attributable the striking results obtained in the experiments previously cited in which isolated foodstuffs were fed. REFERENCES Abderhaldex. Lehrbuch der Physiologische Chemie. Fischer. Untersuchungen iiber Aminosauren, Polypeptide und Proteine. Ceiling. The Nutritive Value of the Diamino-Acids occurring in Proteins for the Maintenance of Adult Mice. Journal of Biological Chemistry, Vol. 31, page 173 (1917)- Hart and Humphrey. The Relation of the Quality of Proteins to Milk Production. Journal of Biological Chemistry, Vol. 21, page 239 (1915) ; Vol. 26, page 457 (1916) ; Vol. 31, page 445 (1917)- Hammarsten.. Textbook of Physiological Chemistry. Hawk. Practical Physiological Chemistry. Jones. The Nucleic Acids; their Chemical Constitution and Physiological Conduct. Kossel. Lectures on the Herter Foundation. The Proteins. Bulletin of the Johns Hopkins Hospital, Vol. 23, page 65 (1912). Mann. Chemistry of the Proteids. Mathews. Physiological Chemistry. McCoLLUM. The Value of Cereal Proteins for Growth. Journal of Bio- logical Chemistry, Vol. 19, page 323 (1914). !McCoLLUM AND Davis. Nutrition with Purified Food Substances. Jour- nal of Biological Chemistry, Vol. 20, page 641 (1915); The Cause of the Loss of Nutritive Efficiency of Heated Milk, Ibid., Vol. 23, page 247 (1915)- Mendel. Nutrition and Growth. The Harvey Society Lectures for 1914-1915, page loi ; and Journal of the American Medical Associa- tion, Vol. 64, page 1539 (1915). Mitchell. Feeding Experiments on the Substitution of Protein by Definite Mixtures of Isolated Amino Acids. Journal of Biological Chemistry, Vol. 26, page 231 (1916). 68 CHEMISTRY OF FOOD AND NUTRITION Osborne. The Vegetable Proteins. OsBORNK. Die Pflanzenproteine. Ergebuisse der Physiologic, Vol. lo, pages 47-215 (1910)- Osborne and Mendel. Feeding Experiments with Isolated Food Sub- stances. Carnegie Institution of Washington, Publication No. 156 (Parts I and II) and a series of subsequent articles : Journal of Biological Chemistry, Vol. 12, page 473; Vol. 13, page 233; Vol. 17, page 325 ; Vol. 18, page i; Vol. 20, page 351; Vol. 22, page 241; Vol. 25, page i; Vol. 26, pages I, 293; Vol. 29, page 69 (1911-1916). Examine also the later issues of this Journal for papers published subsequently to the compiling of this list. Osborne, Van Slyke, Leavenworth, and Vinograd. Some Products of Hydrolysis of Gliadin, Lactalbumin, and the Protein of Rice. Jour- nal of Biological Chemistry, Vol. 22, page 259 (1915). Plimmer. Chemical Constitution of the Proteins, I and II. CHAPTER IV ENZYMES AND DIGESTION The carbohydrates, fats, and proteins as they exist in foods* are in most cases not of a nature to be used by the body tissues in the exact form in which they are eaten, but must usually undergo more or less alteration in the digestive tract to fit them for absorption and utilization. In so far as the changes which the food undergoes in the alimentary tract are chemical they are brought about mainly by the action of digestive enzymes; but the efficiency of the digestive process is also largely de- pendent upon the mechanical factors of digestion which there- fore will also be briefly considered in this chapter. Historical The idea that changes comparable to fermentation are in- volved in the processes of digestion apparently originated with von Helmont about 300 years ago. Sylvius, half a century later, cited alcohoUc and acetous fermentations to illustrate the type of process by which he believed the foodstuffs to be digested. Descartes held that as the result of a peculiar fermentation there was generated in the stomach "an acid of great potency, com- parable to nitric acid." From the standpoint of our present knowledge these early scientists appear to have made con- siderable progress toward a correct interpretation of the digestive process; but in their own times, before the beginning of the * A table showing percentages of proteins, fats, and carbohydrates in foods is given in Appendix B. 69 70 CHEMISTRY OF FOOD AND NUTRITION scientific development of organic or physiological chemistry, the views which they advanced appeared hazy and unscientific compared with those of the physiologists who were studying digestion from the mechanical point of view and by supposedly exact methods. Thus Dr. Archibald Pitcairn (1652-1713) proposed to explain gastric digestion, " without the aid of a Daemon or a Stygian Liquor," as due entirely to the triturating action of the stomach, the power of whose muscular walls he estimated as " equal to 12,951 pounds " (Gamgee). The view that the digestion of food in the stomach is due solely to the mechanical action of the stomach walls was refuted by Reaumur, working with birds, and by Stevens, who experi- mented with a man who was accustomed to swallow small stones and regurgitate them at will. In Stevens' experiments this man swallowed hollow silver balls filled with food and perforated to permit access of the gastric juice but strong enough to resist the muscular contractions of the stomach walls. Food thus introduced was found to undergo digestion in the stomach al- though it was entirely protected from the triturating action of the stomach walls. Furthermore Stevens found that gastric juice obtained from a dog was able to digest meat outside of the stomach. At about the same time Spallanzani also showed clearly that gastric juice can act outside of the body. In addi- tion, he pointed out its antiseptic properties and emphasized the difference between the digestive process and that of alcoholic, acid, or putrefactive fermentation. About fifty years after the work of Spallanzani came the classical observations (1825- 1833) of Dr. Beaumont upon Alexis St. Martin, who, as the result of a gunshot wound, was left after recovery from his injury with a gastric fistula which permitted both the collection of human gastric juice and the direct observation of the processes going on in the stomach of a healthy man "active, athletic, and vigorous, exercising, eating, and drinking, like other healthy and active people." Dr. Beaumont's full and interesting ac- ENZYMES AND DIGESTION 7 1 count of his experiments with St. Martin ^ greatly extended the knowledge both of the muscular behavior of the stomach and of the conditions governing the secretion of the gastric juice and the " chymification " of the food in the stomach. The year after the publication of Beaumont's observations, Eberle showed - that by extracting the mucous membrane of the stomach with dilute hydrochloric acid he could obtain an artificial juice which showed the same digestive action which Spallanzani and Beaumont had observed with the natural se- cretion, and two years later Schwann ^ concluded that gastric juice owed its pecuUar activity to a substance presumably dif- ferent from any substance previously known and to which he gave the name pepsin. Schwann did not claim to have isolated this peculiar substance in a pure state but did effect a partial separation. Subsequently several other investigators attempted to isolate pepsin. Attempts to Determine the Chemical Nature of Enzymes * In 1902 Pekelharing prepared what has generally been re- garded as probably the purest pepsin of which we have record. This product contained carbon, hydrogen, nitrogen, and sul- phur in proportions within the range of variation found among ordinary proteins.f It also behaved like ordinary proteins in the xanthoproteic test and Millon reaction and in showing the presence of the tryptophane group. 1 W. Beaumont. Experiments and Observations on the Gastric Juice and the Physiology of Digestion. Plattsburg, 1833. 2 Eberle. Physiologic der Verdauung nach Versuchen. Wiirzburg, 1834. ' Schwann. Ueher das Wesen der Vcrdauungsproccsse. Miiller's Archiv, 1836, pages 90-138. * Those students not yet familiar with the names of the common enzymes should perhaps read first the sections on classification and terminology below. t A small amount of chlorine shown by Pekelharing's preparation was later found by Dezani to be not an essential constituent but probably due to incomplete removal of the hydrochloric acid with which pepsin is associated in the gastric juice. 72 CHEMISTRY OF FOOD AND NUTRTTTON Dezani, in igio, carried forward Ihe work upon Ihc chemical nature of pepsin by preparing what was beheved to be a sub- stantial duplicate of Pekelharing's product and submitting this to hydrolysis, followed by search for individual hydrolytic products according to the methods which had recently been developed in the study of the structure of the proteins. He demonstrated the presence of leucine, tyrosine, arginine, histidine, and lysine and also found evidence of other amino acids which the limitations of his material and methods did not permit him to identify. Thus pepsin as prepared by Pekelharing and by Dezani is a nitrogenous material not identical with any other known sub- stance but complying with the criteria of our present concep- tion of a protein in elementary composition, in color reactions, and especially in yielding the familiar amino acids upon hy- drolysis. Recent studies by Aldrich also indicate that the chemical nature of pepsin is that of a protein. It must be borne in mind that the criteria of purity usually appHed in chemical investigations are not applicable to enzyme preparations because of their colloidal nature and the readi- ness with which their characteristic properties are destroyed. Yet in view of the fact that, with very few if any excep- tions, the changes by which the organic foodstuffs are pre- pared for absorption in the digestive tract and are utiHzed in the body tissues are dependent upon the presence of enzymes the material for whose synthesis must in the long run be furnished by food, we should not be deterred by the inherent difficulties and uncertainties of the subject from the study of such evidence regarding the chemical nature of the enzymes as can be obtained ; nor are we at present quite so much in the dark as the state- ments in most textbooks would seem to indicate. Several years earlier than Pekelharing's work on pepsin, Os- borne ^ had published an investigation of the chemical nature 1 T. B. Osborne. Journal of the American Chemical Society, Vol. 17, page 587 (1895) ; Vol. 18, page 536 (1896). ENZYMES AND DIGESTION 73 of diastase (malt amylase), which may be regarded as marking the beginning of our modern knowledge in this field. From this work it appeared that the enzymic activity is a property of a definite fraction of the protein material of the malt, or in other words that the enzyme is protein in its chemical nature. Al- though criticized by some, Osborne's findings have been con- firmed by the most recent investigations. Since space permits here only the discussion of those enzymes which are directly concerned in digestion, the reader must be referred to the original papers for an account of Osborne's methods and results. Of the two amylases concerned in the digestive process, ptyalin of saliva and amylopsin of the pancrei'^tic juice, only the pancreatic amylase has been studied by modern methods with reference to its chemical nature. In an investigation ^ in which the attempts at purification were guided and their success largely judged by quantitative determinations of the starch-digesting action of the products there was developed a method of purification which in nu- merous independent experiments yielded a product that was not only extraordinarily active in the hydrolysis of starch but was essentially uniform both in digestive activity and in chemi- cal nature. This result strongly suggests that the product was not merely an indefinite mixture but represented at least some approximation toward an actual isolation of the enzyme. These preparations show the composition and color reactions of typical proteins and, hke Osborne's malt amylase, the material when heated in water solution yields an albumin coagulum and a proteose or peptone which remains in solution. Moreover, on hydrolysis the material yields the same groups of amino acids which are yielded by typical proteins such as casein, which it also resembles in elementary composition. While the chemical nature of the lipases of the digestive tract * Journal of the American Chemical Society, Vol. a, page 1195; Vol. 34, page 1104; Vol. 35, page 1790. 74 CHEIMISTRY OF FOOD AND NUTRITION has not been studied, Falk and Sugiura have shown that the purified Hpase preparations made from castor beans are, Uke the proteases and amylases above mentioned, essentially pro- tein material.* The materials obtained in attempts to isolate enzymes are here called merely products or preparations ; it is not stated that any enzyme has been perfectly separated and purified. As already explained, the familiar criteria of purity are not appli- cable to these unstable colloidal substances. It is possible that the enzymes in the purified preparations mentioned above may still be mixed with considerable amounts of other substances, and it has ev^n been suggested that the protein material of which the above-mentioned enzyme preparations are chiefly composed may be present only as a carrier and that the actual enzyme may be a substance of a different nature. There is, however, no direct evidence in favor of this suggestion. The facts now available make it altogether probable that the typical enzymes concerned in the utihzation of the foodstuffs either are modified proteins or contain protein as an essential component. In this case the food protein must furnish material for body enzymes as well as for body tissue. Classification and General Properties of Enzymes The word "enzyme" (from the Greek "in yeast") was intro- duced by Kiihne as a general designation for the substances formed in plants or animals which had previously been called * Recently Falk has suggested that the lipolytically active grouping is the tautomeric enol-lactim form of the peptide linking which becomes inactive on rearrangement to the keto form. Experiments testing this view resulted in the pro- duction of lipolytically active substances by the action of alkali on castor bean globulin, casein, and gelatin. Further confirming evidence was obtained on study- ing the ester-hydro lyzing action of glycine, glycyl-glycine, and hippuric acid at different hydrogen ion concentrations. Falk holds that "given a definite chemical grouping, the nature of which has been indicated, and which may be present in different classes of substances, certain definite lipolytic actions will result." ENZYMES AND DIGESTION 75 "soluble" or "unorganized" ferments to distinguish them from " organized " ferments (fermentation organisms). As more and more of the acti\dties previously regarded as char- acteristic of organisms have been found to be due to enzymes, the conception of enzyme action has broadened until now the term enzyme is apphed by most writers to all organic catalysts formed in plant or animal cells. Those which are ordinarily secreted from the cell and exert their activities outside of it (as in the case of the digestive ferments) are sometimes called extracellular enzymes, and those which normally perform their functions within the cells in which they are formed (as in yeast or in muscle cells) may be called intracellular enzymes even though it be possible by artificial means to cause them to act independently of living matter. Although each enzyme is generally supposed to be a definite chemical substance, the identification and classification of enzymes are based upon the changes which they bring about. Some of the better-known groups of enzymes are as follows : 1. The hydrolytic enzymes. a. Proteolytic or protein-splitting enzymes. b. Lipolytic or fat-splitting enzymes. c. Amylolytic or starch-splitting enz3^mes. d. Sugar-splitting enzymes. 2. The coagulating enzymes, such as thrombin or thrombase (the fibrin ferment), and rennin, which causes the clotting of milk. 3. The oxidizing enzymes, or " oxidases " (which, if the oxi- dation be accompanied by a splitting off of amino groups, may be called " deamidizing " or " deaminizing " enzymes). 4. The reducing enzymes or " reductases." 5. Those which, like the zymase of yeast, produce carbon dioxide without using free oxygen. * 6. Enzymes causing a breaking down of a larger into a smal- ler molecule of the same composition, as in the production of lactic acid from glucose. 76 CHEMISTRY OF FOOD AND NUTRITION 7. Enzymes causing chemical rearrangement without break- ing down of larger into smaller molecules, " mutases." Terminology of the hydrolytic enzymes. — Except in so far as some familiar enzymes continue to be known by their old estabhshed names (pepsin, rennin, trypsin, etc.), scientific usage now generally follows the suggestion of Duclaux that each hydrolytic enzyme be designated by a name indicating the kind of substance on which it acts, together with the sufl5x ase. Thus starch-splitting enzymes are called amylases; fat-splitting enzymes, lipases; protein-splitting enzymes, proteases. The name showing the activity of the enzyme is often preceded by an adjective to indicate its source ; e.g. salivary amylase (ptya- lin), pancreatic amylase (amylopsin). Such designation does not necessarily imply that the amylase found in the saliva either is or is not the same substance as the amylase of the pancreatic juice. In discussions of enzyme action the substance on which the enzyme acts is sometimes called the substrate. Within the cell producing it an enzyme often exists in an inactive form known as the zym-ogen or antecedent of the active enzyme. The zymogen may be stored in the cell in the form of material which is converted into active enzyme at the time of secretion, or the secretion may be poured out with the zymogen not yet completely changed to active enzyme, or sometimes in a form which requires the presence of some other substance in order to render it active. In this case the latter substance is said to activate the enzyme. Influence of hydrogen ion concentration. — The activity of most enzymes is largely dependent upon the exact acidity or alkalinity of the medium. This is now usually expressed in terms of hydrogen ion concentration. Thus a normal solution of hydrochloric acid would contain, if the HCl were completely ionized, i gram of hydrogen ions per liter ; and in a thousandth- normal solution in which the ionization actually is almost com- plete (actually about 99 per cent of the HCl in such a solution ENZYMES AND DIGESTION 77 is ionized at ordinary temperatures) the concentration of hy- drogen ions is o.ooi gram per liter or i x io~^. Pure water, according to the usually accepted estimates, has a hydrogen ion concentration of i x lo"'' and the same concentration of hydroxyl ions. Thus water which is pure and strictly neutral may also be regarded as being equivalent to a ten-miUionth- normal acid and at the same time a ten-millionth-normal alkali. In order to avoid cumbersome numbers Sorensen has proposed to indicate hydrogen ion concentration by writing the negative exponent as a whole number, e.g. in the case of pure water Ph"^ = 7-0 ; in thousandth-normal hydrochloric acid Ph"*" = 3.0. Thus according to the Sorensen notation, generally indicated by the use of the symbol Ph^, a number lower than 7 shows acidity and the more acid the solution the lower the number ; a number higher than 7 shows alkalinity and the greater the alkalinity the higher the Pg"*" number, since this is the negative exponent of the hydrogen ion concentration. It must be remembered that the Sorensen exponent, or Ph"*" number, varies with the hydrogen ion concentration not arith- metically but logarithmically : i x io~® = Ph"*" 6.0 ; 2 X io~® = Ph" 5-7. f The hydrogen ion concentrations most favorable to the action of certain well-known enzymes have recently been measured with the following results : Enzyme Optimitm H Ion Concentration as Ph+ (Nelson) (Okada) when acting on fibrin (Long) when acting on casein (Long) (Sherman and Thomas) Activity of the Digestive Enzymes That the typical digestive enzymes are very pronounced catalysts may be judged from the relatively large amounts of Invertase (Sucrase) 4-4 Pepsin . . . 1-5 Trypsin . . . 8.0-8.3 Trypsin . . . 5-6-6.3 Malt amylase 4-4 78 CHEMISTRY OF FOOD AND NUTRITION material which they are capable of digesting under favorable conditions. Thus Hammarsten's rennin coagulated 400,000 to 800,000 times its weight of casein ; Petit described a pepsin powder which dissolved 500,000 times its weight of fibrin form- ing 1000 times its weight of peptone; the pancreatic amylase preparation of Sherman and Schlesinger digested 2,000,000 times its weight of starch with the production of 1,200,000 times its weight of maltose. A catalyzer is usually considered to alter the velocity of a reaction but not to initiate it. Thus hydrogen peroxide de- composes spontaneously into water and oxygen. In a pure aqueous solution this change goes on slowly, but it is very greatly accelerated by the presence of a minute amount of colloidal platinum. Blood and tissue extracts contain enzymes which accelerate the decomposition of hydrogen peroxide apparently in much the same way as does platinum, and the present tend- ency is to regard the enzymes generally as acting quite like the inorganic catalyzers in altering by their presence the velocity of certain reactions. Some of the best-known enzyme actions, however, fit into this view only theoretically ; for if the enzyme be considered as simply accelerating a reaction already taking place, it must also be considered that in the absence of the enzyme the reaction is so slow that it cannot be demonstrated. It may perhaps be asked why, if enzymes act by catalysis, there should be any limit to the amount of substrate which the enzyme can hydrolyze. One reason that enzymes cannot hy- drolyze infinite amounts of substrate is that they are them- selves unstable organic substances which undergo decomposition when kept in solution. In most cases the purer the enzyme the more rapidly its solutions lose their activity. Another reason that an enzyme does not continue to hydrolyze substrate indefinitely is that the reaction is progressively retarded by the accumulation of the products formed. The activity of an enzyme may be stopped, even when all ENZYMES AND DIGESTION 79 other conditions are favorable, by the accumulation of the prod- uct of its action ; and in certain circumstances the action of the enzyme may be reversed so as to accelerate a change in the opposite direction to that in which it ordinarily acts. Thus Croft Hill showed it to be possible to reverse the ordinary action of maltase so as to make it bring about a conversion of mono- into di-saccharide ; Pottevin synthesized triolein by means of the pancreas ferment, and Taylor and others have demonstrated a partial reversion of the tryptic digestion of proteins. While the exact significance of these experiments upon the reversi- biUty of the actions brought about by the digestive enzymes has been questioned, there seems to be no doubt that hydrolytic enzymes are widely distributed in active cells and that many of the transformations which take place in the course of the me- tabolism of the foodstuffs in the body are best explained on the ground of the reversibility of enzyme action. Consideration of the tissue enzymes will be left until the study of the fate of the foodstuffs in metaboUsm is taken up. At this point it may be convenient to summarize in tabular form the occurrence and action of the chief digestive enzymes. SxHiiMARY OF Chief Digestive Enzymes Act on Car- bohydrates Enzymes Where Chiefly Found Ptyalin (salivary Salivary secretions amylase) Amj'lopsin (pan- Pancreatic juice creatic amj-lase) Invertase Intestinal juice (Sucrase) Action Act on Fat Maltase Lactase Lipases Intestinal juice Intestinal juice Gastric (?) and pancreatic juices Converts starch to maltose Converts starch to maltose Convert ; sucrose to glucose and fructose Converts maltose to glucose Converts lactose to glucose and galactose Split fats to fatty acids and gb'c- erol 8o CHEMISTRY 01- FOOD AND NUTRITION Summary of Chief Digestive Enzymes {Continued) Enzymes Where Chiefly Found [Action Pepsin Gastric juice Splits proteins to proteoses and peptones . Trypsin Pancreatic juice Splits proteins to Act on Pro- teins proteoses, pep- tones, polypep- tids, and amino acids Erepsin Intestinal juice Splits peptones to amino acids and ammonia With this brief sketch of the nature and action of the diges- tive enzymes, the adequate discussion of which would require a volume in itself, we may now pass to a review of the digestive process, following the course of the food through the human alimentary tract and noting briefly both the mechanical and chemical treatment to which it is subjected. Salivary and Gastric Digestion Since the muscular movements of the digestive tract, par- ticularly of the stomach when empty, play an important part in bringing about the sensations which lead to the taking of food, it may be well to note at this point the results obtained by Cannon and Washburn in their recent investigation of hunger. Lest hunger be confused with appetite, it is essential to clearness that these terms be defined. Some consider that the two experiences differ only quantitatively, appetite being regarded as a mild state of hunger; but Cannon and Wash- burn hold that hunger and appetite are fundamentally differ- ent. In their view : " Appetite is related to previous sensations of the taste and smell of food ; it has therefore, as Pawlow has shown, important psychic elements. It may exist separate from hunger, as, for example, when we eat delectable dainties merely to please the ENZYMES AND DIGESTION 8l palate. Sensory associations, delightful or disgusting, deter- mine the appetite for any edible substance, and either memory or present stimulation can thus arouse desire or dislike for food." " Hunger, on the other hand, is a dull ache or gnawing sen- sation referred to the lower midchest region or epigastrium. It is the organism's first strong demand for nutriment, and, not satisfied, is Hkely to grow into a highly uncomfortable pang, less definitely locaHzed as it becomes more intense. It may exist separate from appetite, as, for example, when hunger forces the taking of food not only distasteful but even nauseating." Hunger is not due merely to emptiness of the stomach. It is true that under ordinary conditions hunger is apt to appear soon after the last food has passed from the stomach to the in- testine, but if the stomach be artificially emptied, the sensation of hunger may not be felt until some hours afterward. Nor is hunger due to hydrochloric acid secreted into an empty stomach, for if the empty stomach of a hungry person be washed out, but little if any acid is found. The explanation of hunger, advanced by Cannon and Wash- burn, is that it is due to the muscular contractions of the walls of the empty stomach. In order to learn whether direct proof of this might be secured experi- mentally in man, one of the investigators accustomed himself to swallowing a small soft rubber balloon attached to the end of a rubber tube by means Qf which it could be withdrawn when desired. The tube and bulb were habitually carried thus in the esophagus and stomach for two or three hours at a time until the experience ceased to have any disturbing effect. Experiments were then made in which the balloon, thus held in the stomach, was partially inflated with air and connected with a manometer and record- ing apparatus by means of which any pressure exerted upon the balloon was recorded automatically. In the actual experiments, the subject sat at rest with his hand on a key which he pressed whenever he experienced the sensa- tion of hunger. This key was connected with a recording device which, like the apparatus recording the muscular contractions of the stomach upon the rubber balloon, was out of sight of the subject. G 82 CHEMISTRY OF FOOD AND NUTRITION Before hunger was experienced the recording apparatus revealed no evi- dence of muscular activity in the stomach. The records of hunger "pangs" and of muscular contractions of the stomach were always approximately simultaneous, that is, when the subject of the experiment felt hungry, power- ful contractions of the stomach were always being registered. The con- tractions were about 30 seconds in duration, with pauses of 30 to 90 seconds between. It was found in almost every case that the contraction reached its greatest intensity just before the record of the hunger sensation began, and that the feeling of hunger disappeared when the contraction ceased although no food or drink had been taken. Cannon considers the evidence conclusive that hunger is caused by the contractions, and not vice versa, as Boldireff had thought. Other observations in the course of Cannon's experiments showed that the lower end of the esophagus also contracts periodically in hunger, an explanation of the fact that sensations of hunger may be felt in cases where the stomach has been removed. Furthermore Cannon considers that vague sensations of hunger may also originate from muscular contractions in the intestine. What causes the stomach contractions which give the sen- sation of hunger has not been determined. They do not seem to be directly related to bodily need. That they usually begin at or soon after the accustomed meal hour may be taken not only as evidence that habit plays an important role, but also as an indication of the desirability of eating at regular times ; for in view of the importance of the muscular tone of the stomach walls, these observations seem to justify the view that the strong muscular contraction of the empty stomach may be regarded as an indication that the condition which causes the first sen- sation of hunger is that in which the stomach is in the best state of readiness to receive the food. There is also direct experi- mental evidence that the stomach digests more expeditiously the food which is " eaten with hunger " (Hudek and Stigler, cited by Carlson). The description of the digestive process which follows presupposes that the food is eaten under favor- able conditions and received by a digestive tract which has been permitted to form good and regular habits. The eating of food induces a flow of saUva from great num- ENZYMES AND DIGESTION 83 bers of minute glands In the lining membrane of the mouth and from the three pairs of large salivary glands. That saliva is secreted in response to psychic as well as chemical stimulation is shown by the fact that actual contact with the food is not necessary, since secretion may be started by the sight or odor or even the thought of food. Mixed human saliva has usually a faintly alkaline reaction and always contains ptyalin (salivary amylase), although its amylolytic power appears to vary con- siderably with individuals and with the same individual at dif- ferent times of the day. As the food comes in contact with saUva, the digestion of starch and dextrin under the influence of the ptyaUn begins at once ; but as mastication is an entirely voluntary act, the thoroughness with which the food becomes mixed with saliva is subject to wide variations. Usually the food stays too short a time in the mouth for the starch to be acted upon there to any great extent, and until recently it was supposed that salivary digestion must cease almost as soon as the food reaches the stomach, since the ac- tivity of ptyalin is quickly checked by even small amounts of free hydrochloric acid. It was supposed that the food mass must soon be mixed with the gastric juice under the influence of the " churning " of the stomach contents by the muscular contraction of the stomach walls, which was so interestingly described by Dr. Beaumont in the account of his classical re- searches already referred to (pages 70-7 1 ) . From the nature of the case Dr. Beaumont's observations were made entirely at one point in the stomach. Here he found during digestion a vigorous mus- cular churning and mixing of the food mass with the gastric juice. For a long time this was supposed to represent the state of the entire stomach contents. This view has now been abandoned as the result of a number of recent investigations, among which those of Cannon and of Griitzner are of especial interest. When a small amount of an inert metallic compound such as bismuth subnitrate is mixed with the food, it becomes possible 84 CHEMISTRY OF FOOD AND NUTRITION to photograph the food-mass within the body by means of the Roentgen rays. By the use of this method Cannon has carried out an extended series of observations upon the movements of the stomach and intestines during digestion/ upon the results of which the statements concerning the mechanism of digestion in this chapter are chiefly based. Cannon's observations, confirmed by those of other investi- gators, show that the vigorous muscular movements described by Beaumont, and which gen- erally begin 20 to 30 minutes after the beginning of a meal, occur only in the middle and posterior, or pyloric, portion of the stomach, while the anterior portion, or fundus, which serves as a reservoir for the greater portion of the food, is not ac- tively concerned in these move- ments and does not rapidly mix its contents with the gastric juice. That there is no general circu- lation and mixing of the entire stomach contents during or immediately following a meal is further shown by the experiments of Griitzner, who fed rats with foods of different colors and on killing the animals and examining the stomach contents found that the portions which had been eaten successively were arranged in definite strata. The food which had been first eaten lay next to the walls of the stomach and filled the pyloric region, while the succeeding por- tions were arranged regularly in the interior in a concentric fashion (Fig. 5). In describing this result Howell says: " Such Fig. 5. — Section of frozen stomach of rat during digestion to show the stratification of food given at differ- ent times. {Griitzner.) The food was given iri three portions and colored differently. Reproduced from Howell's Textbook of Physiology, by permission of the W. B. Saunders Co. 1 These and other investigations are fully discussed in Cannon's Mechanical Factors in Digestion. See also Carlson's Control oj Uiinger in Health and Disease. ENZYMES AND DIGESTION 85 an arrangement of the food is more readily understood when one recalls that the stomach has never any empty space within ; its cavity is only as large as its contents, so that the first portion of food eaten entirely fills it, and successive portions find the wall layer occupied and are therefore received into the interior." The character of the gastric juice secreted in different parts of the stomach varies considerably, especially as regards its acidity. In the middle region the secretion is rich in acid, while both in the cardiac region and at the extreme pyloric end, the " border cells " or " cover cells " (from which the secretion of the acid appears to take place) are few in number or entirely lacking, and the juice secreted in these regions may be neutral or, according to Howell, even slightly alkaline. The nature and extent of the muscular movements also vary greatly in the different regions of the stomach. The peristaltic waves of muscular constriction which bring about the thorough mixing of the food with the gastric juice begin in the middle region and travel toward the pylorus. Over the pyloric part of the stomach when food is present constriction waves are continually coursing toward the pylorus. The food in this region is first pushed forward by the running wave and then by pres- sure of the stomach wall is returned through the ring of con- striction. Thus the food in this portion of the stomach is thoroughly mixed with the gastric juice and is forced by an oscillating progress toward the pylorus. The food in the cardiac end of the stomach is not moved by peristalsis, and so comes only slowly into contact with the gastric juice ; and since the juice secreted here contains Uttle if any free acid, a large part of the food mass remains for some time (variously estimated at from 30 minutes to 2 hours or more) in approximately the same neutral or faintly alkaUne condition in which it was swallowed, and saUvary digestion continues in this part of the stomach without interruption. Thus, if the food has been thoroughly chewed and well mixed 86 CHE^IISTRV OF FOOD AND NUTRITION with saliva before swallowing, much if nol most of its starch may be converted into dextrin and maltose in the cardiac region of the stomach before the activity of the ptyaHn is stopped by contact with the acid of the gastric juice. The fundus, however, is not entirely inactive, but acts as a sort of elastic pouch which is distended by and slowly con- tracts upon the food mass, thus gradually tending to move the posterior portions and particularly the more fluid portion into the pyloric region. As digestion proceeds, the pylorus opens more frequently and the stomach tends to empty itself more and more freely, until finally the pylorus may open to allow the passage of particles which have not been acted upon by the gastric juice. Whether the stomach will thus completely empty itself of one meal before the eating of the next will de- pend of course upon the length of the interval and the amount and character of the food composing the meal. Small test meals may disappear in from i to 4 hours, but meals approxi- mating one third of the day's food may not disappear entirely from the stomach during 6 or 7 hours. In stud^dng the passage of food from the stomach into the in- testine. Cannon found that the pylorus does not open at the approach of each wave of constriction which passes over this part of the stomach, but only at irregular intervals. When the observations made by means of the Roentgen rays were sup- plemented by chemical examinations of stomach and intestinal contents removed at different stages, it appeared that the presence of free acid in the pyloric part of the stomach causes the pylorus to open, and its presence in the small intestine causes the pylorus to close. Thus it would appear that under normal conditions it is only when the protein of the food has become more or less completely saturated with hydrochloric acid and some of the latter remains in the free state, that the food is allowed to pass into the intestine. Ordinarily, when each is fed separately, protein food stays ENZYIMES AND DIGESTION 87 longer in the stomach than carbohydrate, fat longer than pro- tein, and mixtures of fat and protein leave the stomach more slowly than either alone. This is probably because fat tends to retard both the motiUty of the stomach and the secretion of the acid gastric juice. In general the softer or more fluid the fat the more rapidly it will leave the stomach ; also emulsified fats tend to pass on more promptly than fat of the same kind taken in larger masses. The difference noted between protein and carbohydrate is doubtless due to the fact that combination of the acid of the gastric juice with the protein of the food delays the appearance of free acid at the pylorus ; for when protein food was acidulated before feeding and carbohydrate food was made alkaline, the protein was found to leave the stomach more rapidly than the carbohydrate. That the passage of food from stomach to intestine is governed mainly by the degree of acidity reached in the pyloric part of the stomach is of interest in view of the importance to the organism of the action of the acidity of the gastric juice in effecting a partial disinfection of the food. It has been found that when through any cause the hydrochloric acid of the gastric juice is abnormally decreased, the numbers of bacteria in the stomach contents may increase greatly. It will be seen also that the acidity of the chyme as it passes the pylorus has an important influence upon the secretion of the pancreatic juice. The most important characteristics of gastric juice are the presence of free hj^drochloric acid and of pepsin. While other acids may be found in stomach contents, the acidity of gastric juice appears to be due entirely to hydrochloric acid. Normal human gastric juice has been found by different observers to contain about 0.2 to 0.4 per cent of free hydrochloric acid.* ♦According to Carlson, "hunger juice" and "appetite juice" in man contain respectively 0.25 per cent and 0.40 per cent of free hydrochloric acid — averages of hundreds of obser\'ations upon a healthy man having a gastric fistula. 88 CHEMISTRY OF FOOD AND NUTRITION The stimuli which bring about secretion of gastric juice are both psychical and chemical. Psychical stimulation results from the sensations of eating and may also be due to the sight and odor of food. The psychical secretion is studied chiefly by means of the "fictitious feeding" ("sham feeding") experiments in which food is given to dogs which have been prepared with esophageal openings through which the swallowed food escapes without entering the stomach. When such a dog is fed with meat, for example, there is a con- siderable secretion of gastric juice in spite of the fact that no food reaches the stomach. Such a flow of gastric juice is due to impulses received through the nervous system and specifically through the vagus nerve, for fictitious feeding has been found to cause a flow of gastric juice when the vagi are intact, but not after they have been cut. Secretion produced in this way reflexly as the result of the sensation of taste, odor, etc., is called by Pawlow a "psychic secretion" or "appetite juice." When the secretion is once started, even if no food enters the stomach, the flow of juice maj' continue for some time after the stimulus has ceased. On the other hand, the normal secretion of gastric juice may be checked by unpleasant feelings such as fear, anger, or pain. This has been repeatedly observed with frightened or angry animals. Hornborg reports a similar observation upon a small boy. Food was shown but withheld, and the child became vexed and distressed, whereupon no gastric juice was secreted. After he was calmed, and given the food, it was some time before secretion began. Cannon infers, furthermore, that there is a "psychic tone" or "psychic contraction" of the gastro-intestinal muscles, analogous to the psychic secretion. In the same fashion that secretion may be checked, so also the movements of the stomach, bringing about the mixing of food with gastric juice and insuring its passage on into the duodenum, may be stopped during excitement or pain. This fact has been observed many times in experi- ments w^ith various animals, as well as in the case of human beings. If psychic secretion is normally excited, it insures the prompt beginning of gastric digestion. Stimulations arising within the stomach itself supplement the psychic influences and provide for the continued secretion of the gastric juice long after the mental effects of a meal have disappeared. This second stimu- lation is chemical and depends upon the production in the py- loric mucous membrane of a specific substance, or hormone, which acts as a chemical messenger to all parts of the stomach, ENZYMES AND DIGESTION 89 being absorbed into the blood and thence exciting the activity of the various secreting cells of the gastric glands (Starling). Meat extracts, soups, etc., are particularly active in exciting the secretion which depends upon chemical stimulation; milk causes less secretion ; white of egg is said to have no effect. Under normal conditions, the amount of nutritive material absorbed from the stomach is insignificant as compared with the amount absorbed from the intestine. Nearly all the food eaten is passed from the stomach into the intestine in the form of chyme, having been more or less perfectly liquefied and acid- ulated by its thorough mixing with the gastric juice in the middle and pyloric regions of the stomach. The stomach therefore has several functions. It serves (i) as a storage reservoir receiving food in relatively large quanti- ties, say three times a day, and passing it on to the intestine in small portions at frequent intervals, (2) as a place for the con- tinuation of the salivary digestion of starch, and (3) for the beginning of the digestion of proteins and perhaps fats, and finally (4) as a disinfecting station by virtue of the germicidal action of the hydrochloric acid of the gastric juice. Intestinal Digestion Digestion in the small intestine. — When the pylorus opens, food, now reduced to liquid chyme, is projected into the upper part of the small intestine, where it usually lies for some time in the curve of the duodenum, until several additions have been made to it from the stomach. While the food rests here the bile and pancreatic juice are poured out upon it, and here also, as well as in other parts of the small intestine, a certain amount of intestinal digestive juice (" succus entericus ") is secreted by the glands of the Hning membrane and mixed with the intestinal contents. While for purposes of descrip- tion the pancreatic and intestinal juices and the bile may be go CHEMISTRY OF FOOD AND NUTRITION discussed separately, it is to be remembered that in normal digestion they always act together. Cannon's observations showed that after a certain amount of food and digestive juices has accumulated as just described in the first loop of the small intestine, the mass all at once becomes segmented by constrictions of the intestinal walls, and the segmentation is repeated rhythmically for several minutes, so that the in- dividual portions are subjected to relatively extensive and energetic to-and-fro movement, which is doubtless very im- portant in facilitating the emulsification of fat. Other effects of the muscular constrictions which cause the segmentation are (i) a further mixing of food and digestive juices, (2) the bring- ing of the digested food into contact with the absorbing mem- brane, (3) the emptying of the venous and lymphatic radicles in the membrane, the material which they have absorbed being forced into the veins and lymph vessels by the compression of the intestinal wall. After a varying length of time the seg- mentation ceases and the small segments are carried forward individually by the peristaltic movement, or join and move on as a single body. The fluid food mass which the stomach pours into the duo- denum contains a small amount of free hydrochloric acid be- sides a larger amount combined with protein and sometimes or- ganic acids from the food as eaten, or from bacterial fermentation of carbohydrates in the stomach. The pylorus having closed, the alkalinity of the bile, the pancreatic juice, and the intestinal juice combine to neutralize the acids present. In man the main duct of the pancreas and the bile duct unite and empty into the small intestine about 8 to 10 cm. (3 to 4 inches) below the pylorus. The pancreatic juice is a clear liquid having an alkalinity probably equivalent to a 0.5 per cent solution of sodium carbonate and containing three important enzymes or their zymogens — trypsin, amylopsin (amylase), and steapsin or lipase. ENZYMES AND DIGESTION 91 The outflow of the pancreatic juice begins at once when any of the acid stomach contents passes through the pylorus, and has been shown by BayHss and StarUng to be due to a definite chemical substance, secretin, a typical hormone produced as the result of the action of the acid upon some constituent of the intestinal mucous membrane, which is absorbed and carried by the blood to the pancreas and there stimulates the flow of pancreatic juice. Human bile, which, as already stated, enters the intestine through the same duct with the pancreatic juice, is a slightly alkaline solution containing, in addition to water and salts, bile pigments, bile acids (as salts), cholesterin, lecithin, and a pecuHar protein derived from the mucous membrane of the bile ducts and gall bladder. The presence of the bile in the intestinal contents greatly increases the solubility of the fatty acids, while at the same time it diminishes the surface tension between watery and oily fluids. Bile may also accelerate the action of pancreatic lipase in a more direct way. Thus bile aids both the digestion and the absorption of fats. The bile acids are themselves absorbed to a considerable extent and again secreted by the liver. The secretion of bile by the liver, although variable in amount, is continuous. Its ejection from the gall bladder into the intestine occurs, however, only during digestion, and appears to be excited by the passage of chyme through the pylorus, and to run parallel to the outpouring of the pancreatic juice. According to Starling, the rapid flow of bile during intestinal digestion is due not only to the pouring out of what was previously stored in the gall bladder, but also to an increased rate of secretion to which the liver is stimulated by the same chemical mechanism which stimulates the flow of pancreatic juice. The intestinal juice is a distinctly alkaline liquid secreted by the tubular glands (crypts of Lieberkiihn) with which the small intestine is lined. It contains at least five enzymes : entero- 92 CHEMISTRY OF FOOD AND NUTRITION kinase, by the action of which trypsinogen is converted into trypsin , erepsin, which produces further cleavage of the pro- teoses and peptones; and the three enzymes, sucrase (or in- vertase), maltase, and lactase, which hydrolyze respectively the three disaccharides, sucrose, maltose, and lactose. The secre- tion of intestinal juice is probably stimulated by secretin, and possibly also by another hormone whose production is de- pendent upon the presence of pancreatic juice. Careful observations on the reaction of the contents of the small intestine were made by Moore and Bergin in 1897.* Samples taken through a fistula immediately above the ileo- caecal valve were always alkaline to methyl-orange, lacmoid, and litmus, but acid to phenolphthalein. Hence neither hydrochloric acid, nor any appreciable amount of the stronger organic acids such as acetic, butyric, or lactic, could have been present in the free state. The acid reaction shown by phe- nolphthalein was probably due either to traces of organic acids, or possibly to dissolved carbonic acid, or to acid-protein com- pounds not yet completely digested and absorbed. It seems probable that this fairly represents the condition as to reaction which exists throughout the greater part of the small intestine. Under such conditions all three classes of foodstuffs would be readily attacked by the digestive enzymes present, and brought into condition for absorption — the carbohydrates as mono- saccharide; the fats as fatty acid and glycerol; the proteins (chiefly at least) as amino acids. The rate of passage of different foodstuffs through the small intestine has been studied by Cannon with the aid of the Roent- gen rays, according to the general method already described. Fat, carbohydrate, and protein foods, uniform in consistency and in amount (25 cc), were fed to cats which had been fasted * Very recently the subject has been reinvestigated by Long and Fenger, using modern methods for the actual measurement of hydrogen ion concentration. See Journal oj the American Chemical Society, June, 1917. ENZYMES AND DIGESTION 93 for 24 hours. At regular intervals for 7 hours after feeding, the shadows of the stomach and intestinal contents were ob- served by means of the Roentgen rays. The process of rhythmic segmentation above described was seen with all three kinds of foodstuffs, and the frequency of its occurrence corresponded roughly to the amount of food present in the intestine. Absorption takes place very readily in the small intestine — ■ more readily and completely than can be explained by the purely mechanical laws of diffusion. On this account the process is sometimes called " resorption " to distinguish it from passive absorption such as takes place by diffusion through non-living membrane. Observations have been made upon a patient having a fistula at the end of the small intestine. In this case it was found that 85 per cent of the protein matter of the food was absorbed before this point was reached, and the absorption of the other foodstuffs is probably equally complete. For this patient the food began to pass the ileocaecal valve in from 2 to 5! hours after eating, but the time required from the eating of the food until the last portions had passed into the large intestine was 9 to 23 hours. Digestion in the large intestine. — We have seen that in the small intestine the conditions are very favorable both for digestion and for absorption, and that very much the greater part of the available nutrients has been absorbed before the food mass reaches the ileocaecal valve. Hertz has observed, however, that often the ileum is still full at the end of four or five hours after the last trace of chyme has left the stomach. Consequently there may be an accumulation of incompletely digested food and active digestive enzymes in the last few inches of the ileum, where it remains and undergoes digestion for perhaps a longer period than in the stomach. During all this time there is active segmentation, but very little peristalsis. 94 CHEMISTRY OF FOOD AND NUTRITION Beginning at infrequent intervals some time after the chyme first reaches it, the ileocaecal valve relaxes each time a peri- staltic wave passes along the last few inches of the ileum. Can- non finds that the ileocaecal valve is physiologically " com- petent " for food which passes through it normally from the small intestine. This means that the food which has reached the large intestine in the natural way is ordinarily never forced back into the small intestine again. This is important because in the anterior portion of the large intestine the waves which appear most frequently are those of antiperistalsis — i.e. tend to force the food back toward the small intestine. Since the ileocaecal valve prevents the food passing back, these antiperi- staltic waves result in thoroughly churning the food in this part of the large intestine and constantly bringing fresh por- tions in c!ontact with the intestinal wall so that the conditions here are quite favorable for absorption. IVIoreover, the walls of the large intestine furnish an alkaline secretion which further aids the completion of the digestive changes already begun. So far as known the large intestine secretes no digestive enzyme of its own. With the passage of material from the ileum into the caecum, the caecum and ascending colon become gradually filled. Recent observations show that this passive filling takes place very slowly except during and immediately after meals (Hertz). The material remains in the large intestine for a comparatively long time (generally about a day, often longer) ; for the peristaltic movements which carry the material onward, while stronger than the waves of antiperistalsis, are of less frequent occur- rence, at least in the first part of the large intestine. During this time there is a marked absorption of water, along with the remaining products of digestion. The residual material gradually becomes more solid and takes on the character of feces. ENZYMES AND DIGESTION 95 Bacterial Action in the Digestive Tract The digestive tract of an infant contains no bacteria at birth, but usually some gain access during the first day of life. In the average adult it is estimated that each day's food in its passage through the digestive tract is subjected to the action of over one hundred bilHon bacteria, chiefly in the large intestine. Since bacteria are regularly present in the digestive tract in such large numbers, it has been questioned whether they may not perform some essential function in connection with the nor- mal processes of digestion. Experiments to demonstrate whether animals are independent of such bacteria are beset with many difficulties. Nuttall and Thierfelder kept sterile for several days the digestive tracts of young guinea pigs delivered by Caesarean section and fed upon thoroughly sterilized food, and as the animals thus treated lived and gained in weight, the experimenters concluded that intestinal bacteria are not es- sential to normal nutrition. This view has recently received strong support from the observations of Levin, who examined the intestinal contents of Arctic animals in Spitzenberg. The digestive tracts of white bears, seals, reindeer, eider ducks, and penguin were found to be in most cases free from bacteria, sho\ving that the latter are not essential to the normal processes of digestion and nutrition. Kendall, however, in citing the evi- dence presented by Levin, points out that Arctic mammals, as soon as they are brought to temperate regions, rapidly ac- quire intestinal bacteria which do not seem to interfere with the well-being of the host. Furthermore Schottelius claims that the conclusions of Nut- tall and Thierfelder are not justified since their experiments did not cover a long enough period. He himself experimented with chickens from bacteria-free eggs. One group kept in an absolutely bacteria-free environment and fed on sterile food, did well for ten days, but thereafter developed very slowly. When 96 CHEMISTRY OF FOOD AND NUTRITION they were given " infected " food (containing common bacteria), they gained rapidly. Meanwhile a second group which had been kept in a sterile environment but had received " infected " food from the start, grew normally, as did a third group kept throughout under ordinary conditions. From these results Schottelius concluded that intestinal flora seem to be necessary for the normal development of chickens. Similar observations have been made by Madame MetschinikofE using tadpoles, and by Moro using turtles. Notwithstanding this conflicting evidence, it would seem fair to conclude from the observations of Levin that if it were possible to exclude absolutely all bacteria from the digestive tract, the well-being of the body would be in no wise im- paired ; yet under such conditions as ordinarily exist, the bacteria which usually predominate in the digestive tract of the healthy man probably render an important service in helping to protect the body against occasional invasions of obnoxious species. According to Herter, a few species, such as B. lactis aerogenes, B. coli, B. bifidus, have adapted themselves so well to the con- ditions existing in the human digestive tract that they are ordinarily not harmful to the host unless present in abnormally large numbers, and being able to hold their own against new- comers they may act beneficially in giving rise to conditions which check the development of other types of organisms, capable of doing injury, which under ordinary conditions man can hardly prevent from occasionally gaining ingress through food or drink. " The presence in the colon of immense numbers of obligate micro-organisms of the B. coli type may be an important de- fense of the organism in the sense that they hinder the develop- ment of that putrefactive decomposition which, if prolonged, is so injurious to the organism as a whole. We have in this adaptation the most rational explanation of the meaning of ENZYMES AND DIGESTION 97 the myriads of colon bacilli that inhabit the large intestine. This view is not inconsistent with the conception that under some conditions the colon bacilli multiply to such an extent as to prove harmful through the part they take in promoting fermentation and putrefaction." Proteolytic enzymes formed by intestinal bacteria may assist in the digestion of food, and it is conceivable that bac- teria may synthesize proteins or amino acids which may then be absorbed by the host, but the recent experiments of Osborne and Mendel seem to show that this cannot be an important factor in protein metabolism. If for our present purpose we consider only the bacteria which are prominent in producing decomposition of foodstuffs in the digestive tract, and these only with reference to this one prop- erty, we may regard as the three main types: (t) the bacteria of fermentation, such for example as the lactic acid bacteria ; (2) the putrefactive bacteria, such as the anaerobic B. aerogenes capsulatus (B. welchii); (3) bacteria of the B. coll type, showing some of the characters of both the fermentative and putre- factive organisms, but tending in general to antagonize the putrefactive anaerobes. Among cases of excessive bacterial decomposition in the digestive tract the fermentation of carbohydrates with pro- duction of organic acids (and possibly also alcohol) is most likely to occur in the stomach, while the putrefaction of pro- teins occurs mainly in the large intestine. While it is true that in general the products of fermentation tend to restrict putrefaction, yet, since the two processes take place for the most part at such widely separated points of the digestive tract, there may be excessive fermentation and excessive putrefac- tion in the same individual at the same time. Among the con- ditions which favor excessive fermentation are : diminished tone and motihty of the stomach, dilation, diminution or absence of free hydrochloric acid in the gastric juice, and excessive use of H 98 CHEMISTRY OF FOOD AND NUTRITION carbohydrate food — especially sucrose and glucose, which are more susceptible to fermentation in the stomach than are lactose, maltose and starch. In the normal human stomach the conditions are quite unfavorable for the development of anaerobic putrefactive bacteria, not only because of the presence of air, but also because of the action of the gastric juice ; and favorable conditions are not found in the anterior portion of the small intestine. In the lower third of the small intestine the numbers of bacteria increase and among them sometimes putrefactive forms. In the large intestine the conditions are much more favorable for the anaerobic putrefactive bacteria, and these may produce marked decomposition in any protein still remaining unabsorbed. In general the greater the amount of digestible but undigested or unabsorbed protein and the longer the material stays in the large intestine, the greater the amount of putrefactive de- composition. Not infrequently excessive fermentation in the stomach causes local sensitiveness which results in the taking of less bulky food (or such as has less indigestible residue), which in turn tends to stagnate in the intestine and thus render the conditions more favorable for intestinal putrefaction. Ac- cording to Herter there sometimes results from the eating of large quantities of meat and sugar a type of fermentation in which oxaHc acid is produced and which must therefore be highly injurious; but ordinarily the products of fermentation are only irritating, while putrefaction gives rise to products which are more distinctly toxic. These include indol, skatol, phenol, and cresol, which are for the most part absorbed into the system and finally excreted in combination with sulphuric acid as " ethereal " or " conjugated " sulphates. Of these the best-known is potassium indoxyl sulphate, commonly called " indican." The amounts of conjugated sulphates and of in- dican in the urine are valuable indications of the intensity of the putrefactive process in the intestine. ENZYMES AND DIGESTION 99 CoeflBcients of Digestibility of Food The fecal matter passed per day varies considerably in health, but, on an ordinary mixed diet of digestible food materials, is usually between loo and 200 grams of moist substance contain- ing 25 to 50 grams of solids. The feces contain any indigestible substances swallowed with the food and any undigested resi- dues of true food material ; but ordinarily they appear to be largely composed of residues of the digestive juices, together with certain substances which have been formed in metabohsm and excreted by way of the intestine, and bacteria, Uving and dead. Prausnitz studied the feces of several persons placed alter- nately on meat and on rice diets and found that, although the solids of the meat were about ten times as rich in nitrogen as the solids of the rice, the two diets yielded feces whose soHds were of practically the same composition. Some of the data of these experiments are shown in the table. Composition of Feces from Different Diets (Prausnitz) Person Principal FOOD Nitrogen in dry feces per cent Ether Extract in dry feces per CENT Ash in dry FECES per CENT H. ... H. ... M. ... M. ... W. P. . . W. P. . . Rice Meat Rice Meat Rice Meat 8.83 8.75 8.37 9.16 8.59 8.48 12.43 15.96 18.23 16.04 15.89 17-52 15-37 14.74 11.05 12.22 12.58 13-13 In view of such results Prausnitz considers that " normal " feces have essentially the same composition irrespective of the food, the quantity of food residues in such " normal " feces being negligible. From this point of view the feces show not so much the extent to which the food has been absorbed as lOO CHEMISTRY OF FOOD AND NUTRITION whether it is a large or a small feces-former. On the other hand, so far as the nitrogen compounds of the feces are con- cerned, it is probably true, as generally assumed, that they represent material either lost or expended in the work of di- gestion, and therefore that the nitrogen of the feces is to be de- ducted from that of the food in estimating the amount avail- able for actual tissue metabolism. This, however, is by no means equally true of the ash constituents, many of which after being metabolized in the body are eliminated mainly by way of the intestine rather than through the kidneys. On a Uberal diet consisting entirely of non-nitrogenous food the amount of nitrogen in the feces was 0.5 to 0.9 gram per day, which is more than is sometimes found in feces from food furnishing enough protein to meet all the needs of the body. Thus the expenditure of nitrogenous material in the digestion of fats and carbohydrates may be larger than in the digestion of protein food. The feces always contain fat (or at least substances soluble in ether) as well as protein. Fasting men have ehminated 0.57 to 1.3 grams of " fat " per day ; and when the diet is very poor in fat, the feces may contain as much as was contained in the food. As the fat content of the food rises, the actual amounts in the feces increase, but the relative amounts de- crease, so that up to a certain point the apparent percentage utiHzation of the fat becomes higher. The limit to the amount of fat which can be thus well digested varies with the individual and with the form in which the fat is given. Quantities up to 200 grams per day have been absorbed to within 2 to 3 per cent when given in the form of milk, cheese, or butter. In addition to protein and fat the feces always contain various other forms of organic matter which in the routine proximate analyses usually made in connection with feeding experiments are collectively reported as " carbohydrates determined by difference." ENZYMES AND DIGESTION lOI With these facts in mind one may make use of the coefficients of digestibiHty without being misled by them. These co- efficients show the relation between the constituents of the food consumed and the corresponding constituents of the feces. Thus if the feces from a given diet contain 5 per cent as much protein as was contained in the food, this proportion is as- sumed to have been lost or expended in digestion, and the co- efficient of digestibility of the protein of the diet is stated to be 95 per cent. While as just shown this assumption is not entirely correct, yet it is approximately true of the organic nutriments that the difference between the amounts in the food and in the feces represents what is available to the tissues of the body, and thus these coefficients serve a useful purpose in the computation of the nutritive values of foods. From the results of hundreds of digestion experiments At- water computed the coefficients of digestibility of the organic nutrients of the main groups of food materials, when used by man as part of a mixed diet, to be as follows : — Average Coefficients of Digestibility of Foods when Used in Mixed Diet (Atwater) Protein Fat Carbohydrates PER CENT PER CENT PER CENT Animal foods 97 95 98 Cereals and breadstuffs 85 90 98 Dried legumes .... 78 90 97 Vegetables 83 90 95 Fruits 85 90 90 Total food of average mixed diet 92 95 98 In some cases these figures are higher than have been re- ported for similar foods by other observers, the differences being due mainly to the fact (not formerly recognized) that a food may be more perfectly utiUzed when fed as part of a I02 CHEMISTRY OF FOOD AND XUTRFnON simple mixed diet than when fed alone. Milk is an example of such a food, and has when consumed as part of a mixed diet a much higher coefficient of digestibility than is often assigned to it on the basis of earlier experiments. It will be seen that the coefficients differ less for the different types of food than might be expected from popular impressions of " digestibility " and " indigestibility." It is also note- worthy that the coefficients of digestibility are less influenced by the conditions under which the food is eaten and vary less with individuals than is generally supposed. In explanation of this it may be noted that general impressions of digestibility relate mainly to ease of digestion and particularly to ease and rapidity of gastric digestion, and that there is Httle direct relation between the ease with which a food is digested in the stomach and the extent to which it is ultimately digested in its passage through the entire digestive tract. Substances which are resistant to gastric digestion will tend to remain long in the stomach and will probably excite a greater flow of gastric juice. Thus a greater amount of acid chyme will enter the duodenum, and this will result in the secretion of a greater amount of pancreatic juice also. Similarly an increase in the amount of food eaten may have little effect upon the coefficient of digestibility of the foodstuffs. In a series of experiments by the writer it was found that the doubling of a small diet decreased the coefficient of digestibility by less than i per cent. Snyder reports that as between medium and large amounts of oatmeal and milk, the protein was 7 per cent and the fat 6 per cent more completely absorbed in the case of the medium ration. REFERENCES Bayliss. Principles of General Physiology. Bayliss. The Nature of Enzyme Action. Cannon. The Mechanical Factors of Digestion. ENZYMES AND DIGESTION 103 Cannon and Washburn. An Explanation of Hunger. American Journal of Physiology, Vol. 29, page 441 (1911). Carlson. The Control of Hunger in Health and Disease. Effront. Les Catalyseurs Biochemique. EULER. General Chemistry of the Enzymes. Falk and Sigiura. The Esterase and Lipase of Castor Beans. Journal of the American Chemical Society, Vol. 37, page 217 (1915). Falk. An Experimental Study of Lipolytic Actions. Proceedings of National Academy of Science, Vol. i, page 136 (March 1915). Journal of Biological Chemistry, Vol. 31, page 97 (191 7). Fischer. Physiology of Alimentation. Hull and Keeton. The Existence of a Gastric Lipase. Journal of Bio- logical Chemistry, Vol. 32, page 127 (191 7). Herter. Bacterial Infections of the Digestive Tract. Howell. Textbook of Physiology. Mathews. Physiological Chemistry, Chapters 8, 9, 10. Metchnikoff and Woolman. Studies on Intestinal Putrefaction. An- nales de VJnstitute Pasteur, Vol. 27, page 825 (19 12). Nelson and Vosburgh. Kinetics of Invertase Action. Journal of the American Chemical Society, Vol. 39, page 790 (April 191 7). Oppenheimer. Die Fermente. Osborne. The Chemical Nature of Diastase. Journal of the American Chemical Society, Vol. 17, page 593 (1895). • Osborne and Mendel. The Contribution of Bacteria to the Feces. Jour- nal of Biological Chemistry, Vol. 18, page 177 (1914). Pawlow. The Work of the Digestive Glands. Schmidt and Strassburger. Die Faezes des Menschen in normalen und krankhaften Zustande. Sherman and Gettler. The Forms of Nitrogen in Pancreatic and Malt Amylase Preparations. Journal of the American Chemical Society, Vol. 35, page 179 (1913)- Sherman ant) Schlesinger. Pancreatic Amylase. Ibid., Vol. :i^, page 1195; Vol. 34, page 1 104; Vol. 37, page 1305 (1911-1915). Starling. Recent Advances in the Physiology of Digestion. Taylor. Digestion and Metabolism. Vernon. Intracellular Enzymes. CHAPTER V THE FATE OF THE FOODSTUFFS IN METABOLISM CARBOHYDRATES The carbohydrate of the food, having been converted into monosaccharides in the intestine, is taken up by the capillary blood vessels of the intestinal wall and passes from them into the portal vein. After a meal rich in carbohydrate the blood of the portal vein is rich in glucose, sometimes reaching twice its normal glucose content ; and may show levulose and galac- tose as well. In the blood of the general circulation, however, only glucose is found, and this remains small in quantity — about one tenth of one per cent — even after a meal rich in carbohydrates, so that a considerable part of the carbohydrate taken must be stored temporarily in the liver and given up gradually to the blood in the form of glucose, thus keeping nearly constant the glucose content of the blood of the general cir- culation. The carbohydrate thus stored in the liver cells is deposited in the form of glycogen, which, after an abundant meal, may reach lo per cent of the weight of the liver (or, in rare cases, an even higher figure) and may fall to nearly nothing when no carbohydrate food has been taken for some time. To a less extent the muscles store glycogen in a similar way, their glycogen contents varying from traces to about 2 per cent. The fact that the carbohydrate stored in the liver after a meal is so largely converted into glucose and passes into the 104 THE FATE OF THE FOODSTUFFS IN METABOLISM 1 05 blood current before the next meal, while the glucose content of the blood remains small and nearly constant, indicates that the glucose of the blood must be quite rapidly used, and from our present standpoint the most important question of the car- bohydrate metabolism is the fate of the glucose carried to the muscles and other tissues by the blood. Oxidation of Carbohydrate By comparison of the arterial and venous blood, it is plain that in its passage through the muscles the blood becomes poorer in glucose and oxygen and richer in carbon dioxide, and this change is more marked when the muscle is active than when it is at rest. The oxidation of glucose in the muscles is in some way dependent upon the pancreas, but the exact func- tion of the pancreas in this connection is still obscure. It is not to be supposed that the glucose is burned directly to carbon dioxide and water. There is much evidence that the glucose molecule is broken before oxidation, and in all probability this first cleavage yields mainly three-carbon compounds. Some lactic acid is always produced by working muscle and this has long been regarded as a possible intermediate product in the metabolism of glucose.* Lactic acid appears to bear important relationships both to carbohydrate metabolism and to muscle contraction. The discussion of the significance and role of lactic acid cannot be attempted here. It may be said, however, that in recent years much experimental evidence has accumulated in support of the view that lactic acid is not formed directly from glucose, but rather through the inter\en- tion of other three-carbon compounds, probably glyceric alde- hyde or methyl glyoxal (pyruvic aldehyde) or both. * It should perhaps be noted here that lactic acid plays a part not only in the metabolism of carbohydrate but of other foodstuffs as well. It may be formed, for instance, from glycerol and from certain amino acids. Io6 CHEMISTRY OF FOOD AXD NUTRITION If we think of the glucose molecule as first breaking into three-carbon molecules with a minimum of internal rearrange- ment, the most probable primary product would appear to be glyceric aldehyde, the formation of which might be represented crudely as follows : CHoOH • CHOH • CHOj H • CHOH ■ CHOH ■ CHO Or, to write the reaction in a more usual form, CeHiaOe-^ 2 CH2OH • CHOH • CHO Glucose Glyceric aldehyde It is also possible that the first product of cleavage of glu- cose may be pyruvic aldehyde or methyl glyoxal : CeHioOe^ 2 CH3 • CO • CHO + 2H2O Glucose Methyl glyoxal (Pyruvic aldehyde) Both glyceric aldehyde and methyl glyoxal have been shown to result from the cleavage of glucose under the influence of alkali in vitro and there are doubtless enzymes in the tissues which catalyze one or both of these reactions with the result that glucose readily undergoes such cleavage as a preliminary to oxidation in the body. Opinion is at present divided as to whether glyceric aldehyde or pyruvic aldehyde (methyl glyoxal) is to be regarded as the usual first step in glucose metabohsm. In either case it is prob- able that the bulk of the carbohydrate material passes through the form of pyruvic aldehyde (methyl glyoxal) on its way to oxidation. According as we assume the process to go on with or with- out the intermediary formation of glyceric aldehyde, the pro- duction of lactic acid from glucose in the body may be rep- resented in either of the following ways : CeHisOe^ CH2OH • CHOH • CHO^ CH3 ■ CO CHO Glucose Glyceric aldehyde Pyruvic aldehyde THE FATE OF THE FOODSTUFFS IN METABOLISM 1 07 -^ CH3 ■ CHOH • COOH Lactic acid or CeHioOe -^ CH3 • CO • CHO ^ CH3CHOH • COOH Glucose Pyruvic aldehyde Lactic acid Each of these reactions has been brought about in the labora- tory by heating with alkaU and at the lower alkalinity of the body the tissue enzymes are believed to catalyze the same or similar changes. Moreover it has been shown that under suit- able experimental conditions lactic acid is formed from gly- ceric aldehyde and from pyruvic aldehyde by the action of surviving liver tissue ; and the further fact that in experimental diabetes glucose may be formed from glyceric or pyruvic alde- hyde as well as from lactic acid tends also to confirm the belief that these aldehydes are intermediary products between glucose and lactic acid — both in normal metaboHsm and experimental diabetes. Glycerol also when perfused through liver tissue yields lactic acid, and since the first product of oxidation of glycerol is in all probability glyceric aldehyde, we have here a further reason for believing that the latter is a normal precursor of lactic acid. There has been no direct demonstration of the presence of glyceric aldehyde or of pyruvic aldehyde (methyl glyoxal) in the body ; but this is probably due to their unstable or highly reactive nature. The view that glyceric aldehyde passes through pyruvic aldehyde in being transformed into lactic acid is not only probable on stereochemical grounds but is strongly sup- ported by much recent evidence indicating that pyruvic aldehyde occupies a central position in the intermediary me- tabolism. Thus far in our study of the catabolism of glucose we have considered no oxidative changes but only the cleavages and transformations which, from the standpoint of the use of glu- cose as fuel, may be regarded as preliminary to oxidation. Probably the first oxidation product to be formed in glucose Io8 CHEMISTRY OF FOOD AND NUTRITION cataboHsm is pyruvic acid, CH3 • CO • COOH. This may be formed by the oxidation either of pyruvic aldehyde or of lactic acid. The relation of the three substances may be represented thus: CH3 • CO • CHO i CH3 • CHOH ■ COOH Pyruvic aldehyde Lactic acid CH3 • CO • COOH Pyruvdc acid Pyruvic aldehyde and lactic acid are, so to speak, upon the same energy plane. Molecule for molecule they are of equal fuel value and either is readily convertible into the other. The conversion of pyruvic acid into lactic acid or pyruvic aldehyde probably takes place under certain conditions, but this involves reduction and so is not to be expected in the normal course of glucose oxidation. The fate of pyruvic acid under normal con- ditions is probably to undergo further oxidation through acetic acid to carbonic acid and water. It is possible that acetalde- hyde or alcohol or both may intervene between pyruvic acid and acetic acid, and that formic acid may be produced as an intermediate step between acetic and carbonic acids. To summarize what now appears to be the most promising theory of the intermediary metabolism of carbohydrate, we may say that the glucose is first transformed, either directly or through glyceric aldehyde, into pyruvic aldehyde (methyl glyoxal), which may either be changed to lactic acid or oxidized directly to pyruvic acid that readily undergoes oxidation to carbon dioxide and water through steps not yet fully worked out. Lactic acid may also be converted into pyruvic acid and thus ultimately be completely oxidized. In case of excessive formation or inadequate oxidation, as in extreme muscular fatigue or asphyxial conditions, lactic acid may accum.ulate in the body or may be excreted unchanged. THE FATE OF THE FOODSTUFFS IN METABOLISM 109 Glucose Glyceric aldehyde % Methyl glyoxal ^ Lactic acid \ / Pyruvic acid \ (Acetic aldehyde) \ (Acetic acid) \ (Formic acid?) Carbonic acid Whatever the exact mechanism of the process, a large part of the glucose brought by the blood is oxidized in the muscles to furnish energy, which appears as external or internal work. In general, the rate at which combustion takes place in the tissues depends upon the activity of the tissue cells, rather than upon the supply either of combustible matter or of oxygen. When a sufficient supply of oxygen is provided, any further increase has httle effect upon the rate of combustion, and, as we have seen, any excess of carbohydrate instead of being burned is stored as glycogen. But while the absorption of an abun- dance of carbohydrate does not greatly change the amount of combustion taking place in the body, it may result in the use of carbohydrate as fuel almost to the exclusion of fat for the time being, as is shown by observations upon the respiratory quotient. The respiratory quotient is the quotient obtained by di- viding the volume of carbon dioxide given off in respiration by the volume of oxygen consumed. That is — Volume of CO. produced ^ . Respiratory quotient." Volume of O2 consumed no CHEMISTRY OF FOOD AND NUTRITION The numerical value of this quotient will evidently depend upon the elementary composition of the materials burned. Car- bohydrates will yield a quotient of i.o since they contain hy- drogen and oxygen in proportions to form water, so that all oxygen used to burn carbohydrate goes to the making of carbon dioxide, and each molecule of O2 so consumed will yield one molecule of CO2, occupying (under the same conditions of temperature and pressure) the same amount of space as the oxygen consumed to produce it. Thus in burning a molecule of glucose, six molecules of oxygen are consumed and six mole- cules of carbon dioxide produced : CcHioOe + 6 Oo ^ 6 CO2 + 6 H2O. Here the volumes of oxygen and of carbon dioxide are equal and the respiratory quotient is i.e. Fats contain much more hydrogen than can be oxidized by the oxygen present in the molecule, and therefore a part of the oxygen used to burn fat goes to form water, so that the volume of oxygen consumed is greater than the volume of carbon di- oxide produced, which gives a respiratory quotient lower than 1.0. The common fats of the body and of the food give quo- tients approximating 0.7. Thus the oxidation of stearin is represented by the equation : 2C57H110O6 + 16302^ 114CO2 + 110H2O. Since 163 volumes of oxygen are consumed and 114 volumes of carbon dioxide produced, the respiratory quotient is — ^ = 0.699. 163 Proteins give quotients intermediate between those of car- bohydrates and fats, but if the amount of protein used in the body be determined by other methods (see Chapter VIII) and allowed for, one may then deduce from the respiratory quotient the proportions of carbohydrates and fats which are THE FATE OF THE FOODSTUFFS IN METABOLISM III being burned in the body at any given time. The body will show a respiratory quotient of i.o when burning carbohydrate alone, of 0.7 when burning fat alone, and of an intermediate value when both fat and carbohydrate are being burned. If, now, the respiratory quotient rises soon after the eating of carbohydrate food, it is evident that the carbohydrate is being used more freely and fat less freely than before. In an experiment by Magnus-Levy the subject before taking food showed a quotient of 0.77. He then ate 155 grams of cane sugar, after which the quotient was determined at intervals of an hour for 7 hours with the following results: i.oi, 0.89, 0.89, 0.92, 0.82, 0.82, 0.79. The quotient here shows that within an hour after the sugar was eaten the body was making use of the carbohydrate to such an extent that fat either was not being used at all or was being formed from carbohydrate as fast as it was burned; and that for seven hours after the meal the body continued to use carbohydrate to a greater, and fat to a less, extent than was the case at the beginning of the experiment. It has been pointed out that, when carbohydrate is absorbed in larger quantity than is required to meet the body's immediate needs for fuel, the surplus normally accumulates as glycogen, which is stored conspicuously in the Hver, but also to a con- siderable extent in the muscles and other organs. The amount of carbohydrate which will be stored in the entire body after rest and Uberal feeding is estimated at 300 to 400 grams. Thus the total amount of carbohydrate which can be stored as such in the body is no more than is frequently taken in one day's food. When the supply of carbohydrate is so abundant that it continues in excess of the needs of the body and accumulates until the liver and muscles have no tendency to increase their store of glycogen, the further surplus of carbohydrate tends to be converted into fat. 112 CHEMISTRY OF FOOD AND NUTRITION Production of Fat from Carbohydrate Experimental evidence of the transformation of carbohydrate into fat has been cited in Chapter II where it was shown that animals which fatten readily on carbohydrate food may store more body fat than could possibly be derived from the fats and proteins eaten ; that milch cows have yielded more fat in the milk than could be accounted for on any other assumption than that fat was formed from carbohydrate; and that there may be more carbon stored in the body from the carbohydrate food eaten by a fattening animal than can be accounted for in any other way than that a part of the carbon taken into the body as carbohydrate was retained as body fat. Further proof of the ability of the animal body to change carbohydrate into fat is obtained from the respiratory quotient. As noted above, observations made after a fast tend to show quotients approaching that of fat, while after feeding carbohy- drates the quotient may rise rapidly. If the quotient reaches i.o, it shows that the body as a whole is using carbohydrate and not fat as fuel; and a quotient greater than i.o may be taken as evidence that the carbohydrate is itself supplying part of the oxygen which appears as carbon dioxide, or, in other words, that it is breaking down in such a way that a part is burned while another part goes to form in the body a substance more highly carbonaceous and having a lower respiratory quotient than the carbohydrate itself. In many cases it is certain that this substance can be nothing but fat. Respiratory quotients greater than i.o have been observed after Hberal carbohydrate feeding in several species, including man. Each such observation furnishes evidence of a conversion of car- bohydrate into fat. The formation of fat from carbohydrate in the animal body is therefore established by four distinct lines of experimental evidence: (i) by determination of the amounts of body fat THE FATE OF THE FOODSTUFFS IN METABOLISM II3 formed, (2) by determination of the milk fat produced, (3) by observation of the amount of carbon stored, (4) by observations upon the respiratory quotient. Chemical Steps in the Formation of Fat from Carbohydrate While there is no doubt whatever of the ability of the animal to synthesize fat from carbohydrate, the mechanism of the pro- cess is far from clear. As expressed by Leathes, " the chemical changes involved are fascinating in their obscurity." What- ever the exact steps, the transformation of carbohydrate into fatty acid radicles must involve reduction of hydroxyl groups and condensations to form the long chains of the higher fatty acids. We have already seen that in what we believe to be the normal course of carbohydrate catabolism there occurs, either along with or quickly following the breaking of the glucose molecule into three-carbon compounds, a reduction of certain hydroxyl groups with transfer of the oxygen so that substances such as methyl glyoxal, pyruvic acid, and lactic acid are formed. From pyruvic acid or lactic acid acetaldehyde may be formed ; two molecules of acetaldehyde may then undergo aldol conden- sation and the aldol be transformed (by simultaneous reduction and oxidation, or transfer of oxygen from the ^ to the terminal carbon) into butyric acid. Such an hypothesis is consistent with reactions observed in vitro and with the well-known pro- duction of butyric acid in certain bacterial fermentations of sugar and of lactic acid. Leathes favors this hypothesis and comments upon it (in part) as follows : " The biochemical sig- nificance of the synthesis of butyric acid from lactic acid and from sugar by bacteria, becomes greater, however, when it is remembered that in this fermentation normal caproic acid is simultaneously formed, and as Raper showed also, though in still smaller amount, normal octoic or capryHc acid. ... In butyric fermentation it seems that the reactions that lead to the synthesis of butyric acid may lead to the synthesis of acids 114 CHEMISTRY OF FOOD AND NUTRITION of longer chains but still unbranchcd and containing an even number of carbon atoms, in other words, that these acids may be produced by. condensation of two, three, or four acetic alde- hyde molecules. In higher organisms, plants or animals, this same condensation carried further would result as Nencki sug- gested in the formation of the series of acids with straight chains of even numbers of carbon atoms leading up to palmitic and stearic acid." Raper^ has shown experimentally that con- densation of two molecules of aldol in alkaline solution yields a straight chain product which on oxidation and reduction by laboratory methods yields normal octoic (caprylic) acid. Smedley has developed an alternative hypothesis regarding the mechanism of fatty acid synthesis from carbohydrate material. According to Smedley,^ the most probable starting point is pyruvic acid. As an intermediary step in the metabolism of carbohydrate, pyruvic acid is probably formed in large quantities in the body, though its reactivity may prevent it from accumulating in measurable amounts. Pyruvic acid readily breaks down to acetaldehyde and carbon dioxide. It also condenses with aldehydes to form prod- ucts which, under conditions similar to those existing in the body, undergo rearrangements (through simultaneous or suc- cessive oxidation and reduction) which result in the spHtting out of carbon dioxide leaving an acid of two more carbon atoms than were contained in the original aldehyde ; or an aldehyde of two more carbon atoms than the original aldehyde may be formed, and this in turn react with another molecule of pyruvic acid forming a fatty acid or aldehyde of two more carbon atoms. Each of these hypotheses assumes as a starting point only ' /. Chem. Soc, Vol. 91, page 1831 (1907). See also Leathes, The Pats, pages 106-109. "^Journal of Physiology, Vol. 45, Proc. page 26; Biochemical Journal, Vol. 7, pagj 364. THE FATE OF THE FOODSTUFFS IN METABOLISM 115 substances which we have good reason to beheve are regularly formed in carbohydrate metabolism, and both are consistent with the well-known fact that natural fats contain fatty acid radicles having all multiples of two carbon atoms from four to eighteen, but none containing uneven numbers of carbon atoms in the molecule. FAT In digestion the fat is split into fatty acids and glycerol which, however, upon absorption are recombined into neutral fat. It is believed that this recombination occurs during the passage of these digestion products through the intestinal wall. The fat thus absorbed is taken up by the lymph vessels rather than the capillary blood vessels, and is poured with the lymph into the blood. The fat which renders the blood plasma turbid at the height of absorption will usually have passed from the blood into the tissues after a few hours. The fat thus leaving the blood may be burned as fuel, or stored for use as fuel in the future, and a part may be transformed into tissue lipoid or enter into combination with proteins to form some of the chemically more complex substances of cellular protoplasm, cell membrane, or of the central nervous system. The fat burned as fuel serves as a source of energy for muscular work and other activities essentially as does carbohydrate. The average re- sults of a very complete series of experiments by Atwater and his associates indicated that the potential energy of fat was 95.5 per cent as efficient as that of carbohydrates for the pro- duction of muscular work. Oxidation of Fat The glycerol from fat is presumably oxidized to glyceric alde- hyde which passes to methyl glyoxal, whose fate is doubtless the same in this case as when the same substance is formed in carbohydrate metabolism. Il6 CHEMISTRY OF FOOD AND NUTRITION The fatty acid presents a separate problem. Through the work of Dakin, and of Knoop and Embden the " beta-oxidation theory " has been developed and is now generally accepted. Ac- cording to this theory the fatty acid is attacked by oxidation at the ^-carbon atom with the probable formation first of /3-hydroxy, and then of )8-ke tonic acids. Further oxidation at this point must then cause a separation of the a- and y8-carbon atoms ; thus two carbons of the original fatty acid break away, presumably to un- dergo complete oxidation, and there remains a fatty acid with two less carbon atoms than the original. By such a process stearic acid would yield palmitic ; palmitic would yield myristic ; myris- tic, lauric ; and so on to butyric acid. Beta-oxidation of butyric acid would yield successively y8-oxybutyric, and acetoacetic acid. Normally the acetoacetic acid should yield two molecules of acetic, which in turn should burn to carbon dioxide and water. The sequence of changes from caproic acid to the final oxida- tion products would thus be as follows : Caproic ; ^-oxY /3-KETO Butyric j3-0XY AcETO- Acetic Carboni ACID (hydroxy) CAPROIC butyric acetic CH3 ->CH3 ->CH3 1 ' ^CH3 ->CH3 ->CH3 ^2CH3 ->4C02 CH2 CH2 CH2 CH2 CHOH CO2 COOH -I-4H2O 1 1 CH2 1 CH2 CH2 CH2 CH2 CH2 1 1 1 CH2 1 CHOH CO COOH COOH COOH 1 CH2 CH2 CH2 COOH COOH COOH When the normal process is interfered with or overtaxed, an- other reaction may occur with the formation from acetoacetic acid of carbon dioxide and acetone, which latter like acetoacetic acid and j8-oxybutyric acid sometimes appears in the urine, especially in many cases of diabetes mellitus. The acidosis of diabetes is believed to be due to the )8-oxybutyric acid and acetoacetic acid thus formed. Acetone, acetoacetic acid, and THE FATE OF THE FOODSTUFFS IN METABOLISM 1 17 /8-oxybutyric acid are sometimes spoken of collectively as " ace- tone bodies." For further discussion of the intermediary metab- olism of fat and of the evidence that the acidosis of diabetes is cliiefly due to acids arising from fat metabolism, the reader is referred to Dakin's Oxidations and Reductions in the Animal Body and the chapter on diabetes in Lusk's Science of Nutrition. Storage of Food Fat in the Body That fat derived from the food may be stored as body fat has already been shown (Chapter III) and need not be dis- cussed further here. Recently Mills ^ has found that fatty oils injected with antiseptic precautions into the subcutaneous tissue may under favorable conditions be absorbed therefrom and used in the body in the same way as if obtained by feeding. Whether fat once deposited in the tissues will remain and ac- cumulate, or be returned to the circulation and used as fuel, will depend upon the balance between the food consumption and the food requirements of the organism as a whole. In this respect, there is no difference between fat consumed and de- posited as such and fat formed in the body from other food materials. Can Carbohydrate be Formed from Fat? Glycerol is readily convertible into glucose in the body, probably passing through the form of glyceric aldehyde as an intermediate step; but the glycerol radicle represents only about one twentieth of the energy value of the fat molecule. Whether carbohydrate is ever formed from fatty acid in the animal body is an open question. As evidence of such formation of carbohydrate from fat, Hill cites observations upon hibernating animals showing in- crease of glycogen during sleep, accompanied by respiratory quotients lower than 0.7. ^Archives of Internal Medicine, Vol. 7, page 6g4 (igii). Il8 CHEMISTRY OF FOOD AND NUTRITION On the other hand, in phlorizin poisoning * and severe diabetes when it would seem that all material in the body capable of transformation into glucose is being thus changed, there does not appear to be a production of glucose from fat (fatty acid). As this latter type of experimentation has been extensively em- ployed while relatively little evidence of the sort cited by Hill has been presented, the trend of opinion is rather away from the view that the animal body can form carbohydrate from fatty acid radicles, or transform fat into carbohydrate beyond the limited amount obtainable from the glyceryl radicles of the fat. It has been suggested that the low respiratory quotients above mentioned may be due to accidental fluctuations, since the blood does not always show the same carbon dioxide con- tent. The question of actual transformation of fat into car- bohydrate is not of great practical importance in normal nu- trition, because under normal conditions fats may be used interchangeably with carbohydrates as source of energy to a very large, though not unlimited, extent. PROTEINS It is now believed that the hydrolysis of proteins to amino acids in the digestive tract is practically complete. The sig- nificance of this digestive cleavage lies not simply in the for- mation of more soluble and more readily diffusible substances, but also in the resolution of the complex molecules of food protein into their simple amino acid " building stones " (" Bau- steine ") which may be rearranged by the body in the synthesis of its own tissue proteins. * Phlorizin causes very great glj-cosuria and, if the poisoning is continued, the usual symptoms of severe diabetes such as muscular weakness, acidosis, acetonuria, and death in coma. From moderate dosage, however, the animal recovers. The glucose content of the blood falls (instead of rising as in true diabetes). The action of the phlorizin appears to be primarily upon the kidneys, causing them to secrete glucose much more rapidly than usual, thus draining ofif the glucose from the blood and keeping it below the norma! level. THE FATE OF THE FOODSTUFFS IN METABOLISM 119 Absorption and Distribution of Protein Digestion Products The work of the past few years, to be described in the para- graphs which follow, indicates that the amino acids, resulting from digestive hydrolysis of the food proteins, pass through the intestinal wall and into the blood of the portal vein unchanged, are carried through the liver into the blood of the general cir- culation and are thus distributed throughout the body, and are rapidly absorbed from the blood into the various tissues. Thus each tissue receives its protein material in the form of amino acids from which can be synthesized the particular kind of protein characteristic of the tissue in question. In other words, each tissue makes its own proteins from the amino acids brought by the blood. Amino acids not used in synthesizing protein (whether brought by the blood or formed by breakdown of tissue material) are broken down or deaminized in the tissues in the manner described beyond. A brief account of recent work on the distribution and im- mediate fate of the amino acids may serve to give a more ade- quate impression of the modern view. In 1906 Howell obtained a qualitative reaction for amino acids in the blood, but conclusive evidence of the relation of these amino acids to metabolism required the development of better methods than were then available for the estimation of amino acid nitrogen in the fluids and tissues of the body. Such methods were developed and appHed independently and almost simultaneously in 191 2 by FoUn and Denis and by Van Slyke and Meyer. Folin and Denis distinguished between the nitrogen of pro- teins, non-proteins, ammonia, and urea. The non-protein nitrogen includes that of amino acids and they were able to show that this form of nitrogen increased in the blood and tissues when glycine or a mixture of amino acids resulting from pancreatic digestion of protein was undergoing absorption from the small intestine. Moreover the increase in the non- 120 CHEMISTRY OF FOOD AND NUTRITION protein nitrogen of the blood and muscles was nearly suflEicient to account for the nitrogenous material absorbed from the intestine, from which it appeared that they had traced the ab- sorbed amino acids and found them to be carried through the blood and to the muscles without being either built up into protein or broken down into ammonia or urea on the way. Urea formation was found to follow distinctly later than the absorption and distribution of the amino acids. Van Slyke and Meyer estimated amino acids by quantita- tive determination of the nitrogen present as amino groups in the non-protein fraction of the blood or tissue. They found that, during the digestion of protein, amino acids pass through the intestinal wall and appear not only in the portal blood but also in the blood of the general circulation, showing that the amino acids, for the most part at least, pass both the intestinal wall and the liver unchanged. Closely following the work of Folin and of Van Slyke, Rona (191 2) demonstrated by experiments upon isolated segments of intestine that the amino acids pass unchanged through the intestinal wall ; Abel (1913) dialyzed free amino acids from the circulating blood of living animals by means of his vivi-diffusion apparatus and actually separated alanine in crystaUine form; and Abderhalden (1914) separated glycine, alanine, valine, leucine, aspartic acid, glutamic acid, lysine, arginine, histidine, and tryptophane from large quantities of shed blood. Soon afterward (191 5) Henriques and Andersen showed that dogs and goats could be kept in a normal condition of nutrition and might even store nitrogen and gain weight when they were nourished exclusively by intravenous injection of a food solution containing nitrogen only in the form of completely digested protein — a strong confirmation both of the completeness of cleavage of protein in normal digestion and of the fact that the body is nourished by free amino acids carried by the blood with- out intervention of chemical changes in the intestinal wall. THE FATE OF THE FOODSTUFFS IN METABOLISM I2l Van Slyke (working upon dogs) continued his investigation of the fate of the amino acids and found that they are rapidly taken up from the blood by the tissues where they seem to be held by adsorption. Since the amino acids can be extracted by means of cold water or alcohol they do not seem to be held in chemical combination with the tissue proteins nor can simple diffusion account for the extent to which they enter the tissues, because they rapidly attain a higher concentration in the muscle and liver cells than in the blood with which these are in contact. The extent to which this concentration of amino acids in the muscles may go seems to have a fairly definite limit at about 75 milligrams of amino acid nitrogen per loo grams of muscle. In the case of liver tissue this " saturation capacity " seems somewhat more elastic and the concentration may reach about twice the maximum observed in muscle, i.e. up to 150 miUi- grams of amino acid nitrogen per 100 grams of liver. In the muscles the amino acids taken up as just described disappear only very gradually and may not seem to be appreciably changed for several hours ; in the liver they disappear rapidly ; in the kidney, pancreas, and spleen they disappear less rapidly than in the liver. The disappearance of the amino acids from the tissues may be due either to a building up into protein or a breaking down with the formation of ammonia and urea or both. It seems probable that in general both processes go on in all tissues, each tissue building its own proteins and each also taking part in the deaminization of amino acids with formation of am- monia or urea. The more rapid disappearance of amino acids from the Hver tissue is probably due to the greater activity of the liver in deaminization and urea formation, especially since Van Slyke has recently measured the increase of urea in the blood on its passage through the Hver and shown that the passage of the blood through the muscle under parallel con- ditions does not increase its urea content to a measurable extent. 122 CHEMISTRY OF FOOD AND NUTRITION Van Slyke's experiments also show that the blood contains amino acids at all times and that the tissues are not freed from amino acids by fasting, while on the other hand high protein feeding does not result in any great accumulation of amino acids as such either in the blood or tissues. All these observa- tions confirm the view that amino acids are the normal inter- mediary products in both the building up and breaking down of body protein and that any large storage of nitrogen in the body must be due to formation of body protein and not to mere ac- cumulation of free amino acids. Utilization of Protein in the Tissues The proteins of the digested food, absorbed and distributed in the form of amino acids as described above, soon become available for nutrition ; and among other functions they, like the carbohydrates and fats, may be burned * as fuel for muscular work. Pfiuger proved that protein may serve as a source of muscular energy by feeding a dog for 7 months exclusively upon meat practically free from fat and carbohydrate, and requiring it throughout the experiment to do considerable amounts of work, the energy for which must in this particular case have been derived largely from the protein consumed. The experimental facts and theoretical explanations regard- ing the breaking down of proteins (or of the amino acids arising from them) in the body tissues must now be considered. By experiment it has been found that if a meal extra rich in pro- tein be eaten, an increased elimination of nitrogenous end pro- ducts can be observed within 2 or 3 hours, and probably much the greater part of the surplus nitrogen will have been excreted within 24 hours of the time it was taken into the stomach. It does not follow, however, that the whole of the protein mole- * It will of course be understood that the protein is not supposed to be burned directly. Protein is split to amino acids, the amino acids deaminized, and the non- nitrogenous residues of the amino acids are burned. THE FATE OE THE FOODSTUFFS IN METABOLISM 1 23 cule is broken down and eliminated so quickly, and many ex- periments have shown that the carbon often does not leave the body so rapidly as does the nitrogen. Evidently, the nitrog- enous radicles of the protein may be split off in such a way as to leave a non-nitrogenous residue in the body, and the study of protein metabolism involves a consideration of the fate of both the nitrogenous and the non-nitrogenous derivatives. The fate of the latter may conveniently be considered first on account of its relation to the metabolism of carbohydrates and fats. Of special interest is the problem to what extent the deaminized cleavage products of protein may be actually trans- formed into carbohydrate or fat in the body. Formation of Carbohydrate from Protein As early as 1876 Wolff berg tested the formation of carbohy- drate from protein by fasting fowls for two days in order to free them from glycogen and then feeding for two days with meat powder which had been washed free from carbohydrate. Two of the fowls were killed soon after this protein feeding and showed more glycogen in their livers and muscles than could be accounted for except as derived from the protein fed. Two similar fowls killed 17 and 24 hours after feeding showed much less glycogen. This formation of glycogen from protein was fully confirmed by Kulz in a long series of experiments in which the food consisted of chopped meat thoroughly extracted with warm water (Lusk). Independent evidence of the production of carbohydrates from protein is found in the work of Seegen, who chopped and mixed the liver of a freshly killed animal and determined the amount of carbohydrate in it by analysis of a portion, while the remainder was kept at body temperature and sampled for analysis from time to time. The percentage of carbohydrate was found to increase, showing that the Hver cells can form car- bohydrate from their own protein substance. 124 CHEMISTRY OF FOOD AND NUTRITION The most striking evidence of the origin of carbohydrate from protein in the animal body is found in the many observations and experiments which have been made in cases of diabetes, and in experimental glycosuria produced either by administra- tion of phlorizin or by removal of the pancreas. In such cases large amounts of carbohydrate may be given off in the form of glucose even when there is Httle body fat and no carbohydrate or fat is fed. The glucose must therefore result from the me- tabolism of protein. In Lusk's exhaustive experiments upon dogs rendered diabetic by phlorizin, 58 per cent of the total weight of protein broken down in the body (whether in fasting or on a meat diet) Was eliminated in the form of glucose. According to Lusk : " After ingestion of protein in the normal organism this sugar becomes early available and may be burned before the nitrogen belonging to it is ehmi- nated, or, if the sugar be formed in excess, it may be stored as glycogen in the liver and muscles for subsequent use. In this way it is obvious that at least half the energy in protein may be independent of the curve of nitrogen elimination, but may rather act as though it had been ingested in the form of carbohydrates." The way in which the production of carbohydrate from pro- tein may take place has received much attention. Lusk dem- onstrated experimentally that alanine, one of the cleavage prod- ucts of all known proteins, may yield glucose abundantly in the body ; and he suggested that the change might occur through the formation of lactic acid as an intermediary product, since he had already shown that lactic acid is convertible into glu- cose. The work of Dakin has thrown further light on the intermediate steps of this transformation. He has shown that glyoxals have been formed from a-amino and a-hydroxy acids, in vitro — e.g. pyruvic aldehyde (methyl glyoxal) from alanine and lactic acid ; and on the other hand a-hydroxy acids have been formed from glyoxals, both in vivo and in vitro. THE FATE OF THE FOODSTUFFS IN METABOLISM 125 CH3— CHNH2— COOH^ CH3— CO— CHO + NH3 Alanine Methyl glyoxal CH3— CHOH— COOH t CH3— CO— CHO + H2O Lactic acid Methyl glyoxal Attempts, however, to synthesize amino acids directly from glyoxals in vitro were not successful. There is some evidence of that synthesis in vivo, but it cannot be considered as fully established whether it takes place directly by the addition of ammonia to free glyoxals, or whether the a-amino acid is formed secondarily from the a-ketonic acid, resulting from the oxidation of glyoxals. The work of Knoop and of Embden and Schmitz leaves no doubt of the ability of the liver cells to form amino acids from the ammonium salts of the corresponding a-ketonic acids. Alanine, phenylalanine, and tyrosine were produced in this way.* It is of course possible that there may have occurred, in these liver perfusion experiments, intermediate steps not recognized by the investigators, but this does not detract from the significance of the fact that the synthesis of amino acids from ammonium salts has now been repeatedly demonstrated by experiment. The relations emphasized by Dakin may be represented as follows : Glucose Protein I i II (Glyceric aldehyde ?) (Amino acids including) II II Lactic acid ^ Methyl glyoxal J5?^ Alanine ^\ I ^^^ ^^ Pyruvic acid^-""'^ I to further oxidation * Embden also obtained alanine after perfusion of ammonium lactate, but the lactate may have been first changed to pyruvate and the alanine formed from the latter. 126 CHEMISTRY OF FOOD AND NUTRITION Attention may be called in passing to the possible importance of the interrelations of alanine, methyl glyoxal, and lactic acid to the regulation of neutrality, not only in the body as a whole (Chapter IX) but also in the particular cells in which deamination may be more active than oxidation. It will be noted that alanine (a nearly neutral substance) yields on de- amination another neutral suljstance (methyl glyoxal) and a base (ammonia) CH3— CHNHo— COOH -^ CH3— CO— CHO + NH3 And furthermore that the neutral substance methyl glyoxal may react with water to form lactic acid CH3— CO— CHO + H-.O ->- CHa- CHOH— COOH Experiments in vitro have shown that the production of lactic acid from methyl glyoxal is promptly checked unless the free acid is quickly neutraHzed ; also that the conversion of alanine into methyl glyoxal and ammonia is accelerated by acids (Dakin). Thus far the possible mechanism of formation of carbohydrate from protein cleavage products has been considered here chiefly in terms of alanine. To what extent is its behavior representa- tive of that of the other amino acids? Experiments in vitro show that the transformation of an a-amino acid into the cor- responding a-ketonic aldehyde is a very general reaction. Dakin and Dudley demonstrated it for all the amino acids with which they worked — glycine, alanine, phenylalanine, vaUne, leucine, and aspartic acid. Experiments in vivo (chiefly on dogs ren- dered diabetic by phlorizin poisoning) have shown that glycine, alanine, serine, cystine, aspartic acid, glutamic acid, arginine, and proline are all capable of yielding large amounts of glu- cose. Leucine, tyrosine, and phenylalanine when similarly administered to phlorizinized dogs increase the elimination of acetoacetic acid rather than glucose. Valine, lysine, and THE FATE OF THE FOODSTUFFS IN METABOLISM 1 27 tryptophane yield neither glucose nor acetoacetic acid to any important extent (Dakin). The amino acids which yield glucose are called glucogenetic, and the amount of glucose which a given protein can yield in the body will naturally depend upon the glucogenetic amino acid radicles which it contains. Since the amino acids result- ing from protein hydrolysis cannot be quantitatively recovered by any laboratory method thus far developed, it is not yet possible to calculate just how much carbohydrate a given protein should theoretically yield. For meat protein and some others the yield has been determined experimentally as in Lusk's in- vestigations cited above. For further discussion of this point see Lusk's Science of Nutrition. We have therefore abundant evidence from the work of in- dependent investigators, using different methods, that the animal body may form carbohydrates readily and in large pro- portion from the protein of the food; and the mechanism of the process is beginning to be fairly well understood. Production of Fat from Protein There has been much controversy regarding the formation of fat from protein in the animal body. A number of observa- tions by Voit which were believed to demonstrate such a pro- duction of fat were subjected to vigorous criticism by Pfliiger and apparently shown to be capable of other interpretations. Later experiments by Cremer in Voit's laboratory appear, however, to estabHsh the 'formation of body fat from protein food beyond reasonable doubt. Thus in one of these experiments a cat after a preliminary period of fasting was placed in a respiration apparatus and fed hberally with lean meat for eight days. The amount of protein broken down in the body was estimated from the nitro- gen eliminated. The carbon eliminated was also measured, 128 CHEMISTRY OF FOOD AND NUTRITION and it was found that 58.4 grams of carbon had been retained in the body. This would correspond to 130 grams of glycogen, but the total amount of glycogen in the body at the end of the experiment was only 35 grams, hence about three fourths of the carbon retained by the cat from the protein food must have been stored as body fat. The evidence of formation of milk fat in part from protein, while perhaps not amounting to a mathematical demonstration, is still very strong. Since there is already abundant experimental evidence of the production of carbohydrate from protein and of the transfor- mation of carbohydrate into fat, it is evident that protein food can indirectly, if not directly, contribute to the formation of fat in the body. The Fate of the Nitrogen in Protein Metabolism It has already been shown that the nitrogen of the protein of food enters the circulation chiefly, if not wholly, as amino acids and is taken up as amino acids by the various body tissues. The amino acids thus obtained by the tissues from the food serve as material for the building up of body proteins; but in the breaking down of body proteins there is doubtless a liber- ation of amino acids of the same kinds. Amino acids from either source are subject to deaminization in the tissues, and in so far as a-amino groups are concerned the process doubtless consists chiefly in the splitting out of the nitrogen as ammonia, most of which is later changed to urea. Nitrogen in other forms than a-amino acids may be expected to undergo a some- what different metabolism, and it is well known that the urine always contains other nitrogen compounds in addition to ammonium salts and urea. Much light has been thrown upon the chemistry of protein metabolism by the study of the quantitative relations existing THE FATE OF THE FOODSTUFFS IN METABOLISM 1 29 among the different forms of nitrogen in the urine under dif- ferent conditions. For our present purpose it will be sufficient to consider only the more important of the nitrogen compounds of the urine and the relations which they are beUeved to bear to the processes of normal metabolism. Urea. — The proteins, on being metaboUzed in the body, yield varying amounts of arginine, which may undergo hydroly- sis into ornithine and urea. In this way a small part of the nitrogen of protein may reach the urea stage through a series of direct cleavages. It is altogether probable, however, that much the greater part of the urea ehminated arises as follows : The protein in cataboHsm is split to amino acids, which are deaminized (as in the conversion of alanine to methyl glyoxal above mentioned), the nitrogen of the amino group being spHt out as ammonia, which with the carbonic acid constantly being produced in metabolism forms ammonium carbonate.* Loss of one molecule of water yields ammonium carbamate, which in turn on loss of one molecule of water yields urea. (NH4)2C03 ^ NH4CO2NH2 + H2O NH4C02NH2^CO(NH2)2 + H2O Ammonium chloride or sulphate evidently cannot be changed to urea in this way ; and experiments show that if hydrochloric or sulphuric acid is introduced into the blood, it is eliminated by the kidneys largely as ammonium salt, and the quantity of urea is correspondingly decreased. In diseased conditions of the liver the organic salts of ammonia (which normally should be burned to carbonate and then converted as above) may also pass through and be eliminated without being changed to urea. In health and on a full protein diet (say about 100 grams protein per day) from 82 to 88 per cent of the total nitrogen excreted by the kidneys is usually in the form of urea. On a low protein diet this percentage is lower. * If ammonium salts of organic acids are first formed, the complete oxidation of the organic acid radicle will bring this ammonia also into the form of carbonate. K I30 CHEMISTRY OF FOOD AND NUTRITION Ammonia. — As already noted, ammonia is evidently a normal precursor of urea, being changed to the latter in part in the muscles and other tissues generally and in part during its passage through the liver. In accordance with this view we find that the eUmination of nitrogen as ammonia may be notably increased at the expense of urea : (i) in structural dis- eases of the liver; (2) after injecting mineral acids which combine with ammonia in the body, forming stable ammonium salts ; (3) in cases of a pathological excess of acids in metabolism, such as often occurs in diabetes and in fevers. All of these are, of course, abnormal conditions. Normally, about 2 to 6 per cent of the total nitrogen eliminated is in the form of ammonium salts, the amount depending largely upon the relation between the amounts of acid-forming and of base-forming elements in the food, which will be discussed in connection with the study of the ash constituents of food and of mineral metabolism (Chapter X). Uric acid and the purine bases (nucleic acid metabolism). — A part of the nitrogen of human urine is always in the form of uric acid and purine bases. These owe their origin either to the free purine substances of the food, such as the guanine and hypoxanthine of meat extract, or to the metabohsm of nucleic acid derived from the nucleoproteins of the food or of the body tissues. The constituent groups of the nucleic acids and the order of their liberation on hydrolytic cleavage such as occurs in metabolism may be represented by the following diagram adapted from the works of Wells and of Jones : THE FATE OF THE FOODSTUFFS IN METABOLISM 131 Nucleoprotein Protein Nuclein Protein Nucleic acid {Nucleotide) Phosphoric acid Nucleoside Carbohydrate Purine bases Adenine Guanine Base<| Pyrimidine bases Cytosine Thymine Uracil Explanation of diagram. — The distinction between nucleo- proteins and nucleins is somewhat arbitrary and perhaps of doubtful value. Wells regards nucleoproteins simply as com- plexes containing a larger proportion of protein than is con- tained in nucleins or vice versa. Jones prefers to discuss nuclein metabolism entirely in terms of nucleic acid in order to avoid the danger of unnecessary confusion with protein metabolism. The nucleic acids do not contain any radicles found in simple proteins ; they are compounds of phosphoric acid and carbohy- drate with purine and pyrimidine bases in which the acid and base radicles are not linked to each other but both to the car- bohydrate radicle. Phosphoric acid-carbohydrate-base chains of this sort are called nucleotides, and the nucleic acids contain- ing four such chains in the molecule are, in this terminology, tetranucleotides. Nucleotidases are enzymes which split nucleic acids liberating the phosphoric acid and leaving compounds of carbohydrate with base which are collectively known as nucleo- 132 CHEMISTRY OF FOOD AND NUTRITION sides. Nucleosidases are enzymes splitting nucleosides into their constituent carbohydrates and bases. In the case of plant nucleic acid the carbohydrate is a pentose ((/.ribose) and the bases are adenine, guanine, cytosine, and uracil. In animal nucleic acid the carbohydrate is that of a hexose and the bases are adenine, guanine, cytosine, and thymine. Lusk summarizes the hydrolysis of yeast nucleotides as follows : Nucleotide — H3PO4 ->- Nucleoside — c?.ribose ->■ Base Adenylic acid ->- Adenosine ->■ Adenine Guanylic acid ->■ Guanosine ->■ Guanine Cytodin-nucleotide ->- Cytodine ->- Cytosine Uridin-nucleotide ->- Uridine ->- Uracil And to show at a glance the characteristic cleavage products of the two types of nucleic acid : Animal nucleic acid Plant nucleic acid (Thymus) (Yeast) Phosphoric acid Phosphoric acid Guanine Guanine Adenine Adenine Cytosine Cytosine Thymine Uracil Hexose Pentose Formulce and relationships. — The chemical relationships of the purine bases and uric acid so far as these are shown by empirical formulae are as follows : Purine, C5H4N4 Adenine, C5H3N4NH2, amino-purine Guanine, C5H3N4ONH2, amino-oxy-purine Hypoxanthine, C5H4N4O, oxy-purine Xanthine, C5H4N4O2, dioxy-purine Uric acid, C6H4N4O3, trioxy-purine THE FATE OF THE FOODSTUFFS IN METABOLISM 1 33 Uric acid, the most highly oxidized of these purines, is the one chiefly found in the urine. The chemical relations of these substances to each other are more fully shown by the structural formulae given on this page. The chemical structure of the pyrimidine bases is indicated by the following formulae : Cytosine Thymine Uracil N= C— NH2 NH— CO NH— CO II II II CO CH CO C— CH3 CO CH I II ' I II I II NH— CH NH— CH NH— CH 6-amino, 2-oxy-pyrimidine 5-methyl, 2, 6-dioxy-pyrimidine 2, 6-dioxy-pyrimidine Since these substances do not yield uric acid or purine bases their fate will not be discussed here. The mode of origin of uric acid from nucleic acid through the purine bases is as follows : Nucleic acid N = C— NHo HN— CO HC C— NH >CH N— C— N Adenine (6-amino purine) H2NC C— NH II II >CH N— C— N"^ Guanine (2-amino, 6-oxy purine) HN— CO NH— CO HN— CO HC C— NH— > II II >CH N— C— N^ Hypoxanthine (6-oxy purine) OC C— NH— > I II >CH HN— C— N^ Xanthine (2, 6-dioxy purine) OC C— NH I II >co HN— C— NH Uric acid (2, 6, 8-trioxy purine) 134 CHEMISTRY OF TOOD AXl) XUTRITION Not only is uric acid the most highly oxidized of the purines, but it represents the highest degree to which oxidation can be carried without breaking the purine ring. The extent to which the purine ring is broken and uric acid destroyed in the body varies with the species. In most mammals such " uricolysis " is an important feature of the purine metabolism. In man the power to destroy uric acid seems to have been almost or en- tirely lost, many recent investigations tending to show that the human body does not contain uricolytic enzymes and that all of the uric acid formed in the body must be transported and excreted either through the kidneys (chiefly in the form of acid urates) or through the intestinal wall. Purines undergoing metabolism in the body may be derived either (i) from the catabolism of nucleoprotein of body tissue or (2) from the food which may contain both nucleoproteins and free purines. Sometimes the term " endogenous uric acid " is applied to that fraction having the former origin, while " ex- ogenous uric acid " indicates that fraction which is directly due to the food. The endogenous uric acid in the urine of man of average size amounts usually to about 0.3 to 0.4 gram per day ; the exogenous varies from mere traces to 2 grams or more accord- ing to the kind and amount of food consumed. On ordinary mixed diet the total urinary output of uric acid averages about 0.6 to 0.7 gram per man per day. The usual range is about 0.5 to i.o gram of uric acid per man per day, in which case the uric acid nitrogen constitutes about i to 3 per cent of the total nitrogen of the urine. Recent investigations of Jones, Levene, and others have greatly elaborated the theory of nucleic acid structure and purine metaboHsm outHned above. For full discussion the reader is referred to the works of Jones (1914) and Jones and Read (1917). Creatine and creatinine. — Chemically creatinine is the anhydride of creatine : THE FATE OF THE FOODSTUFFS IN METABOLISM 135 N (CH3)— CH2— C =0 N (CH3)— CH2— CO I I I HN = C OH HN = C I I NH2 NH' Creatine Creatinine The biochemical relationships and physiological significance of these substances have been much studied in recent years, and the literature of the subject is far too extensive to be summarized satisfactorily here. The main facts with regard to their ehmination as end products of metaboHsm are : that crea- tine appears in the urine of children normally and in that of adults during starvation, fevers, and other wasting diseases and when there is impaired functioning of the liver ; that normal adults ordinarily excrete little or no creatine but a considerable amount of creatinine. The quantity of creatinine excreted is fairly constant for the individual, averaging about 0.02 gram per kilogram of body weight per day. On ordinary mixed diet the creatinine nitrogen usually constitutes 3 to 7 per cent of the total nitrogen of the urine. Distribution of excreted nitrogen as influenced by level of pro- tein metabolism. — The above statements regarding the dis- tribution of the eliminated nitrogen among the different end products refer to results obtained upon an ordinary mixed diet containing the usual amount of protein. FoUn has shown by a careful and extended study of the urines of healthy men living first upon high and then upon low protein diets, that the distribution of the nitrogen between urea and the other nitrog- enous end products depends very largely upon the absolute amount of nitrogen metaboUzed. In the case of a man who on one day consumed high protein diet free from meat, and a week later was living on a diet of starch and cream, which furnished in all about 6 grams of protein per day, the distribution of end products was changed as shown in the following table : 136 CHEMISTRY OF FOOD AND NUTRITION Total nitrogen . . Urea nitrogen . . Ammonia nitrogen Uric acid nitrogen Creatinine nitrogen Undetermined nitrogen On High Protein Diet (Free from Meat) Grams 16.8 14.7 0.49 0.18 0.58 0.85 Per cent 87.5 2.9 i.i 3.6 4-9 On Low Protein Diet (Starch and Cream) Grams 3.6 2.2 0.42 0.09 0.60 0.27 Per cent 61.7 "•3 2-5 17.2 7-3 Thus, on passing from the high protein to the low protein diet (both being free from meat products) there was a marked de- crease in both the absolute and the relative amounts of urea, and a decrease in the absolute, but increase in the relative, amount of uric acid, while the absolute amount of creatinine remained unchanged, so that its relative amount was greatly increased. REFERENCES Abderhalden. Lehrbuch der Physiologische Chemie, Dritte Aufl. Abel, Rowntree and Turner. The Removal of Diffusible Substances from the Circulating Blood of Living Animals by Dialysis. Journal of Pharmacology, Vol. 5, page 275 (1913). Ackroyd ANT) Hopkins. Feeding Experiments with Deficiencies in the Amino Acid Supply : Arginine and Histidine as Possible Precursors of Purines. Biochemical Journal, Vol. 10, pages 551-576 (December, 1916). Allen. Glycosuria and Diabetes. Benedict. Uric Acid in Its Relations to Metabolism. The Harvey Lectures, 1915-1916. Dakin. Oxidations and Reductions in the Animal Body. Dakin and Dudley. (A series of papers on intermediary metabolism.) Journal of Biological Chemistry, Vol. 14, pages 321, 423, 555; Vol. 15, pages 127, 463; Vol. 16, page 505; Vol. 17, page 451; Vol. 18, page 29 (1912-1913). Embden and Schmitz. Synthesis of Amino Acids in the Liver. Bio- ckemische Zeitschrifl, Vol. 29, page 423; Vol. 38, page 393 (1910-1912). THE FATE OF THE FOODSTUFFS IN METABOLISM 137 FoLiN. A Theory of Protein Metabolism. American Journal of Physi- ology, Vol. 13, page 117 (1905). FoLiN AND Denis. Protein Metabolism from the Standpoint of Blood and Tissue Analysis. Journal of Biological Chemistry, Vol. 11, pages 87, 161; Vol. 12, pages 141, 253, 259; Vol. 14, page 29 (1912-1913). Henriques and Andersen. Nutrition through Intravenous Injection. Zeilschrifl fur physiologische Chemic, Vol. 88, page 357 (1913). Janney. The Metabolic Relationship of the Proteins to Glucose. Journal of Biological Chemistry, Vol. 20, page 321 ; Vol. 22, page 203; Vol. 23, page 77 (iQiS)- Jones. Nucleic Acids ; their Chemical Properties and [Physiological Conduct (19 14). Jones and Read. (On the structure of yeast nucleic acid.) Journal of Biological Chemistry, Vol. 29, pages 111-122, 123-126; Vol. 31, page 337 (1917)- Knoop ANT) Kertes. Behavior of a-Amino Acids and a-Ketonic Acids in the Liver. Zeitschrifl fiir physiologische Chemie, Vol. 71, page 252 (1911). Levene and Meyer. (Intermediary metabolism of carbohydrate.) Jour- nal of Biological Chemistry, Vol. 1 1 , page 361; Vol. 1 2, page 265 ; Vol. 1 4, pages 149, 551; Vol. IS, page 65; Vol. 17, page 442; Vol. 18, page 469 (1912-1914). LuSK. Science of Nutrition. Lyman. Metabolism of Fats. Journal of Biological Chemistry, Vol. 32, pages 7, 13 (1917)- Mathews. Physiological Chemistry, Chapter 18. Ffluger. Glycogen. Archiv fiir die gesammte Physiologic, Vol. 96, pages 1-398 (1903). Rose. Creatinuria in Women. Journal of Biological Chemistry, Vol. 31, page I (1917)- Underbill. Studies on the Metabolism of Ammonium Salts. Journal of Biological Chemistry, Vol. 15, pages 327, 337, 341 (1913). Van Slyke. The Significance of Amino Acids in Physiology and Pa- thology. Harvey Lectures, 1915-1916. Van Slyke et al. The Fate of Protein Digestion Products in the Body. Journal of Biological Chemistry, Vol. 12, page 399; Vol. 16, pages 187, 197, 213, 231 (1912-1913). Proceedings of the Society for Experimental Biology and Medicine, Vol. 12, page 93 (1915). Wells. Chemical Pathology. WooDYATT. Studies on Intermediary Carbohydrate Metabolism. Harvey Lectures, 1915-1916. CHAPTER VI THE FUEL VALUE OF FOOD AND THE ENERGY REQUIREMENT OF THE BODY We have seen that carbohydrate after its absorption into the body may either be oxidized, or stored as glycogen, or trans- formed into fat ; that fat may be oxidized or stored and that at least its glyceryl radicle may be converted into carbohydrate ; and that protein absorbed as amino acids may either be built up into body protein, or deaminized and oxidized, or may yield carbohydrate, or may (either directly or indirectly) contribute to the production of fat. It has also been shown that any or all of these foodstuffs may be utiUzed as fuel for muscular work. Thus the body is not restricted to the use of any one food- stuff for the support of any one kind of work, but on the contrary has very great power to convert one nutrient into, or use it in place of, another, and so to utilize its resources that the total potential energy of all of these nutrients is economically em- ployed to support the work of all parts of the organism. The carbohydrates, fats, and proteins stand in such close mutual relations in their service to the body that for many purposes we may properly consider the food as a whole with reference to the total nutritive requirements, provided a common meas- ure of values and requirements can be found. Since the most conspicuous nutritive requirement is that of energy for the work of the body, and since these organic nutrients all 138 THE FUEL VALUE OF FOOD 1 39 serve as fuel to yield this energy, the best basis of comparison is that of fuel value, expressed most conveniently in terms of Calories. Heats of Combustion of the Foodstuffs The calorific value or heat of combustion of any substance, i.e. the amount of energy liberated by the burning of a given quantity of the combustible material, is best determined by means of the bomb calorimeter devised by Berthelot. The particular form of Berthelot bomb which has been most used in the examination of food materials and physiological products is that of Atwater and Blakeslee, fully described by Atwater and Snell in the Journal of the American Chemical Society for July, 1903. In outHne it consists of a heavy steel bomb with a platinum or gold-plated copper lining and a cover held tightly in place by means of a strong screw collar. A weighed amount of sample is placed in a capsule within the bomb, which is then charged with oxygen to a pressure of at least 20 atmospheres (300 pounds or more to the square inch), closed, and immersed in a weighed amount of water. The water is constantly stirred and its temperature taken at intervals of one minute by means of a differential thermometer capable of being read to one thousandth of a degree. After the rate at which the temperature of the water rises or falls has been determined, the sample is ignited by means of an electric fuse, and, on account of the large amount of oxygen present, undergoes rapid and complete combustion. The heat liberated is communicated to the water in which the bomb is immersed, and the resulting rise in tem- perature is accurately determined. The thermometer read- ings are also continued through an " after period," in order that the " radiation correction " may be calculated and the observed rise of temperature corrected accordingly. This corrected rise, multiplied by the total heat capacity of the ap- paratus and the water in which it is immersed, shows the total I40 CHEMISTRY OF FOOD AND NUTRITION heat liberated in the bomb. From this must be deducted the heat arising from accessory combustions (the oxidation of the iron wire used as a fuse, etc.) to ob- tain the number of Calories * arising from the combus- tion of the sample. More recently the adiabatic form of the bomb cal- orimeter (a modifi- cation which avoids the necessity of cor- rections for heat loss) is coming into more general use. See, for example, the paper by Riche, in the Journal of the A merican Chemical Society for Novem- ber, 1913. * When the term " Cal- orie" is used in this work it will be understood to mean the "greater cal- orie," or "kilogram cal- orie," i.e. the amount of heat required l,to raise the temperature of one kilogram of water one degree centigrade. This is very nearly the same as the heat required to raise four pounds of water The Atwater bomb calorimeter. one degree Fahrenheit. THE FUEL VALUE OF FOOD 1 41 The heat of combustion of organic substances is closely connected with their elementary composition. One gram of carbon burned to carbon dioxide ^-ields 8.08 Calories and I gram of hydrogen burned to water yields 34.5 Calories. If a compound consisting of carbon and hydrogen only be burned, it gives nearly the amount of heat which these would give if burned separately. On the other hand, carbohydrates and fats, being com- posed of carbon, hydrogen, and oxygen, the carbon and hydrogen are already partly oxidized by the oxygen present in the molecule ; so that 100 grams of glucose, for example, containing 40 grams carbon, 6.7 grams hydrogen, and 53.3 grams oxygen, would yield considerably less heat than would be obtained by burning 40 grams of pure carbon and 6.7 grams of pure hydrogen to carbon dioxide and water respec- tively. Proteins when burned in the calorimeter give ofiE their carbon as carbon dioxide, their hydrogen as water, and their nitrogen as nitrogen gas.* Thus the nitrogen con- tributes nothing to and takes nothing from the heat of com- bustion; and the latter is dependent here, as in the case of carbohydrates and fats, upon the amount of carbon and hydrogen present and the extent to which they are already combined with oxygen. A Httle additional heat is obtained by the burning of the small amount of sulphur present in the protein. The relation between the elementary composition and heat of combustion will be made clearer by the following table, which includes a number of typical compounds found in the food or formed in the body. * As a matter of fact a small part of the nitrogen is oxidized to nitric acid in the bomb calorimeter, but this is determined and its heat of formation subtracted, so that the final results are as stated above. 142 CHEMISTRY OF FOOD AND NUTRITION Heats of Combustion axd Approximate Elementary Composition of Typical Compounds Substance Heat OF COMBUS- TION Calories PER GRAM Carbon PER CENT Hydro- gen PER CENT Oxygen PER CENT Nitro- gen PER CENT Sul- phur PER CENT Phos- phorus PER CENT Glucose 3-75 40.0 6.7 53-3 Sucrose 3-96 42.1 6.4 51-5 Starch Glycogen J 4.22 44.4 6.2 49.4 Body fat . 9.60 76.5 12.0 II. 5 Butter fat 9-30 75-0 II. 7 133 Edestin 5-64 514 7.0 22.1 18.6 0.9 Legumin S.62 51-7 7.0 22.9 18.0 0.4 Gliadin 5-74 52.7 6.9 21.7 17.7 I.O Casein . 5.85 53-1 7.0 22.5 15-8 0.8 0.8 Albumin 5.80 52.5 7.0 23.0 i6^o 1-5 Gelatin 15-30 50.0 6.6 24.8 18.0 0.6 Creatinine 4-58 42.5 6.2 14.1 37-2 Urea . . 2-53 20.0 6.7 26.7 46.6 Since the energy used in the body is obtained from the oxi- dation of the same kinds of compounds which exist in food, i.e. from carbohydrates, fats, and proteins (or their cleavage products), we can estimate the amount of energy transformed in the body if we know the amount of each kind of foodstuff oxi- dized. Account must, however, be taken of the completeness of the oxidation in each case. When undergoing complete oxidation in the bomb calorimeter the foodstuffs yield the following average heats of combustion : Carbohydrates Fats Proteins 4.1 Calories per gram. 9.45 Calories per gram. 5.65 Calories per gram. In the body carbohydrates and fats are oxidized to the same products as in the calorimeter and so yield the same amounts of heat. Protein, however, which burns in the bomb to carbon dioxide, water, and nitrogen, yields in the body no free nitrogen. THE FUEL VALUE OF FOOD 1 43 but urea and other organic nitrogen compounds which are eliminated as end products. These organic nitrogenous end products are combustible ; they represent a less complete oxi- dation of protein in the body than takes place in the bomb. The loss of potential energy calculated on the assumption that all nitrogen left the body as urea would be about 0.9 Calorie per gram of protein, but on account of the elimination of other substances of higher heat of combustion (creatinine, uric acid, etc.), the actual loss in the form of combustible end products is considerably greater and averages about 1.3 Calories for each gram of protein broken down in the body. Hence, when the body burns material which it has previously absorbed, it obtains : From carbohydrates 4.1 Calories per gram. From fats 9.45 Calories per gram. From protein (5.65 — 1.30 = ) 4.35 Calories per gram. In calculating the fuel value of the food, however, allow- ance must be made for the fact that a part of each of the ma- terials is lost in digestion.* The approximate averages on a mixed diet are : Carbohydrates 2% lost, 98% absorbed. Fats s% lost, 95% absorbed. Protein 8% lost, 92% absorbed. The approximate physiological fuel values of the food constit- uents are then : Carbohydrates 4.1 X 98% = 4. Calories per gram. Fats 9.45 X 95% = 9. Calories per gram. Protein 4.35 X 92% =4. Calories per gram. The figures given by Rubner as representing the fuel values of food con- stituents are as follows : '"^ Carbohydrates 4 . i Fats 9.3 Protein 4.1 * The expression "lost in digestion " is here used in the sense explained in Chapter IV. r 144 CHEMISTRY OF FOOD AND NUTRITION These were derived from experiments with dogs fed on meat, starch, sugar, etc., and therefore do not allow for so much loss in digestion as has been found to occur with men living on ordinary mixed diet. Fuel Value of Food Materials If the composition of a food is known, its approximate fuel value is easily computed by means of the above factors. Thus milk of about average composition contains : Protein, 3.3 per cent; fat, 4.0 per cent; carbohydrate, 5.0 per cent. One hundred grams of such milk will furnish in the form of protein {t,.;^ X 4. =) 13.2 Calories; of fat (4.0 X 9. =) 36.0 Calories; of carbohydrate (5.0 X 4. = ) 20.0 Calories; total for 100 grams of milk, 69.2 Calories. Eggs contain * on the average, in the edible portion, 13.4 per cent protein, 10.5 per cent fat, and no appreciable amount of carbohydrate. They would then furnish per 100 grams (13.4 X 4) + (io-5 X 9) = 148. 1 Calories. Milk and eggs are sufficiently similar to be used interchange- ably in the adult dietary within reasonable Hmits, but evi- dently they furnish, weight for weight, very different amounts of nutrients and energy. Ordinarily the quantities to be taken as equivalent or mutually replaceable are those which furnish equal fuel value, e.g. loo-Calorie portions, the weights of which may be calculated directly from the fuel values of 100 grams. Thus, for milk — 100 grams furnish 69.2 Calories; then, if X be the number of grams which furnish 100 Calories : 100 : 69.2 ::x: 100 ; x = 145. f Similarly for eggs: 100 : 148 :: X : 100 ; x = 68. * These and all similar statements of average composition are based on Bull. 28, Office of Experiment Stations, U. S. Dept. Agriculture. t It is considered sufficiently accurate to state these quantities to the nearest whole number of grams. THE FUEL VALUE OF FOOD 145 And since the two extremes in the proportion are always the same, the weight in grams of the loo-Calorie portion may al- ways be found by dividing 10,000 (the product of the extremes) by the number of Calories per 100 grams. The fuel value of foods is often stated in Calories per pound. Thus in the same table (Bull. 28) from which the above figures for composition are taken, the fuel value of milk is given as 325 Calories per pound. Since 453.6 grams furnish 325 Calo- ries, — 453-6 : 325 : : -'^" : 100 1 x = 139.6, the number of grams required to furnish 100 Calories. This figure is about 3 per cent less than the one found above be- cause it is based on a fuel value computed by Rubner's factors, which are 2.5 to 3.3 per cent higher than the factors based on more recent work. (See above.) 1^ The following figures for a few common food materials* are based upon the more recent factors, and show the weight of the loo-Calorie portion in grams and ounces, and the distribu- tion of the calories between proteins, fats, and carbohydrates: Table of ioo-Calorie Portions f of Food Material Based on the Factors — Protein, 4 ; Fat, 9 ; Carbohydrate, 4 Food Material (Edible Portion) Weight of Portion Grams Ounces Distribution of Calories In protein In fat In carbo- hydrates Beef, free from visible fat Beef, round steak . . . Beef, corned Ham, lean Ham, fat 64 33 37 19 3-0 2-3 1-3 1.2 0.7 80.4 54-5 20.9 29.7 II. I 19.6 45-5 79.1 70-3 88.9 * Arranged according to the classification used in the bulletins of the U. S. Department of Agriculture and in Konig's well-known reference work Die Chemie der Menschlichen Nahrungs- und Genussmillel, viz. meats, fish, eggs, dairy products, grain products, sugars and starches, vegetables, fruits, nuts, oils. t Table i of Appendix B shows loo-Calorie portions of a much larger number of food materials. 146 CHEMISTRY OF FOOD AND NUTRITION Table of ioo-Calorie PoRTioNst of Food Material Based on the Factors — Protein, 4; Fat, 9; Carbohydrate, 4 {Continued) Food Material (Edible Portion) Bacon, smoked . . . Codfish Salmon Eggs Milk Butter Corn meal .... Oatmeal Rice Wheat, "entire" . . Wheat flour . . . . Bread, white . . . Sugar Asparagus . . . . Beans, dried . . . . Beans, string . . . Beets Cabbage Carrots Celery Corn, green or canned Lettuce Potatoes Spinach Tomatoes Turnips Apples Bananas Currants, dried . . , Oranges . . . . " , Peaches .... Pineapple .... Plums Prunes, dried . . Raisins .... Almonds .... Chestnuts . . . Peanuts .... Olive Oil .... Weight of Portion Grams Ounces 16 143 49 67 145 14 27 25 28 28 28 38 25 450 29 240 216 317 220 540 99 523 120 418 438 253 159 lOI 31 194 242 232 118 33 29 15 43 18 0.6 5-0 1-7 2-3 51 0-5 i.o 0.9 1.0 1.0 1.0 1-3 0.9 16.0 1.0 8.4 7-4 II. I 7-7 19.1 3-2 18.4 4.2 14.7 15-5 8.9 5-6 3-5 1. 1 6.8 8.5 8.2 4-1 1.2 1.0 0.5 1-5 0.6 0.4 Distribution of Calories In protein In fat 6.7 95-0 43-3 36.1 19.0 0.5 9.0 16.1 9.1 14-7 11.8 14.1 32.4 26.1 22.2 13-8 20.3 9-7 23.8 12.2 25.2 lo.s 35-1 iS-7 13.2 2-5 S-2 3-0 6.2 6.8 3-7 4-7 2.8 30 13-0 10.7 18.8 93-3 5-0 56.7 639 52.0 99-5 11.4 16.2 0.7 3-5 2.8 4-5 8.2 4-7 6.5 2.0 8.6 7.9 4.8 9.8 14.1 1.2 "•3 15.7 4.6 7.2 5-4 4-7 3-5 2.2 6.3 8.6 76.4 16.6 634 In carlx)- hydrates 29.0 79.6 67.7 90.2 81.8 854 81.4 100. o 594 69.2 713 84.2 71. 1 82.4 71.4 78.0 60.7 88.3 53-6 68.6 82.2 90.3 894 92.3 90-3 91.0 90.0 95-3 97.2 88.4 10.6 72.7 17.8 THE FUEL VALUE OF FOOD 147 Since proteins and carbohydrates have the same average fuel value and the ash of food does not as a rule constitute a large percentage, the striking differences in the weights of the various foods required to furnish 100 Calories are usually referable to differences in water content or fat content or both. That beans have nearly 20 times the fuel value of celery is essentially due to the difference in moisture, while the differ- ence in fuel value between lean beef and bacon, or between codfish and salmon, is chiefly a matter of fat content. Meat free from fat is about three fourths water and one fourth pro- tein, and so has a fuel value of about one Calorie per gram, while clear fat has a fuel value about nine times as great. Fuel values of meats as given in the standard tables are apt to be somewhat misleading, inasmuch as they allow for all the fat ordinarily found on the various cuts as taken from the animal, whereas in many cases a considerable part of this fat is trimmed off by the butcher and treated as a by-product; and often much of the remaining fat is removed either in the kitchen or at the table. If a pound of steak consists of 14 ounces of clear lean, and 2 ounces of clear fat, and the fat is not eaten, at least half of the total fuel value of the pound of steak is lost. Many vegetables are more watery than lean meats and so contrast even more strikingly with the fats. An ounce of clear fat pork is equal in fuel value to about two pounds of cabbage ; an ounce of olive oil to over three pounds of lettuce. In connection with such comparisons of fuel value, however, it should be emphasized that the fuel value of a food, while of primary importance, is not alone a complete measure of its nutritive value, which will depend in part also upon the amounts and forms of nitrogen, phosphorus, iron, and various other essential elements furnished by the food. In order to indicate relative richness in nitrogenous constituents (pro- tein), it is not uncommon to state the "nutritive ratio" along with the fuel 148 CHEMISTRY OF FOOD AND NUTRITION value of a food. The "nutritive ratio" or "nutrient ratio" is the ratio of non-nitrogenous to nitrogenous nutrients, compared on the basis of fuel values. Since the fuel values of carbohydrates and protein are taken as equal (4 Calories per gram), and that of fats as 2^ times as great (9 Calories per gram), the nutritive or nutrient ratio may be shown as follows : Carbohydrate + 2j Fat : Protein :: x : i ; or the ratio may be expressed in the form of a fraction : Carbohydrate + 2J Fat Protein These expressions can, of course, be appUed equally well to percentages or to weights of nutrients. The same information as is given by the statement of fuel value per pound and nutritive ratio may be obtained by comparing the weight of loo-Calorie portions and the percentages of calories supplied by protein as shown in the above table. The statement that 19 per cent of the calories of milk are furnished by protein is equivalent to giving the nutritive ratio of milk as 4.3. ENERGY REQUIREMENT IN METABOLISM — METHODS OF STUDY AND AMOUNTS REQUIRED FOR MAINTENANCE AT REST We know definitely from accurate experiments that the " physiological fuel values " which have been deduced repre- sent the energy which is actually obtained by the body from the food and which appears as muscular work or as heat; and we have every reason to suppose that under ordinary conditions the carbohydrates, fats, and proteins each supply the body with the kinds of energy needed for its maintenance and for its work, approximately in proportion to their fuel values as calculated above. We do not now believe that any one nutrient is used to the exclusion of others as a source of energy for any particular function, nor indeed that the body makes any particular distinction between the foodstuffs as sources of energy. The fuel value of the diet as a whole is utilized to meet the energy requirements of the whole body. For the present, therefore, it is the fuel value of the day's THE FUEL VALUE OF FOOD 149 dietary which we have to consider rather than the distribution of this as regards protein, fats, and carbohydrates. The total food (or energy) requirement is best expressed in Calories per day, either for the whole body or per kilogram of body weight, and for convenience of discussion it is usually assumed that the average body weight (without clothing) is for men 70 kilograms (154 pounds) and for women eight tenths as much, 56 kilograms (123 pounds). There are four important methods of studying the food requirements of man : * 1. By observing the amount of food consumed (dietary studies). 2. By observing the amount of oxygen consumed — pref- erably also the respiratory quotient (respiration experiments). 3. By determining the balance of intake and output (car- bon and nitrogen metabolism experiments). 4. By direct measurement of heat given off by the body (calorimeter experiments). Dietary studies. — Most dietary studies give Httle more than a general indication of the food habits of the people studied ; but in cases where persons have maintained for a long time the same dietary habits and other conditions of life, and the body weight has remained practically constant, it may be fairly safe to assume that the food has furnished just about the right amount of energy for the maintenance of the body under the observed conditions. Great care must be taken in drawing inferences from the body weight because of the readiness with which the body gains or loses moisture. Athletes often lose 2 or 3 pounds in an hour of vigorous exercise and regain it in less than a day. Gain or loss of body weight during short periods, therefore, * For an account of the historical development of the principles which underUe the measuremetit of metabolism, see the introductory chapter of Lusk's Elements oj the ScietKe oj Nutrition. 150 CHEMISTRY OK FOOD AND NUTRITION does not by any means necessarily imply a corresponding gain or loss of fat. The body may lose fat and at the same time maintain its weight through gaining water, or vice versa. When, however, the weight remains nearly the same for months at a time, it may usually be assumed that there is no impor- tant gain or loss of tissue and that the body is receiving just about the proper amount of total food for its needs. Under these conditions an accurate observation of the food consumed may give valuable indications as to the actual food requirement. Of such dietary studies perhaps the most useful individual ex- ample is that of Neumann, who reduced his diet to what ap- peared to be just about sufficient for his needs and then recorded all food and drink taken during a period of 10 months in which the body weight remained nearly constant. The average daily food furnished : * Nutrients Factors Calories Total Calories per Day Protein 66.1 grams X 4. = 264.4 Fat 83.5 grams X 9. = 751.5^2242 Carbohydrate t . ■ • 306.5 grams X 4. = 1226.0 The 2242 Calories per day were evidently fully sufficient to meet the energy requirements of this man, whose weight was 66.5 to 67 kilograms (about 147 pounds) and who was en- gaged at his usual (mainly sedentary) professional work in the Hygienic Institute at Kiel. Later, when his weight had increased to 71.5 kilograms (157 pounds) as the result of following for a time a more liberal diet (furnishing about 2600 Calories per day), he again observed his dietary while taking what was supposed to be an amount of food sufficient for the maintenance of the body and no more. This second dietary study was continued for 8 months, during which the average daily food consumption was found to be : * The data are taken from Chittenden's Nutrition of Man, page 2cS6. t Including some alcohol (taken in the form of beer), which is estimated as equivalent in fuel value to 1.75 times its weight of carbohydrates. THE FUEL VALUE OF FOOD 151 Nutrients Factors Calories '^%\\%^°'''^^ Protein 76.2 grams X 4- = 304-8 Fat 109.0 grains X 9. = 981.0 Carbohydrates* . . . 178.6 grams X 4. = 714.4 The body weight remained nearly constant. These results indicate that this subject, a man of average size, living a normal professional life involving no manual labor in the ordinary sense, but not excluding such muscular movements as are naturally incidental to a sedentary occupa- tion, found his energy requirements satisfied with food furnish- ing 2000 to 2250 Calories per day. Respiration experiments. — Since the foodstuffs yield their energy through being oxidized in the body, it is evident that a measure of the energy metabolism can be obtained by finding either the amount of foodstuffs oxidized or the amount of oxy- gen which is consumed in the process. The apparatus devised and used by Zuntz for this purpose provides a mask, fitting air- tight over the mouth and nose and connected by means of valved pipes with apparatus for measuring and analyzing the inspired and expired air. In this way one can determine the volume of oxygen entering, and the volume leaving, the lungs. The difference is the volume consumed in the body. Benedict has devised an improved form of respiration ap- paratus in which the subject breathes, either through a mouth- er nose-piece, from a current of air which is purified and kept in circulation in the same manner as that of the respiration calorimeter chamber described below. The carbon dioxide which the man produces is absorbed quantitatively and the oxygen which he consumes is exactly replaced by admitting measured volumes of analyzed oxygen gas from a cylinder of compressed oxygen, * Including some alcohol (taken in the form of beer), which is estimated as equivalent in fuel value to 1.75 times its weight of carbohydrates. 152 CHEMISTRY OF FOOD AND NUTRITION A given volume of oxygen used in the body may liberate somewhat different amounts of heat, according as it oxidizes fat, carbohydrate, or protein. For accurate estimations of the energy liberated it is therefore necessary to know the kind Spirometer Lungs 0. Pump Absorbed Absorbed Fig. 7. Diagram of Benedict respiration apparatus. Benedict. Courtesy of Dr. F. G. of material oxidized, as well as the amount of oxygen con- sumed. This is calculated from the respiratory quotient. Since the amount of protein broken down in the body can be estimated from the nitrogen excretion, the determination of the respiratory quotient along with the o.xygen consumption shows the extent of the combustion in the body and the pro- THE FUEL VALUE OF FOOD 153 portions of fat and carbohydrate burned.* From these data the energy can be calculated. As a matter of fact it is not necessary to go through the actual calculation of the amounts of fat and carbohydrate burned since the energy derived from a Hter of oxygen when used to burn carbohydrate and fat in different proportions can be calculated once for all and expressed in relation to the respiratory quotient as shown in the accompanying table. Energy Values of Oxygen and Carbon Dioxide at Different Respiratory Quotients (Zuntz and Schumberg) Respiratory Quotient Calories PER Liter of Oxygen Calories PER Liter of Carbon Dioxide Calories PER Gram of Carbon Dioxide 0.70 4.686 6.694 3.408 0.71 4.690 6.606 3-363 0.72 4.702 6.531 3.325 0.73 4.714 6.458 3.288 0.74 4.727 6.388 3.252 0.7S 4-739 6.319 3.217 0.76 4-752 6-253 3.183 0.77 4.764 6.187 3.150 0.78 - 4.776 6.123 3.117 0.79 4.789 6.062 3.086 0.80 4.801 6.001 3.055 0.81 4.813 S-942 3.025 0.82 4.82s 5.884 2.996 0.83 4-838 5.829 2.967 0.84 4.850 S-774 2-939 0.85 4.863 S-72I 2.912 0.86 4.87s 5.669 2.886 0.87 4-887 5-617 2.860 0.88 4.900 S.568 2.835 0.89 4.912 5.519 2.810 * Or, with very little error, it may be assumed that 15 per cent of the oxygen goes to bum protein and the rest is divided between fat and carbohydrate. The values given in the table herewith agree with this assumption. Attention should be called to the fact that estimates of energy metaboUsm based on carbon dioxide production alone involve larger errors than those based on oxygen consumption alone. 154 CHEMISTRY OF FOOD AND NUTRITION Energy Values of Oxygen and Carbon Dioxide at Different Respiratory Quotients (Zuntz and Schumberg) (Continued) Respiratory Quotient Calories Calories Calories PER Liter of PER Liter of PER Gram of Oxygen Carbon Dioxide Carbon Dioxide 0.90 4.924 5-471 2.78s 0.91 4-936 5-424 2.761 0.92 4.948 5-3/8 2-738 0.93 4.960 5-333 2-715 0.94 4-973 5.290 2.693 0.95 4-985 5-247 2.671 0.96 4-997 5-205 2.650 0.97 5.010 5-165 2.629 0.98 5.022 5.124 2.609 0.99 5 -034 5-085 2.589 1. 00 5-047 S-047 2.569 It is then only necessary to determine the respiratory quotient and the volume of oxygen used in order to know the number of Calories of energy metabolized. This is sometimes called the method of indirect calorimetry. This method of studying the total metabolism permits of experiments being carried out very quickly, and is therefore especially useful for the direct investigation of conditions which affect metabolism promptly, such as muscular work or the eating of food. The periods of observation cannot be very long, but the probable results for the 24 hours' metaboUsm can be estimated by the data obtained during frequent short periods at different times of the day and night. For a critical com- parison of this method with the Pettenkofer and Voit method of studying metabolism by the determination of the carbon balance, the reader is referred to the discussion by Magnus- Levy in Von Noorden's Metabolism and Practical Medicine, Vol. I, pages 186-198. From the results of many observations by the Zuntz method Magnus-Levy estimates the minimum metabolism of a man of average size kept absolutely motionless and fasting at 1625 THE FUEL VALUE OF FOOD 155 Calories per day. Food barely sufficient for maintenance would increase this by 175, and such incidental muscular movements as would ordinarily be made by a man at rest in bed would in- volve another 200, making a total of 20CK) Calories as the esti- mated food requirement of a man at rest with a maintenance diet. Magnus-Levy further estimates that the man, if doing no work (in the ordinary sense), but allowed to move about the room in- stead of remaining in bed, would require 2230 Calories per day. Carbon and nitrogen balance experiments. — From a com- parison of the constituents of the food consumed (" intake ") and of the substances eliminated from the body (" output "), the material actually oxidized and the energy liberated in the oxidation may be determined. The intake is found by weighing and analyzing all food eaten; the output by collecting and determining the end products eliminated through the lungs, the kidneys, the intes- tines, and sometimes (in very exact experiments) the skin. The time unit in experiments upon the intake and output is almost always 24 hours, the experimental day beginning preferably just before breakfast. The feces belonging to the experimental days are marked, usually by giving a small amount of lampblack with the food as in ordinary digestion experiments, separated and analyzed. The end products given off by the lungs and kidneys during an experimental day are taken as measuring the material broken down in the body during the same period. Some time is of course required for the elimination of the nitrogenous end products through the kidneys. This un- avoidable " lag " in the eHmination of nitrogen may intro- duce an error in determining the nitrogen balance unless the subject has been kept for a few days in advance upon the same diet which is to be used in the experiment. Assuming that the total nitrogen and carbon of the ab- sorbed food existed in the form of protein, fat, and carbo- hydrate, and that the amount of carbohydrates in the body is iS6 CHEMISTRY OF FOOD AND NUTRITION constant from day to day, it is only necessary to determine the carbon dioxide of the expired air and the carbon and nitrogen of the waste products, in order to calculate the amounts of material oxidized and of energy liberated in the body. Ex- periments of this sort have played a most important part in the development of our knowledge of nutrition. The cal- culations are usually based on the following average analyses of protein and body fat: Fat Carbon Nitrogen Hydrogen Oxygen . Sulphur 76.5 12 "•5 The following data were obtained with a man on ordinary mixed diet : Calculation of Energy Metabolism from Carbon and Nitrogen Balance. Max of 64 Kilograjis at Rest in Atwater Respiration Apparatus, Grams per Day Intake Protein Fat Carbo- hydrate Nitrogen Carbon Total in food . . . Lost in digestion Absorbed .... 94.4 5-4 89.0 82.5 3-7 78.8 289.8 3-2 286.6 151 0.9 14.2 16.2 16.2 — 2.0 239.0 7-4 231.6 Output Bj' lungs 207.3 12.2 B}'' kidneys Metabolized 219-5 + 12. 1 Balance THE FUEL VALUE OF FOOD 157 A loss of 2.0 grams body nitrogen indicates (2.0 X 6.25 =) 12.5 grams body protein burned. Also there were 89.0 grams absorbed from food, and, therefore, in all 101.5 grams total protein burned. Since the respiratory quotient showed that the body was in carbohydrate equilibrium at the beginning and end of each experimental day, i.e. at seven o'clock each morning, it may be concluded that the amount of carbohydrate burned was the same as that absorbed from the food, viz. 286.6 grams per day. From the carbon balance, therefore, we estimate the amount of fat burned as follows : 12.5 grams body protein yield (12.5 X 53 per cent = ) 6.6 grams carbon and there were in the absorbed food . . . . 231.6 grams carbon .'. total available was 238.2 grams carbon But total catabolized was only 219.5 grams carbon .'. the body stored in the form of fat 18.7 grams carbon Since fat contains 76.5 per cent carbon, i gram carbon =0= 1.307 grams fat. .*. 18.7 grams carbon = 24.4 grams fat. The body therefore absorbed 78.8 grams fat stored 24.4 grams fat burned 54.4 grams fat In all the body burned per day 101.5 grams protein, yielding (101.5 X 4.35 * = ) 442 Calories 54.4 grams fat, yielding (54-4 X 9.45 * = ) 515 Calories 286.6 grams carbohydrate, yield- ing (286.6 X 4.1 * = ) 1 1 75 Calories 2132 Calories By means of the carbon and nitrogen balance Sonden and Tigerstedt studied the energy metaboHsm of eight resting men between nineteen and forty-four years of age, with results which varied for the different subjects from 1853 to 2292 Calories * Here the factors for fuel value are not reduced to allow for loss in digestion, because this loss has already been deducted in computing the amount of each nutrient actually absorbed and rendered available. 158 CHEMISTRY OF FOOD AND NUTRITION per day. Many other experimenters have used the same method with similar results. Calorimeter experiments. — The most direct, and in some respects most convincing, way of ascertaining the energy me- tabolism is by the method oj direct calorinietry. This consists in measuring the total energy expenditure of the body as heat or as heat and mechanical work by confining the subject in a chamber permitting of actual measurement of the heat produced. It was not until the development of the Atwater-Rosa- Benedict respiration calorimeter that complete and satisfactory data covering periods of one to several days were obtained. This apparatus consisted of an air-tight copper chamber, sur- rounded by zinc and wooden walls with air-spaces between, and was large enough for a man to live in without discomfort, being about 7 feet long, 4 feet wide, and 6^ feet high. An opening in the front of the apparatus, which was sealed during an experiment, serves as both door and window and admits suf- ficient light for reading and writing. A smaller opening, having tightly fitting caps on both ends, was used for passing food, drink, excreta, etc., into and out of the chamber. The chamber was furnished with a folding bed, chair, and table, and was ventilated by means of a current of air which passed usually at the rate of about 2^ cubic feet per minute. At first this venti- lating air current was maintained and measured by means of a specially constructed meter pump which also automatically took samples of the air for analysis. Later the apparatus was so modified as to make use of the same air throughout an experiment, the carbon dioxide and water given off by the sub- ject being removed by circulating the air through purif>ang vessels, and the oxygen which the subject uses being replaced by adding weighed amounts of oxygen to the air current as required.* By this means it is possible to carry out, in the * Figure 8 indicates diagrammatically the ventilating system as applied in one of the later forms of apparatus. THE FUEL VALUE OF FOOD 159 ^ \ \ \ \ O TENSION EQUALIZER 2J' 0, INTRODUCED" yy vv r^ H2O ABSORBED H,S 0, CO, ABSORBED POTASH LIME H^O ABSORBED H2S O4 BLOWER Fig. 8. — Diagram of ventilation of respiration calorimeter. The air is taken out at lower right-hand corner and forced b> the blower through the apparatus for absorbing water and carbon dioxide. It returns to the calorimeter at the top. O.xj-gen can be introduced into the chamber itself as need is shown by the tension equalizer. Courtesy of Dr. F. G. Benedict and the Carnegie Institution of Washington. l6o CHEMISTRY OF FOOD AND NUTRITION calorimeter, metabolism experiments in which the oxygen and hydrogen as well as the carbon and nitrogen balances are determined, and from these .data the gain or loss of carbohy- drate as well as of protein and fat can be determined. The ventilating air current is so regulated that it enters and leaves the calorimeter at the same temperature; and between the copper and zinc walls are placed a large number of thermo-electric junctions connected with a deHcate gal- vanometer by means of which each wall is tested every four minutes, day and night, during the progress of an experiment, and the minute amounts of heat which may pass to or from the calorimeter through its walls are quickly detected and made to balance each other. Thus there is no gain or loss of heat either through the walls of the chamber or by the venti- lating air current, and the heat given off by the subject can leave only by the means especially provided for carrying it out and measuring it. A part of the heat Uberated is carried from the chamber in latent form by the water vapor in the outgoing air, which is accurately determined. The rest of the heat is brought away by means of a current of cold water circulating through a copper pipe coiled near the ceiling of the chamber. The quantity of water which passes through the pipe and the difference between the temperature at which it enters and that at which it leaves the coil are carefully determined and show how much heat is thus brought out of the chamber. In recent years several different calorimeters, based on the principles of the apparatus just described but adapted in size and shape to different types of experimentation, have come into use. Notable among these are the " chair " and the " bed " calorimeters, which are so constructed as to accommodate a subject in the sitting or reclining position in comfort but in a minimum of space; for only by making the calorimeter chamber small is it practicable to obtain a high degree of THE FUEL VALUE OF FOOD i6i 240 Calories Industrial painting] "Active exercise" (bicycle ergometer) 290 Calories Walking actively (about 3! miles per hour) 300 Calories Stoneworking 4°° Calories "Severe exercise" (bicycle ergometer) 450 Calories Sawing wood 480 Calories Running (about 55 miles per hour) 500 Calories "Very severe e.xercise" (bicycle ergometer) 600 Calories By the use of these estimates the probable food requirement for a person of 70 kilograms (154 pounds) may be calculated very simply, as, for instance, in the following example : 8 hours of sleep at 65 Calories = 520 Calories 2 hours' Hght exercise * at 1 70 Calories = 340 Calories 8 hours' carpenter work at 240 Calories = 1920 Calories 6 hours' sitting at rest at 100 Calories = 600 Calories Total food requirement for the day, 3380 Calories Tigerstedt, in his Textbook of Physiology, gives estimates of food requirements for different degrees of activity as indicated by means of typical occupations, which may be useful in check- ing results calculated as above. * Going to and from work, for example. CONDITIONS GOVERNING ENERGY METABOLISM 187 According to Tigerstedt : 2000-2400 Calories per day suffice for a shoemaker. 2400-2700 Calories per day suffice for a weaver. 2700-3200 Calories per day suffice for a carpenter or mason. 3200-4100 Calories per day suffice for a farm laborer. 4100-5000 Calories per day suffice for an excavator. Over 5000 Calories per day are required by a lumberman. Lusk gives the following summary of energy requirements of women at work at typical occupations as investigated by Becker and Hamalainen in Finland: A seamstress sewing with a needle required 1800 Calories. Two seamstresses, using a sewing machine, required 1900 and 2100 Calories, respectively. Two bookbinders required 1900 and 2100 Calories. Two household servants, employed in such occupations as cleaning windows and floors, scouring knives, forks, and spoons, scouring copper and iron pots, required 2300 to 2900 Calories. Two washerwomen, the same servants as the last named, required 2600 and 3400 Calories in the fulfillment of their daily work. Benedict and Cathcart find that when muscular work is severe there is a rise in the respiratory quotient, the rise being greater the more severe the work. In such cases the respiratory quotient is found to fall during the rest period following the work, and usually to a lower figure than that observed before the work was begun. They interpret this to mean that hard muscular work draws upon the stored carbohydrate of the body in sHghtly greater proportion than upon the stored fat. That the work is performed at the expense of both carbohydrate and fat is shown by Benedict and Cathcart's data as well as by those of many previous experiments. Apparently it is only severe muscular activity which has any appreciable influence 1 88 CHEMISTRY OF FOOD AND NUTRITION upon the relative proportions of fat and carbohydrate burned. In the experiments cited on page i8i, for example, the respira- tory quotient was not changed by walking either on a hori- zontal surface or up an inclined plane. It should also be noted that Benedict and Cathcart found the same mechanical effi- ciency in work whether preceded by a carbohydrate-rich or a carbohydrate-poor diet. Influence of Food upon Energy Metabolism Atwater and Benedict determined directly by means of the respiration calorimeter the heat production of the same man during five fasting experiments of one to two days each, and during a four-day experiment with food about sufficient for maintenance. The average total metabolism on the fasting days was about 9 per cent lower than on the days when food was taken. In longer fasts there may be a somewhat greater decrease in heat production. Thus, Benedict found that a man who weighed at the start 59.5 kilograms (131 pounds) metabolized, on the successive days of a seven-day fast, 1765, 1768, 1797, 1775, 1649, 1553, and 1568 Calories respectively. Naturally in long fasts factors other than the simple sparing of the direct effect of food come into play.* Tigerstedt studied by means of the carbon and nitrogen bal- ance the metaboUsm of a man who fasted for five days and for the next two days took a very liberal diet. The following data were obtained: * For a detailed account of the results obtained in a fasting experiment of 31 days' duration, see Benedict, .1 Study of Prolonged. Fasting, Publication No. 203 of the Carnegie Institution of Washington. CONDITIONS GOVERNING ENERGY METABOLISM 1 89 ist fast day 2d fast day 3d fast day 4th fast day 5th fast day Fed 4141 Calorics . . . Fed 4 14 1 Calories (2d day) Body Weight Kilos 67.0 65-7 64.9 64.0 63.1 64.0 65.6 Calculated Total Metabolism Calories 2220 2102 2024 1992 1970 2437 2410 Calories PER Kilo 33-2 * 32.0 * 31-2 3I-I 31-2 38.1 36.8 These results show for man (as had previously been shown with dogs) that in fasting the total metabolism continues at a fairly constant rate in spite of the fact that the energy is ob- tained entirely at the expense of body material. In this case, the diet given at the end of the fasting period (4141 Calories) was approximately double what would have been required for maintenance, but the increase in energy metaboHsm was only 22.5 per cent over that of fasting. The results of fasting experiments thus make it evident that the body has but Httle power in the direction of adjusting its energy metaboUsm to the energy value of its food supply. Rubner found that each type of food exerted a more or less specific influence upon the energy metabolism, so that when the foodstuffs were fed separately, somewhat different energy values were required for the maintenance of body equihbrium. Thus, if the total metabolism of a dog fasting at 33° C. be represented by 100 Calories, he must be fed, in order to prevent loss of body substance, about 106.5 Calories of sugar, or 114.5 Calories of fat, or 140 Calories of protein. A man observed by Rubner metabolized in fasting 2042 Calories ; when fed 2450 Calories in the form of sugar alone, he metabolized 2087 Calories; when * These figures are slightly too high because the loss of carbon on these days was due in part to combustion of glycogen, but is calculated as if due simply to pro- tein and fat. IQO CIIKMISTRY OF FOOD AXD XL'TRITKJN fed 2450 Calories in the form of meal alone, he metabolized 2566 Calories. Recently Lusk and his coworkers have investigated the in- fluence of the foodstuffs upon metabolism (" specific dynamic action ") very extensively and have developed the subject to such an extent that for an adequate discussion of their results the original articles ' or Lusk's own summary - should be con- sulted. It appears from this work that when the digestion products of carbohydrate or fat are carried by the blood to the tissues the energy metabolism (rate of oxidation) rises simply because of the increased concentration of oxidizable material ; but that some of the products of the digestion and intermediary metabolism of protein increase metabolism not only to a greater extent, but also in a somewhat different manner, since they seem to act as stimuli rather than merely as fuel. On an ordinary mixed diet, however, this apparent loss of energy due to eating of protein is not a very large factor in the total metabolism, since the total specific dynamic action makes the metabolism of energy for the day only about one tenth higher on a full maintenance ration than when no food is eaten. Benedict and Roth have studied the energy metabolism of vegetarians as compared with non-vegetarians of the same height and weight in order to determine whether or not the former maintain a lower plane of basal metabolism than do people who eat meat and who are sometimes held to be unduly stimulated by the protein of their food. The energy metabolism was computed from the carbon dioxide production and oxygen consumption determined when the subjects were at complete rest and in the " post-absorptive condition," i.e. at least 12 hours after the last meal, the immediate specific dynamic action of the food being thus practically excluded. Under these con- • Lusk. Journal of Biological Chemistry, Vol. 20, pages vii-.xvii and 555-617. Murlin and Lusk, Ibid., Vol. 22. pages 15-2Q. 2 Lusk. Science of Nutrition, Third Edition, Chapter X'lL CONDITIONS GOVERNING ENERGY METABOLISM 191 ditions the vegetarian men and women showed average basal metaboHsm of 1.06 and 1.025 Calories per kilogram of body weight per hour respectively, while the corresponding data for non- vegetarian men and women were i.io and 1.04 Calories respectively. Benedict holds that these differences are too small to establish any essential difference in the basal energy metabolism of vegetarians and non-vegetarians of like height and weight. It is sometimes thought that superior preparation or very thorough mastication of food results in such improvement in its utihzation that a material saving may be effected in the amount of food required. But it will be remembered that under average conditions only about 5 per cent of the energy value of the food is lost in digestion or expended upon the di- gestive process. Any improvement in those conditions through superior preparation or mastication of the food can therefore at most effect a saving of less than five per cent of the energy value. Thus the influence upon total food requirement is scarcely appreciable. The advantages of good preparation and thorough chewing of the food are very important, but they lie in other directions than reduction in the amount of food required. Recent scientific evidence supports the view that chronic undernutrition or even simple restriction of food consumption in health, if continued sufficiently, may bring the organism to a lower level of energy metabolism than would be indicated by the weight or surface. Regulation of Body Temperature Climate, season, housing, clothing, are all factors which may influence energy metabolism through their bearing upon the regulation of body temperature.* It is evident that the main- * For full discussion of the influence of surrounding temperature upon metabolism and the relation of metabolism to the regulation of body temperature the reader is referred to Lusk's Science of Nutrition. 192 CHEMISTRY OF FOOD AND NUTRITION tcnance of the body at a temperature above that of its or- dinary environment involves a continual output of heat. This output of heat may be regulated in either of two ways: (i) By variations in the quantity of blood brought to the skin, which tend to control the loss of heat by radiation, conduction, and sweating; this is called " physical regulation." (2) By an in- crease in the rate of oxidation in the body in response to the stimulus of external cold ; such a change in the rate of oxidation is known as " chemical regulation." The extra heat production which follows the taking of food (the specific dynamic action of the foodstuffs) may take the place of the " chemical regu- lation " and so help to protect the body from the necessity of burning material simply for the maintenance of its temperature. Muscular work, by increasing the production of heat in the body, may also render chemical regulation unnecessary ; but ap- parently the specific dynamic action does not furnish energy which can be utilized for muscular work.^ The presence of a layer of adipose tissue under the skin as well as the custom of covering the greater part of the external surface with clothing also tends to keep down the loss of heat to the point where " physical regulation " will suffice. Lusk cites experiments by Rubner upon a man whose metabolism was determined when kept in the same cold room but with dif- ferent amounts of clothing, and observes that when the man was sufficiently clothed to be comfortable the " chemical regu- lation " was eliminated (Science of Nutrition, 3d edition, page 149). In general it seems probable that people warmly clothed and living in houses which are heated in winter are not called upon to exercise " chemical regulation " to any considerable extent; in other words, they probably do not burn any considerable amount of material merely for the production of heat, the heat required for the maintenance of body temperature being ob- 1 See Lusk's Science of Nutrition, 3d edition, pages 311-313. CONDITIONS GOVERNING ENERGY METABOLISM 193 tained in connection with the metabolism which is essential to the maintenance of the muscular tension and the various other forms of internal work. If, however, the body be exposed to cold, it may be forced to employ " chemical regulation " with a resulting increase of the food requirement, and this will occur more readily in a thin person than in one who is well protected by subcutaneous fat. The extra heat required in cold weather is probably obtained for the most part through the activities of the muscles. It is a matter of general experience that one instinctively exercises the muscles more vigorously in cold weather than in warm, and if one attempts to endure much cold without muscular exer- cise there results shivering — a peculiar involuntary form of muscular activity whose function appears to be to increase heat production through increasing the internal work of the body. To a large extent, therefore, the regulation of body tempera- ture, in case of exposure to cold, is accomplished through the activity and tension of the muscles. The foregoing discussion has reference primarily to adults. In the case of the infant whose surface is much greater in pro- portion to his weight and whose muscular tone is not yet fully developed, the loss of heat to the surroundings is not so readily checked by " physical " nor so easily made good by " chemical " regulation. Unless the infant is either warmly clothed or sup- plied with an artificial source of heat in cold weather he may be forced to burn, for warmth, material that might better be em- ployed for growth. The Influence of Age and Growth From the fact that in animals of the same species, but of different size the heat production is proportional to the surface rather than to the weight, it would follow that children must have a greater food requirement per unit of weight than adults. o 194 CHEMISTRY OF FOOD AND NUTRITION In a child 2 years old weighing 25 pounds the energy metabolism is approximately half as great as in an adult of six times this weight, i.e. the energy expenditure per unit of weight is three times as great for the young child as for the resting man, and while for the man the expenditure may be taken as a measure of the requirement, in the case of the child an additional allowance must be made to provide the material retained in the body for growth. In studies of infants 7 to 9 months old, Rubner and Heubner found a storage of 12.2 per cent of the energy value of the food consumed, and Camerer found a storage of 15 per cent of the energy and 40 per cent of the protein of the diet. The following data from Tigerstedt illustrate the relative intensity of metaboUsm at dififerent ages : Weight Kgm. Metabolism per Day Subject Total Calories Per Kgm. Calories Per Square Meter Calories Child, 2 weeks . . . Child, 10 weeks . . . Child, 10 years . . . Man at rest .... 3-2 23.2 70.0 258 420 1462 2240 81.0 84.0 63.0 32.0 1000 1200 1499 1071 According to these observations the metabolism per unit of weight is greatest in infancy and declines steadily with increas- ing size ; but calculated per unit of surface it is distinctly less in infancy than in children of 10 years, probably because the infant sleeps a greater proportion of the time and the tension (tonus) of its muscles is not yet fully developed. As between children and adults the energy metabolism is more nearly proportional to the surface than to the weight ; but among children of about the same age the energy require- ment may be computed on the basis of weight about as well as on th2 basis of surface. CONDITIONS GOVERNING ENERGY METABOLISM 195 Murlin and Bailey estimate from their own observations, and the earUer ones of Benedict and Talbot, that the energy requirement of the newborn baby kept comfortably warm and sleeping quietly may be placed tentatively between 1.7 and 2.0 Calories per kilogram per hour, the lower figure for a very fat (10 lb.) child and the higher for a thin (6 lb.) child. Accord- ing to these authors even vigorous crying does not raise this figure more than 40 per cent. Benedict and Talbot in their later pubhcation ^ give measurements of minimum heat pro- duction of 94 newborn infants (2 hours to 6 days old) which range from 1.33 to 2.17 Calories, averaging 1.75 Calories per kilogram per hour. " Maximum " energy metaboHsm, chiefly due to vigorous crying, was also observed in 93 of these cases and found to average 65 per cent above the resting value, while in several instances (10 out of 93) " crying and extreme rest- lessness " resulted in energy expenditure more than double that of the same infant at rest. With the development of the musculature and of muscular tonus, the energy expenditure of the normal infant increases for a time even more rapidly than his body weight, so that at from 2 nionths to i year of age the expenditure of energy while sleeping averages 2.7 Calories per kilogram per hour (average of Howland's, Benedict and Talbot's, and Murlin and Hoobler's data as summarized by the latter). During the waking hours the rate of expenditure is of course materially higher, and in calculating food requirements allowance must be made for growth and for the possibility of losses through imperfect utihzation of the food. In order to provide adequately for all contingencies and support the rapid growth which is normal at this age, it is estimated that a vigorous child will require during the greater part of the first year about 100 Calories of food per kilogram of his body weight per day. But in cases 1 Physiology oj the New Born Infant, Publication No. 233, Carnegie Institution of Washington, 1915. 196 CHEMISTRY OF FOOD AND NUTRITION of artificial feeding, since the digestive tract must be gradually educated to handle the milk of a different species, it will often be necessary to feed much less than 100 Calories per kilogram per day at first, perhaps for several months, and only very gradually increase the food allowance. From the end of the first year until growth is completed the food requirement increases, but not so rapidly as does the body weight, so that while the allowance of food becomes larger per day it becomes smaller per kilogram. On the latter basis the energy requirement at the different ages may be estimated approximately as follows : Under i year 100 Calories per kilogram (45 Calories per lb.) 1- 2 years 100-90 Calories per kilogram (45-40 Calories per lb.) 2- 5 years 90-80 Calories per kilogram (40-36 Calories per lb.) 6- 9 years 80-70 Calories per kilogram (36-32 Calories per lb.) 10-13 years 75-60 Calories per kilogram (34-27 Calories per lb.) 14-17 years 65-50 Calories per kilogram (30-22 Calories per lb.) 18-25 years 55-40 Calories per kilogram (25-18 Calories per lb.) Children who are very active or growing very rapidly may require even more food than the table just given suggests. Such cases are perhaps most frequently found among boys between 10 and 15 years of age. DuBois finds in boys 12 and 13 years old an average basal metabolism (complete rest and almost complete fasting) of 1.76 Calories per kilogram per hour, or about 75 percent above that of healthy adults.* Assuming average activity for boys of this age the energy expenditure during 24 hours would probably amount to 60 to 70 Calories per kilogram and as this is a period of rapid growth the require- ment would be materially higher than the rate of expenditure. Assuming average size at the different ages the allowances in Calories per day become about as follows : f * Per unit of surface the basal energy metabolism of these boys was about 25 per cent higher than that of healthy men. t See also the more detailed table of energy allowances for children in Chapter XIV. CONDITIONS GOVERNING ENERGY METABOLISM 197 Children of i- 2 years inclusive Children of 2- 5 years inclusive Children of 6- 9 years inclusive Girls of 10-13 years inclusive Boys of 10-13 years inclusive Girls of 14-17 years inclusive Boys of 14-17 years inclusive 1000-1200 Calories per day 1 200- 1 500 Calories per day 1400-2000 Calories per day 1800-2400 Calories per day 2300-3000 Calories per day 2200-2600 Calories per day 2800-4000 Calories per day In estimating the food requirement of a family it is usually preferable to consider each child's energy requirement directly rather than to count the children as equivalent to fractions of the hypothetical '' average man." Above the age of 17 years, although there is still some growth, differences in activity due to occupation become so great that the food requirement will usually depend as much upon occupation as upon age. The fuel value of children's dietaries should always be liberal in order to provide amply for muscular activity and for a more intense general metabolism than that of the adult. Furthermore, throughout the period of growth the food must supply a certain amount of material to be added to the body in the form of new tissue in addition to all that which is oxidized to support metabolism. Age Height Weight Food Requirement WITHOUT Muscular Labor Years Meters Feet and inches Kilos Lbs. Total per day Calories Per. Kgm. per day Calories I 5 10 15 20 30 40 60 70 80 0.70 1. 00 1.28 1. 71 1.72 1. 71 2:3 4:2 5:7 + 5:8- 5:7 + 10 17 26 50 65 69 70 68 65 63 22 37 57 no 143 152 154 150 143 139 1000 1400 1800 2800 3000 2750 2500 2300 2000 1750 100 82 70 56 46 40 36 34 31 28 198 CHEMISTRY OF FOOD AND NUTRITION With the elderly, on the other hand, the intensity of metab- oHsm is diminished and the body not only needs less food, but has less ability to deal with excess, so that the food re- quirement gradually declines and may become 10 or 20 per cent, or possibly even 30 per cent, lower than in middle life. In the table on page 197 are given the estimated height, weight, and food requirement of an average man at different ages, the figures for height and weight being based upon the data given by Hill for males of the Anglo-Saxon and Teutonic races {Recent Advances in Physiology and Biochemistry, page 284). These estimates of food requirements are intended to repre- sent approximate averages of available data and to allow for such exercise as would naturally be taken at the age, exclusive of anything which would ordinarily be considered physical labor. They thus illustrate in an approximate way the rate at which the amount of food required for healthy maintenance per unit of body weight declines from infancy to old age. DuBois has recently published in graphic form his estimates of the basal energy metabolism per unit of body surface at different ages. The graph is reproduced by Lusk {Science of Nutrition, 3d edition, page 128). The average basal metaboHsm, per unit of surface, found by DuBois in boys of 12 to 13, in men, and in women was as follows : Average Basal Metabolism of Boys, Men, and Women (DuBois) Age in Years Calorfes per Hour per Square Meter Subjects Computed accord- ing to Meeh's formula Computed by Dubois height- weight formula Boys Men Women Men Women Men 12-13 20-50 20-50 50-60 50-60 77-83 45-7 34-7 32.3 30.8 28.7 49-9 39-7 36.9 35-2 32.7 35-1 CONDITIONS GOVERNING ENERGY METABOLISM 199 Influence of sex. — Whether sex shall be said to influence the energy requirement will depend upon our use of terms. Boys spend on the average more energy than girls, and men more than women, but it is doubtful if the differences are due to other causes than have been considered above. In experi- ments in which children were allowed to move about in a small respiration room, boys were found to expend decidedly more energy than girls of the same age and weight ; but this was probably due to the greater restlessness and muscular tension of the boys, for in another series in which both boys and girls were kept motionless and relaxed during the obser- vations the difference was not found. Benedict and Emmes found, as noted above, a slightly higher basal metabolism in men than in women of the same height and weight, but attribute this to a difference in the average composition of the body. While sex alone seems not to be a measurable factor in energy metabolism, the performance of the reproductive func- tions may make large demands upon the maternal organism. As weight increases during pregnancy energy metabolism increases in at least equal proportion. In the last two weeks of human pregnancy Murhn finds the energy metabolism per unit of weight about 4 per cent higher than for non-pregnant women. During lactation, when the entire nutritive require- ment of the nursing infant is being met through the mother, the energy needs of the latter are greatly increased. Pro- duction of milk involves an extra energy requirement much beyond the actual energy value of the milk secreted. While accurate determinations are not at hand, it seems safe to conclude that the nursing mother taking only moderate exercise may need as much food as a man at muscular work. Liberal feeding of the nursing mother {e.g. up to 2800 to 3000 Calories for a woman with moderate muscular exercise) is not only important for the conservation of her own bodily 200 CHEIMTSTRY OF FOOD AND NUTRITION resources but may prolong the period of lactation and thus be of great value to the child as well.^ REFERENCES Anderson and Lusk. The Interrelation between Diet and Body Condi- tion and the Energy Production during Mechanical Work. Journal of Biological Chemistry, Vol. 32, page 421 (1917). Armsby. Principles of Animal Nutrition, Chapters 6 and 11. Armsby and Fries. Influence of Standing or Lying upon the Metabolism of Cattle. American Journal of Physiology, Vol. 31, page 245 (1912). Atwater. Neue Versuche ueber Stoff- und Kraft-vvechsel. Ergebnisse der Physiologic, Vol. 3 (1904). Atwater, Benedict, et al. Respiration Calorimeter E.xperiments. Bul- letins 44, 63, 69, 109, 136, 175, Office of Experiment Stations, United States Department of Agriculture. AuB AND DuBois. The Basal MetaboUsm of Old Men. Archives of Internal Medicine, Vol. 19, page 823 (191 7). Bailey and Murlin. The Energy Requirement of the Newborn. Amer- ican Journal of Obstetrics, Vol. 71, page 526 (1915). Becker et al. Energy Metabolism during Different Kinds of Work. Skandinavisches Archiv der Physiologic, Vol. 31, (a series of papers) — (1914). Benedict. Metabolism during Fasting. Carnegie Institution of Wash- ington, Publication Nos. 77 and 203. Benedict. Factors Affecting Basal Metabolism. Journal of Biological Chemistry, Vol. 20, page 263 (191 5). Benedict and Carpenter. The Metabolism and Energy Transformations of Healthy Man during Rest. Carnegie Institution of Washington, Publication No. 126. Benedict and Carpenter. The Influence of Muscular and Menf-.l Work on Metabolism and the Efficiency of the Human Body as a Machine. Bulletin 208, Office of Experiment Stations, United States Department of Agriculture. Benedict antj Cathcart. Muscular Work : A Metabolism Study with Special Reference to the Efficiency of the Human Body as a Machine. Carnegie Institution of Washington, Publication No. 1S7. Benedict and Emmes. The Influence upon Metabolism of Non-oxidizable ' For general discussion of the problem of maintaining breast feeding, see papers by Sedgwick, Abt, and Hoobler in the Journal of the American Medical Association for Aug. II, 1917 (Vol. 69, pages 417-428). CONDITIONS GOVERNING ENERGY METABOLISM 20I Material in the Intestinal Tract. American Journal of Physiology, Vol. 30, page 197 (1912). Benedict and Emmes. A Comparison of the Basal Metabolism of Normal Men and Women. Journal of Biological Chemistry, Vol. 20, pages 253-262 (1915). Benedict ant) Roth. The Metabolism of Vegetarians as compared with Non-Vegetarians of Like Height and Weight. Journal of Biological Chemistry, Vol. 20, pages 231-241 (1915). Benedict and Smith. The Metabolism of Athletes. Journal of Biological Chemistry, Vol. 20, pages 243-251 (1915). Benedict and Talbot. Respiratory Exchange of Infants. American Journal of Diseases of Children, Vol. 8, pages 1-49 (1914). Carpenter. Increase in Metabolism during the Work of Typewriting. Journal of Biological Chemistry, Vol. 9, pages 231-266 (191 1). Carpenter and Murlin. Energy Metabolism of Mother and Child just before and just after Birth. Archives of Internal Medicine, Vol. 7, pages 184-222 (1911). DuBois. Respiration Calorimetry in Clinical Medicine. Harvey Lec- tures, 1915-1916. DuBois. The Basal Energy Requirement of Man. Journal of Washington Academy of Sciences, Vol. 6, page 347 (1916). DuBois. The Metabolism of Boys 12 and 13 Years Old as Compared with Metabolism at Other Ages. Archives of Internal Medicine, Vol. 17, page 887 (1916). DuBois AND Associates. (A Series of Articles on Metabolism in Disease.) Archives of Internal Medicine, Vols. 15 and 17 (1915, 191 6). (The reader may also find other papers of this series in volumes published subsequently to the compiling of this list.) DuBois ANT) DuBois. Measurement of the Surface .A.rea of Man. Ar- chives of Internal Medicine, Vol. 15, pages 868-881 (1915). DuBois AND DuBois. A Formula to Estimate the Approximate Surface Area if Height and Weight Be Known. Archives of Internal Medicine, Vol. 17, pages 863-871 (1916). Gephart ANT) DuBois. Basal Metabolism. Archives of Internal Medicine, Vol. 15, page 835; Vol. 17, page 902 (1915, 1916). Krogh. The Respiratory Exchange of Animals and Man. LowY ANT) ZuNTZ. Influence of War Diet upon the Metabolism. Berlin klinische Wochenschrift, Vol. 53, page 825 (191 6). LusK. Elements of the Science of Nutrition, 3d edition. LusK. The Influence of Food on Metabolism. Journal of Biological Chemistry, Vol. 20, pages vii-xvii and 555-617 (1915). 202 CHEMISTRY OF FOOD AND NUTRITION Mathews. Physiological Chemistry, Chapter XIII. Means. Basal Metabolism and Body Surface. Journal of Biological Chemistry, Vol. 21, pages 263-268 (1915). Means, Aub, and DuBois. The Effect of Caffeine on the Heat Production. Archives of Internal Medicine, Vol. 19, page 832 (191 7). MORGULis. The Influence of Underfeeding and of Subseciuent Abundant Feeding on the Basal Metabolism of the Dog. Biochemical Bulletin, Vol. 3, page 264 (1914). MuRLiN. A Respiration Incubator for the Study of the Energy Metabolism of Infants. American Journal of Diseases of Children, Vol. 9, pages 43-58 (January, 1915). MuRLiN AND HooBLER. The Energy Metabolism of Ten Hospital Chil- dren between the Ages of Two Months and One Year. American Journal of Diseases of Children, Vol. 9, pages 81-119 (February, 1915). Murlin and Lusk. The Influence of the Ingestion of Fat. Journal of Biological Chemistry, Vol. 22, page 15 (1915). SjosTROM. The Influence of the Temperature of the Surrounding .Vir on the Carbon Dioxide Output in Man. Skandinainsches Archiv der Physiologic, Vol. 30, pages 1-72 (1913). SoDERSTROM, Meyer, AND DuBois. A Comparison of the Metabolism of Men Flat in Bed and Sitting in a Steamer Chair. Archives of Internal Medicine, Vol. 17, page 872 (191 6). Talbot. Twenty-four-Hours Metabolism of Two Normal Infants with Special Reference to Total Energy Requirement. American Journal of Diseases of Children, Vol. 14, page 25 (1917). Tashiro. Carbon Dio.xide Production from Nerve Fibers when Resting and when Stimulated. American Journal of Physiology, Vol. 32, pages 107-145 (1913). See also: Proceedings of the National Academy of Sciences, Vol. i, page no (1915). VoN Noorden. Metabolism and Practical Medicine, Vol. i, pages 20S-282. ZuNTZ AND MoRGULis. Influence of Chronic Undernutrition on Metabohsm. Biochemische Zeitschrift, Vol. 55, pages 341-354 (1914). CHAPTER VIII FACTORS DETERMINING THE PROTEIN REQUIREMENT Animal cells under all conditions of life are constantly break- ing down proteins into simpler substances which the body elimi- nates. Since this breaking down or " catabolism " of protein does not stop either in fasting or under the most liberal feeding with fats and carbohydrates, it follows that there is always a need for protein whatever the supply of other food. Protein metabolism differs widely from energy metabolism in the conditions which determine its amount, for protein metab- olism is governed mainly by the kind and amount of food, and to only a slight extent if at all by muscular exercise ; whereas energy metabolism is governed mainly by the muscular exer- cise, and to only a relatively small extent by the food. By giving food rich in fats and carbohydrates but poor in protein, the protein metabolism of a healthy man can easily be brought to less than 50 grams per day, and then by changing to a diet rich in protein, it may be increased to 150 or even 200 grams per day ; i.e. the rate of protein metabolism can be increased 200 to 300 per cent in a few days by a change in diet alone, all other conditions remaining the same. Protein Metabolism in Fasting Since the diet has such a great influence upon the amount of protein metabolized, it might be expected that the basal protein metaboHsm could be observed best in fasting. But in fasting the energy metabolism of the body is only a little lower than 203 204 CHEMISTRY OF FOOD AND NUTRITION with food; the amount of combustion continues nearly the same although only body material is available; and since the body must consume so much of its own substance to obtain the energy needed, there is always a chance that in fasting some pro- tein may be burned simply as fuel. Accordingly the protein metabolism in fasting may be greater than that which repre- sents the needs of the body when properly fed, while on the other hand it may be abnormally low through the effort of the body to adjust itself to the abnormal condition. The amount of protein broken down in fasting is much in- fluenced (i) by the previous habit as regards protein consump- tion, and (2) by the metabohsm of stored glycogen and stored fat. The direct effect of the level of protein metabolism on the days preceding the fast is shown in the following data obtained by Voit in experiments upon a dog weighing 35 kilograms : Influence of Previous Diet on Nitrogen Elimination in Fasting (Voit) Foods of Preceding Days and Graj£S of Urea PER Day Meat 2scx> grams Meat 1500 grams Bread Last day with food . First day of fasting . Second day of fasting Third day of fasting Fourth day of fasting Fifth day of fasting . Sixth day of fasting 180.8 60.1 24.9 19.1 17.3 12.3 13-3 110.8 29.7 18.2 17-5 14.9 14.2 I3-0 24.7 19.6 15-6 14.9 13-2 12.7 130 The influence of the metabolism of the previously stored glycogen upon the amount of protein metabolized in fasting is well illus- trated by the following three experiments with one individual : ^ 1 Benedict, Influence of Inanition on Metabolism. Carnegie Institution of Wash- ington (1907). FACTORS DETERMINING PROTEIN REQUIREMENT 205 First Day of Fasting Second Day of Fasting Experiment Glycogen metabo- lized Nitrogen elimi- nated Glycogen metabo- lized Nitrogen elimi- nated I II III grams 181. 6 135-3 64.9 grams 5-84 10.29 12.24 grams 29.7 18.I 23.1 grams 11.04 11.97 12.45 It will be seen that the nitrogen output was less when there was available for metabolism a considerable supply of previously stored glycogen. Since most of the stored glycogen is used up on the first day of fasting, its influence upon the protein metaboHsm is short-lived as compared with that of the stored fat. The influence of the available supply of body fat upon the protein metabolism of fasting is shown by the following obser- vations of Falck, on the protein metaboHsm of two fasting dogs — the one lean, the other fat : Falck 's Lean Dog Falck 's Fat Dog Fasting days Grams protein catab- olized per day Fasting days Grams protein catab- olized per day 1-4 26.1 1-6 29.9 5-8 24.6 7-12 26.7 9-12 33-9 13-18 26.1 13-16 38.0 19-24 22.3 17-20 31-9 25-29 20.0 21-24 3-9 30-34 16.8 35-38 iS-7 On the 25th day the dog died. 40-44 130 45-50 13.6 55-60 12.2 Dog still healthy after 60 days. 2o6 CHEMISTRY OF FOOD AND NUTRITION A rise in protein metabolism of the lean dog after the 8th day showed that from this time he used protein largely as fuel — so largely that the results were fatal in 25 days of fasting. The fat dog, having plenty of other fuel in the form of fat, used protein to a much smaller extent, so that he was able gradually to accommodate himself to a lower level of protein metabolism and to endure a fast of 60 days' duration. The professional faster, Succi, starting with a good store of body fat, fasted 30 days* with the following results: Five days on ordinary food . . . loi. 4 grams protein per day I- 5th days fasting 80.4 grams protein per clay 6-ioth days fasting 53.1 grams protein per day I i-i 5th days fasting 36.2 grams protein per day i6-2oth days fasting 33.1 grams protein per day 2 1-2 5th days fasting 29.3 grams protein per day 26-3oth days fasting :i3.3 grams protein per day Since Succi's health remained good throughout his fast, it might be thought that the true protein requirement of his body was not greater than the smallest figure found for any period — ■ in this case about 30 grams per day. On the other hand, it may well be supposed that, since the body increases its protein metabolism to an abnormally high rate under influence of exces- sive protein feeding, so under the influence of fasting the body may be able to adjust itself to an abnormally low rate of pro- tein metabolism ; and the fact that the protein metaboHsm con- tinues to diminish for such a long time in fasting gives weight to the supposition that the body is here gradually adapting itself to an abnormal condition. One might assume that in some par- ticular period of Succi's fast, the effect of previous feeding might no longer be apparent and the conditions had not yet become abnormal as the result of the fasting, in which case the ex- penditure of protein during one of these periods would represent his normal requirement. Any such assumption must, however, * The output of nitrogen and of several other elements during a 31-day fast recently described by Benedict may be found in Chapter IX. FACTORS DETERMINING PROTEIN REQUIREMENT 207 be more or less arbitrary. A much more definite idea of the normal dietary need is obtained by determining experimentally how much protein must be contained in the daily food in order to keep the body in protein (or nitrogen) equilibrium. Nitrogen Balance Experiments and the Tendency toward Equilibrium at Different Levels of Protein Intake The estimation of the nitrogen balance has already been re- ferred to as one factor in the determination of the total food re- quirement by means of metabolism experiments ; and it has been shown that the balance may be found either by comparing the total intake with the total output, or by comparing the amount absorbed with the amount cataboHzed and eUminated through the kidneys.* When intake exceeds output, there is a plus balance which indicates a storage of nitrogen and therefore of protein in the body ; a minus balance (greater output than intake) indicates a loss of body protein. When the balance is 0, or so near o as to be within the limits of experimental error, the body is said to be in nitrogen {or protein) equilibrium. The healthy full-grown body tends to establish nitrogen equilibrium by adjusting its rate of protein metaboHsm to its food supply within wide limits. The time required by the body for this adjustment depends mainly upon the extent to which the diet is changed. The following observations by Von Noorden illustrate the estab- lishment of equiHbrium after only moderate changes in the diet : A young woman weighing 58 kilograms (128 pounds) at rest in bed was given food furnishing protein, 106 grams ; f at, 7 1 .6 grams ; carbohydrate, 200 grams; fuel value, i860 Calories per day. * Theoretically the elimination through the skin should also be determined and included in the calculation ; practically this is usually neglected unless on account of warm weather or vigorous exercise the subject has perspired profusely. For data on nitrogen in perspiration see Benedict, Journal of Biological Chemistry, Vol. 1, page 263 (1906) and A Study of Prolonged Fasting, Publication No. 203 of the Car- negie Institution of Washington, pages 233-235. 2o8 CHEMISTRY OF FOOD AND NUTRITION Example of Adjustment to Diminished Intake Total nitrogen of food 16.96 grams Lost in digestion (nitrogen in feces) .94 gram "Absorbed" 16.02 grams Nitrogen Catabolized AND ElIKONATED THROUGH Kidneys Nitrogen Balance I St day 2d day 3d day 4th day 5 th day grams - 2.18 - 0.98 + 0.22 + 0.02 + 0.32 Here there was practical equilibrium after the second day. The small amount of nitrogen represented as stored on the third, fourth, and fifth days was very likely lost through the skin. This was a case of adjustment to a lowered protein in- take, for the food previously taken, although not accurately observed, was known to have been rich in protein. Another experiment was made by Von Noorden with the same patient to show the time required to reach equilibrium after increasing the intake of protein. In this case the food furnished 2030 Calories per day and the nitrogen balance was as follows : Example of Adjustment to Increased Intake Day NrrROGEN IN Food NriROGEN m Feces Nitrogen •' Absorbed " Nitrogen Catabolized Nitrogen Balance grams gram grams grams grams I 14.40 0.70 13-70 13.60 + O.IO 2 14.40 0.70 13-70 13-80 — 0.10 3 14.40 0.70 13-70 13.60 + O.IO 4 20.96 0.82 20.14 16.S0 + 3-34 5 20.96 0.82 20.14 18.20 + 1.94 6 20.96 0.82 20.14 19-50 + 0.64 7 20.96 0.82 20.14 20.00 + 0.14 FACTORS DETERMINING PROTEIN REQUIREMENT 209 Here where the amount of protein fed was increased from 90 to 130 grams without change in the total fuel value of the diet, the body reached equihbrium on the fourth day after the increase. It is apparent therefore : (i) That the body tends to adjust its protein metabolism to its protein supply. (2) That when the body is accustomed to a certain rate of protein metaboHsm, it requires an appreciable length of time to adjust itself to a materially higher or lower rate. Hence the rate of protein metaboHsm on any given day will depend in part upon the rate of metaboHsm to which the body has been accustomed and in part upon the protein intake for the day. When the protein supply varies from day to day, the metabolism for each day is influenced by both the factors, with the net result that the elimination equals the intake when averaged for a sufficiently long period, although the data for any particular day might show a distinct gain or loss. When the protein supply is constant for a few days, the effect of pre- vious habit usually disappears and equilibrium is established as in the above cases. A transitory loss of nitrogen from the body is apt to be due simply to the taking of less than the usual amount of protein food, but a persistent loss indicates that the diet is insufl&cient, either in total food (calories) or in protein, to enable the usual adjustment to take place. A transitory storage of nitrogen in the body may occur as the result of an increase either of the protein or of the total fuel value of the food ; but a persistent storage occurs, as Von Noor- den has pointed out, only under the following conditions : (i) In the growing body (or in pregnancy) where new tissue is being constructed. (2) In cases where increased muscular exercise calls for en- largement of the muscles. 2IO CHEMISTRY OF FOOD AND NUTRITION (3) In cases where, owing to previous insufficient feeding or to wasting disease, the protein content of the body has been more or less diminished and consequently any surplus available is utilized to make good the loss. Protein-sparing Action of Carbohydrates and Fats It has been shown above that, in fasting experiments, the amount of stored glycogen and fat in the body exerts a " spar- ing " influence upon protein metabolism, the amount of protein cataboUzed being smaller when the supplies of glycogen and fat are more abundant. Similarly the amounts of carbohydrates and fats in the food influence the rate of protein metabolism as indicated by the nitrogen excretion. The loss of protein which occurs on an insufficient diet may be diminished or even stopped by adding carbohydrates or fat to the food ; and if carbo- hydrate or fat be added to the diet of a man in nitrogen equi- librium, there results a temporary decrease in nitrogen output with a corresponding storage of protein in the body. The former observation could be interpreted as meaning simply that the body draws upon its stored protein for energy so long, and only so long, as the fuel value of the food is insufficient ; but the fact that addition of carbohydrate or fat to a diet already sufficient may cause an actual storage of protein indicates that the " pro- tein-sparing action " or " protein-protecting power " of carbohy- drates and fats involves something more than merely the question whether the body " needs " to burn its stored protein as fuel. As this is a matter of great importance, it may be well to consider somewhat carefully (i) the experimental evidence, and (2) the theoretical explanations, regarding the protein-sparing action of the carbohydrates and fats. For an account of the earher experiments on this subject, especially those of Voit and Rubner upon dogs, the reader is referred to Lusk's Elements of the Science of Nutrition. Only some of the more important of the experiments upon men can be described here. FACTORS DETERMINING PROTEIN REQUIREMENT 21 1 Lusk/ experimenting upon himself, showed the susceptibility of the protein metaboUsm to the sudden withdrawal of carbo- hydrate food. In one experiment a liberal mixed diet contain- ing 20.55 grams of nitrogen was taken until the body was nearly in nitrogen equilibrium, and then, without any other change, 350 grams of carbohydrate were withdrawn from the daily food. On the first day the body protein was largely protected by the carbohydrate previously stored in the body in the form of glycogen, but on the second day the nitrogen metabolism had risen from 19.84 to 27.00 grams per day. In another experi- ment, upon a diet containing less protein, withdrawal of carbo- hydrate increased the nitrogen excretion from 11.44 to 17.18 grams per day. In these cases, as in the fasting experiments, the loss of body protein was less in those subjects having a good store of body fat than in those which were thin. Kayser compared the efficiency of carbohydrates and fats as sparers of protein by observing the effect upon the nitrogen balance of replacing the carbohydrates of the food by such an amount of fat as would furnish the same number of calories, and then' after three days resuming the original diet. This experiment and that of Tallquist which follows are given some- what fully, as they illustrate well the methods and results of investigations based mainly upon the question of nitrogen equi- librium. The observer, who served as his own subject, was twenty-three years old, of good physique, with a small store of body fat, and weighed 67 kilograms. In the first and third periods he ate meat, rice, butter, cakes, sugar, oil, vinegar, and salad. In the second period the diet was changed so as to con- sist of meat, eggs, oil, vinegar, and salad, so that practically all the carbohydrate was withdrawn and replaced by fat. The two diets had practically the same fuel value and protein content. The results of this experiment are shown in the following table : ' Zeitschrijt fur Biologie, Vol. 27, page 459 (1890). 212 CHEMISTRY OF FOOD AND NUTRITION Nitrogen Balance when Feeding Isodynamc Quantities of Carbo- hydrate AND Fat (Kayser) Intake Output Nitrogen Day Total Fat Carbo- Fuel Total Balance nitrogen hydrates value nitrogen grams grams grams Calories grams grams I 21.15 71. 1 338.2 2590 18.66 + 2.46 2 21.15 71.8 338.2 2596 20.04 + I.II 3 21.15 71.8 338.2 2596 20.59 + 0.56 4 21.31 71.8 338.2 2600 21.31 =fc 0.00 5 21.51 221. 1 000.0 2607 23.28 - 1.77 6 21.55 217.0 000.0 2570 24.03 - 2.48 7 21.5s 215.5 000.0 2556 26.53 -4.98 8 21.10 70.4 338.2 2581 21.65 -0.5S 9 21.10 70.4 338.2 2581 19.20 + 1.89 lO 21.10 70.4 338.2 2581 19.65 + 1.4s It is evident from the nitrogen balance of the first period that the amount of protein in the food was here greater than neces- sary, but that equiUbrium was fully established in four days. On substituting fat for carbohydrate there is a marked increase of protein catabolism with corresponding loss of nitrogen from the body, and what is especially noteworthy, there was no evi- dence of any tendency to regain equilibrium during this period, but on the contrary the loss of nitrogen became greater each day the fat diet was continued ; whereas, upon returning to the mixed diet, not only was the loss of protein stopped, but the body almost at once began replacing the protein it had lost, although the nitrogen and calories of the food were practically unchanged. Tallquist ^ compared the protein-protecting powers of iso- dynamic amounts (amounts having equal energy value) of car- 1 Archiv Jtir Hygiene, Vol. 41, page 177. FACTORS DETERMINING PROTEIN REQUIREMENT 213 bohydrates and fats when only a part of either was replaced by the other. The subject was Tallquist himself, a man twenty- eight years old, in good health, and weighing about 80 kilograms. The experiment was performed in Rubner's laboratory, and the diet contained such an amount of total food as was estimated by Rubner to be just about sufficient to supply the energy require- ments of the body, viz., 36 Calories per kilogram per day. The experiment covered 8 days divided into two equal periods. In the first four-day period the diet was rich in carbohydrates, in the second period it was rich in fats. An excellent feature of this experiment is that there was no change in the nature of the protein fed. All foods furnishing any significant amount of nitrogen were the same in the two periods of the experiment. The food of the first period consisted of meat, milk, butter, bread, sugar, coffee, beer. That of the second period contained the same amounts of meat, milk, bread, coffee, and beer, but less sugar, more butter, and some bacon. The same amount of salt was taken in each case. The principal data of the experi- ment may be summarized as follows : Nitrogen Balance when Feeding Isodynamic Quantities of Carbo- hydrate AND Fat (Tallqutst) Intake Output Nitrogen Balance Day Total nitrogen Fat Carbo- hydrates Alcohol Fuel value Nitrogen grams grams grams grams Calories grams I 16.27 44.0 466 18.5 2867 17. II - 0.84 2 16.27 44.0 466 i8.5 2867 14.40 + 1.86 3 16.27 44 -O 466 18.5 2867 14.65 + 1.62 4 16.27 44.0 466 18.5 2867 15.58 + 0.69 S 16.08 140.0 250 19.0 2873 17.66 -1.58 6 16.08 140.0 250 19.0 2873 17.32 - 1.24 7 16.08 140.0 250 19.0 2873 15.94 + 0.14 8 16.08 140.0 250 19.0 2873 16.22 — 0.14 214 CHEMISTRY OF FOOD AND NUTRITION Here only a part of the carbohydrate, about half of that present, and an amount representing about one third of the total fuel value of the diet, was replaced by fat. The change evi- dently had an unfavorable influence upon the nitrogen balance but the loss of body protein was relatively small and continued only 2 days. Atwater ^ compared the protein-sparing action of carbo- hydrate and fat in experiments in which the subject, an athletic young man of 76 kilos, performed a considerable amount of work. The experiments were carried out in the respiration calorimeter and covered in all 15 experimental days upon a diet rich in carbohydrates, arranged in four periods which were alter- nated with four equal periods in which the diet was rich in fats. The change from carbohydrate to fat and vice versa involved about 2000 Calories or nearly half the fuel value of the diet. The average results per day for the entire series of experiments were as follows : On Diet Rich in Carbohydrates On D ffiT Rich in Fat Available Calories in food 4532 4524 Heat equivalent of work per- formed, Calories . . . 558 554 Nitrogen in food, grams . 17-5 17. 1 Nitrogen in feces, grams . 2.5 1-7 Nitrogen in urine, grams . 16.6 18.1 Nitrogen balance, grams . - 1.6 - 2.7 Here again there is a difference in favor of the carbohydrate, but one which is so small as to be of almost no practical sig- nificance. It appears that the carbohydrate of the food cannot be en- tirely replaced by an equal number of calories in the form of fat without an unfavorable effect upon the nitrogen balance; but I Ergebuissc dcr Physiologic, Vol. 3, Part I, page 497. FACTORS DETERMINING PROTEIN REQUIREMENT 21 5 that when the replacement is such as to affect not over one half of the total calories, the difference in protein-sparing action is but slight. Ordinarily on a normal mixed diet the same num- ber of calories has about the same protein-sparing effect. Landergren ^ also found that it is only when the carbohydrate of the diet is entirely replaced by fat that the comparison is so strikingly against the fat as it seemed to be in Kayser's experi- ment. In Landergren's experiments the condition studied was not one of approximate equihbrium, but rather of nitrogen hunger. He fed men diets of adequate fuel value but contain- ing only about one gram of nitrogen daily, and found that by four days of such feeding the urinary nitrogen may be reduced to about 4 grams per day. In one experiment in which the daily food contained 750 grams of carbohydrates the urine of the fourth day showed 3.76 grams of nitrogen. The carbohydrate was then entirely replaced by fat, with the result that the fol- lowing days' urines contained respectively 4.28, 8.86, and 9.64 grams of nitrogen. Evidently in the case of a man accustomed to feeding largely upon carbohydrates the complete replacement of carbohydrate by fat leads to a loss (or an increased loss) of body protein. But by subsequent experiments of the same series it was found that a diet containing nearly half its calories in carbohydrate, and nearly half in fat, had apparently the same protein-sparing power as one made up almost exclusively of carbohydrates. The explanation offered by Landergren is that when the diet suppHes no carbohydrate, the glycogen of the body soon be- comes exhausted, and the carbohydrate needed, to keep up the constant glucose content of the blood is obtained largely by the breaking down of proteins. This might suffice to explain the difference in effect of carbo- hydrate and fat, but not the fact that addition of a non-nitroge- • Skandinai'isches Archiv fur Physiologic, Vot 14, page 112 (1903) ; Abstract Ex- periment Station Record, Vol. 14, page 1099. 2l6 CHEMISTRY OF FOOD AND NUTRITION nous nutrient to a diet already sufficient may cause storage of nitrogen in the body.* A satisfactory explanation of both sets of facts appears to be afforded by the recent advances in our knowledge of the fate of foodstuffs in metabolism which were outlined in Chapter V. The outstanding relationships of the three groups of foodstuffs in the intermediary metabolism may be indicated schematically as follows : CARBOHYDRATE Glucose FAT e.g., Stearin ^ \ Stearic acid Glycerol "^ Glyceric aldehyde ^Methyl glyoxal By /8-oxidation, finally, to carbon dioxide and water PROTEIN .tl. Amino acids, (Among which) Alanine 4 Lactic acid H NH3 ^ I Pyruvic acid ' I By oxidation, finally, to carbon dioxide and water Since ammonia is always being formed in protein catabolism (by deaminization of amino acids), and since the ammonium salts of a-ketonic acids, such as pyruvic acid, are convertible into amino acids which are building materials for body protein, we have here a mechanism by which an intermediary product of carbo- hydrate metabolism (pyruvic acid) takes up a " waste product " of protein metaboHsm (ammonia) and turns it back into amino acid again. Thus carbohydrate, in undergoing metabohsm, " spares " protein, not only by serving as fuel so that protein * Furthermore Lusk points out that Landergren's explanation is hardly ade- quate to cover the results obtained in gelatin-feeding experiments. FACTORS DETERMINING PROTEIN REQUIREMENT 217 need not be drawn upon for this purpose, but also by furnishing material which in combination with ammonia (otherwise a waste product) can actually be converted in the body into some of the amino acids of which body proteins are composed and with which they are in equilibrium. This explains how an in- creased intake of carbohydrate, with resulting increase of pyruvic acid, naturally leads to increased synthesis of amino acids and thus to a tendency toward protein storage, or, to express the same thing in somewhat different terms, tends to push the re- action, Amino acids ^ Protein, toward the right. According to present theory, most, if not all, of the energy of the carbohydrate becomes available through oxidation processes which involve the intermediate production of pyruvic acid, an a-ketonic acid whose ammonium salt is capable of conversion into amino acid. Of the fat only the glyceryl radicle (about one twentieth of the fuel value) is oxidized through pyruvic acid, while the fatty acid radicles, representing about nineteen twentieths of the energy of the fat, are metabolized through /8-oxidation processes which yield, so far as we know, no product whose ammonium salt is convertible into amino acid in the body. Hence complete withdrawal of carbohydrate, even though sub- stituted by sufficient fat to yield an equal number of calories, must be expected to result in increased excretion of nitrogen; but when no more than half of the carbohydrate is replaced by fat there seems to be enough pyruvic acid produced to meet the practical requirements of economical metabolism of protein. Protein Requirement in Normal Nutrition From what has been said above it will be apparent that, within rather wide limits, the greater the amounts of carbo- hydrates and fats eaten, the smaller will be the amount of pro- tein required to maintain nitrogen equilibrium. For practical purposes, however, we may eliminate the ques- tion of the extent to which protein metabolism can be restricted 2l8 CHEMISTRY OF FOOD AND NUTRITION by the use of excessive amounts of other food and reduce the problem to this : When the total food is properly adjusted to the size and activity of the subject so that there is sufficient but not excessive fuel to meet all the energy requirements, how much protein must the daily food contain in order to keep the body in nitrogen equilibrium? The most extended investigation on the protein requirement of man is that of Chittenden.* The general plan followed in this investigation was to have each man reduce his protein food gradually without any great change in his other habits. This gradual reduction of the protein intake was continued usually for some weeks, sometimes for several months, before any com- parison of intake and output was attempted. During this pre- liminary period upon a restricted diet there was in almost every case a loss of weight, and from previous observations f under similar conditions we may safely assume that there was a considerable loss of body protein. After a suf&cient period of adjustment there was usually a tendency for the body weight and the rate of protein metabolism (measured by the amount of nitrogen eliminated through the kidneys) to become fairly constant, indicating that the body had adapted itself to the new conditions. When this point had been reached, a nitrogen balance experiment was made, the intake and output being determined by weighing and analyzing for nitrogen all food consumed and all nitrogenous material given off from the body except that in the perspiration. The fuel value of the food consumed during the same period was calculated by means of figures taken from standard tables. From these calculated fuel values it would appear that the energy of food consumed by Chittenden's subjects was in general about equal to the usual estimates of the energy requirements for similar occupations, * See Chittenden's Physiological Economy in Nutrition and Nutrition of Man. t Neumann, for example, in 35 days on insufficient diet lost 96 grams of nitrogen corresponding to 600 grams of protein, equivalent to about 2.5 kilograms (5.5 pounds) of muscle tissue. FACTORS DETERMINING PROTEIN REQUIREMENT 219 though in several specific instances the subject may have unduly restricted his total food intake and thus created an energy deficit and a tendency toward negative nitrogen balance. Chittenden bases his estimate of the protein requirement, not only upon the nitrogen balances, but also upon the amounts of nitrogen observed to be eliminated daily through the kidneys over long periods in which the body may or may not have been in cornplete equihbrium, but in which health and efficiency were certainly maintained. The first men to serve as subjects in this investigation were Chittenden himself and his associates, who all continued their professional work and either reported no effect or felt benefited by the change to the low protein diet. Similar experiments were then made upon a squad of soldiers, who during the test were quartered near the laboratory and were given regular exercise in the gymnasium in addition to light duties about their quarters. These men showed marked im- provement in physical condition during the test, probably due in part to their more regular habits of life and their gymnastic exercises. In order to eliminate this latter factor while still applying the low protein diet to young and physically active men, the investigation was extended to cover a group of uni- versity athletes who were already well-trained and in prime physical condition at the beginning of their dietary experiment. These athletes not only maintained, but in many cases improved, their gymnastic records while on the low protein diet, one of them winning an all-round gymnastic championship during the time. Chittenden states ^ that his data " are seemingly harmonious in indicating that the physiological needs of the body are fully met by a metaboHsm of protein matter equal to an exchange of o.io to 0.12 gram of nitrogen per kilogram of body weight per day, provided a sufficient amount of non- nitrogenous foods is taken to meet the energy requirements of the body." This would correspond to 44 to 53 grams of pro- 1 Nutrition of Man, pages 226, 272. 220 CHEMISTRY OF FOOD AND NUTRITION tein per day for a man of average weight (70 kilograms, 154 pounds, without clothing), and Chittenden considers that for such a man an allowance of 60 grams of protein per day should certainly be entirely adequate. In a recent examination of the available Hterature upon this subject there were found 86 experiments upon adults showing no abnormality of digestion or health, in which the diet was sufficiently well adjusted to the probable requirement and the nitrogen balance showed sufficient approach to equilibrium to make it appear that the total output of nitrogen might be taken as an indication of the protein requirement. These experi- ments are taken from 20 independent investigations in which 41 different individuals (37 men and 4 women) served as subjects. For purposes of comparison the daily output of total nitrogen in each experiment was calculated to protein and this to a basis of 70 kilograms of body weight. Reckoned in this way, the ap- parent protein requirement as indicated by the data of individual experiments ranged between the extremes of 20.0 and 79.2 grams, averaging 49.2 grams of protein per man of 70 kilograms per day. Thus the average falls well within the range of Chittenden's estimate of the amount of protein required for normal nutrition when the energy value of the diet is adequate. Examination of the data recorded in the original papers indi- cates that the wide differences in amounts of protein catabolized in the different experiments cannot be attributed primarily to the kind of protein consumed nor to the use of diets of fuel values widely different from the energy requirements. Ap- parently the most influential factor was the extent to which the subject had become accustomed to a low protein diet. Difference between Minimum Requirement and Standard Allowance of Protein It may be well to point out here the distinction between the amount of protein actually required on the one hand, and, on FACTORS DETER^IINING PROTEIN REQUIREMENT 2 21 the other hand, the amount which it may be thought best to allow in the planning of dietaries. The term " requirement " should preferably be applied only to the former; the latter would better be called the protein allowance or the standard for protein. The difiference between the amount actually re- quired and the amount which would ordinarily be allowed in planning a dietary is much greater with protein than with fuel value. Surplus fuel is stored as fat, and if excessive fatness is to be avoided, the fuel value of the food must not greatly exceed the energy requirements of the body; but surplus nitrogen is rapidly eliminated from the body and, so long as no injury to health results, leaves no evidence of having been taken in excess of body needs. The eating of a considerable surplus of protein has become habitual, and such a surplus of protein in the food is beheved by many people to constitute a desirable " factor of safety," if not indeed to exert a directly beneficial effect upon health and stamina. Hence there is a tendency to set the pro- tein allowance or standard for protein considerably higher than the actual requirement. If the average daily food requirement of a man at rest be taken as 2000 Calories including 50 grams of protein, the same man at work may require 3000 or 4000 Calories while his actual requirement for protein will not be appreciably increased. If the protein be held at 50 grams while the food is increased from 2000 to 3000 to 4000 Calories, the protein in percentage of total calories would be in the three cases 10 per cent, 7 per cent, and 5 per cent respectively. Thus it is plain that when the energy requirement is subjected to considerable variations by differ- ences in muscular activity, the protein requirement cannot be taken as constituting a fixed proportion of the total calories, since muscular work increases the energy requirement very greatly and the protein requirement very little if at all. In practice, however, a diet of 2000 Calories would usually contain somewhat over 50 grams of protein ; and when the man increased 222 CHEMISTRY OF FOOD AND NUTRITION his activity and his total food consumption, he would probably increase his protein intake in almost the same proportion, for he would in most cases simply eat a larger quantity of his usual kind of food. Moreover, those differences in food requirement which are due to differences in age and size will usually afTect the energy requirement and the protein requirement in about the same proportion ; and, as the majority of dietaries are planned for family groups, the differences in age and size are usually quite as important as the differences in muscular activity. Thus there is rational basis for the custom of allowing enough pro- tein to furnish from lo to 15 per cent of the total energy value of the diet. Influence of the Choice of Food When isolated proteins are fed singly, striking differences in nutritive value appear, as has been shown in Chapter III. In view of this fact it may seem strange that in the experiments hitherto conducted to determine the protein requirement of man the kind of protein fed has not exerted a more striking influence upon the results obtained. There is, however, no real discrepancy between the two sets of findings. The experi- ments described in Chapter III were for the purpose of compar- ing individual proteins isolated even from the other proteins which always accompany them in natural or commercial food materials, and were conducted largely upon rapidly growing young animals, in which there is an active synthesis and reten- tion of protein, so that a deficiency in the supply of any amino acid which is required in the construction of body protein is apt to be quickly and plainly reflected in a diminution or cessation of growth. On the other hand, in experiments like those de- scribed in the preceding section, where the purpose is not to compare proteins but to measure the normal protein require- ment, the diet is naturally made up, not of isolated proteins or FACTORS DETERMINING PROTEIN REQUIREMENT 223 even of single or unusual foods, but (ordinarily at least) of such combinations of staple foods as is believed to represent a normal diet, so that even a relatively simple ration arranged for the purposes of such an experiment would probably contain a num- ber of different proteins among which any peculiarities of amino acid make-up would be apt to offset each other. Moreover the experiments of the latter group have been made entirely upon adults whose protein requirement was limited to that of main- tenance. In such cases there is no longer a demand for amino acids to be built into new tissue, but only to maintain the equi- librium which now exists between amino acids and proteins in the tissues already full grown. Any of the amino acids whose radicles are contained in tissue proteins may be expected to contribute something to the maintenance of such an equilibrium, whereas there can be no growth unless all the necessary amino acids are present. In a corresponding series of experiments upon growing children or nursing mothers the influence of food selection would probably be much more pronounced. Even for the maintenance of adults protein requirement may be found to be considerably influenced by food selection when experiments suitably planned to test the question are carried out. The inadequacy of gelatin as a sole protein food and its inferiority to meat or milk protein when substi- tuted beyond a certain proportion are well known. A series of experiments, designed to demonstrate differences in nutritive efficiency for man of the protein supplied by different staple articles of food, was carried out by Karl Thomas in Rubner's laboratory and the striding results obtained have been widely quoted, often on Rubner's authority. These results, however, have as yet failed of confirmation, and on some important points have been so directly refuted by later workers using longer experimental periods, as to make it appear that Thomas's plan of experimenting and mode of inter- pretation were not entirely suited to the solution of the question at issue. Thomas! thought he had demonstrated that meat protein was greatly superior to bread or potato protein for the maintenance of body tissue ; but Hindhede finds no such difference, being able to maintain normal nutri- tion \\ith either bread or potato nitrogen in relativ'ely small amounts. * Thomas, Archiv JUr Anatomic und Physiologic, igog, pages 210-302. 224 CHEMISTRY OF FOOD AND NUTRITION Rose and Cooper * have also demonstrated the high value of potato nitrogen in the maintenance of nitrogen equilibrium, and preliminary ex- periments in the writer's laboratory f have tended to confirm Hindhede's finding that nitrogen equilibrium may be maintained on a relatively low intake of protein in the form of bread. Of greater practical importance than the experiments with bread alone are those f which show the maintenance of nitrogen equilibrium over long periods on low protein diets in which bread is the chief source of protein, but is supplemented by small amounts of milk. Since estimates of protein requirement, in order to be of general application, should provide for the needs of growth, reproduction, and lactation, as well as for maintenance, it will be well to consider more fully the results obtained in feeding experimental animals upon known rations throughout the period of growth or the entire life cycle. It will be remembered that Osborne and Mendel, feeding isolated proteins in liberal proportion (i8 per cent) in diets adequate and well balanced as regards all other factors, found that edestin, a typical vegetable globulin, was able to supply all the protein requirements of maintenance, reproduction, and growth, even through three generations of rats. With gliadin as the sole protein, maintenance was satisfactory but growth was inhibited ; but an addition of lysine to this diet caused an immediate resumption of growth. When the supply of lysine was cut off, growth again ceased. A ration containing zein as the sole protein did not suffice even for maintenance ; but when tryptophane was added to it, or gliadin, which contains trypto- phane, it served to maintain body weight, and on further addition of lysine, growth ensued. In order to emphasize such differences as these it is some- times thought advantageous to classify proteins as : A. Complete : Capable of maintaining adults and providing for normal growth of the young when used as a sole protein * Rose and Cooper, Journal of Biological Chemistry, Vol. 30, pages 201-204. t Not yet published. FACTORS DETERMINING PROTEIN REQUIREMENT 225 food. Casein and lactalbumin of milk; ovalbumin and ovo- \dtellin of egg ; glycinin of soy bean ; excelsin of Brazil nut ; edestin, glutenin, and maize-glutelin of the cereal grains. B. Partially Incomplete: Capable of maintaining life but not of supporting normal growth. Gliadin of wheat is the well- demonstrated example of this class. C. Incomplete : Incapable either of maintaining Hfe or of supporting growth, when fed as the sole protein. Zein of corn (maize), and gelatin are the conspicuous examples. Any such grouping of the proteins, however, must be used with much discrimination, and with great care to insure an understanding of the quantitative aspects of the experimental data, if misconceptions are to be avoided. Edestin is a con- spicuous example of a " complete " protein, having served as above noted as the sole protein food of a family of rats for three generations; but when the percentage of edestin in the food mixture was considerably reduced, results like those above described for gliadin were obtained — the diet did not support a normal rate of growth, but this could be secured by adding lysine to the food mixture. Similarly casein when fed in reduced proportion to the total food mixture did not support normal growth ; but growth became normal when cystine was added. Thus " complete " proteins may behave as " partially incom- plete " when fed in reduced proportion. It is also to be remem- bered that varying rates of growth in different species (not to mention other differences) make inadmissible any broad generaUzations as to the proportion in which any protein should be fed to species other than that with which its " completeness " or " incompleteness " has been demonstrated. . In some of their most recently published experiments (1916) Osborne and Mendel give quantitative measurements of the relative efficiency (for support of growth in young rats) of some of the " complete " proteins. The rate of gain obtained with 8 per cent of lactalbumin required 12 per cent of casein or 15 Q 2 26 CHEMISTRY OF FOOD AND NUTRITION per cent of edestin ; or, as they also state the result, " to pro- duce the same gain in body weight 50 per cent more casein than lactalbumin was required, and of edestin nearly go per cent more." In maintenance experiments, 2.4 to 3 per cent of lactalbumin was as effective as 3.5 to 4 per cent of casein or edestin. On extending their experiments from rats to chicks, Osborne and Mendel again found that proteins rich in lysine are much more effective for growth than those in which the proportion of lysine is much smaller. McCollum found milk protein much more efficient than wheat or maize protein in supporting the growth of young pigs. As in growth, so in lactation, the demand for material for the construction of new protein creates a condition in which differ- ences of value in the protein fed may readily become more ap- parent than when only maintenance is involved. Hart and Humphrey find that in meeting the protein requirements of milch cows, milk protein and the protein of flaxseed, " oil meal," are about 50 per cent more efficient than the proteins of the corn (maize) or of the wheat kernel; and Hoobler has shown that milk is the best form of food protein for the production of human milk and the protection of the body protein of the nurs- ing mother. Influence of Muscular Exercise At one time it was supposed that muscular power was gener- ated at the expense of muscle substance and this, of course, necessitated the belief that muscular work always increased pro- tein metabolism. Since we now know that the muscles work quite as well at the expense of carbohydrates and fats as of pro- tein, the conclusion that muscular work necessarily increases the metabolism of protein is far from inevitable. It is only necessary to observe the effects of regular muscular exercise, FACTORS DETERMINING PROTEIN REQUIREMENT 227 either in athletic training or in normal labor, to see that the muscles do not waste away when thus used, but rather tend to become larger. Such a growth of the muscles tends toward a storage rather than a loss of protein. Usually, however, mus- cular work also results in increased appetite, and it is difficult to separate the effects of the exercise from those of the extra food. Whether muscular work acts directly to increase the amount of protein metabolized in the body can only be determined by experiments in which sufficient extra fats and carbohydrates are fed to furnish the extra fuel required on the working days. But since fats and carbohydrates spare protein, the feeding of these in any excess over just what is necessary to provide for the increased energy requirement would tend to decrease the metab- olism of protein and counteract any effect which the muscular work might otherwise have in increasing protein metabolism. Hence, in order to show conclusively whether muscular work of itself has any influence upon the protein metabolism, it would be necessary to determine the mechanical efficiency of the man, then to bring him into equihbrium with an amount of food just sufficient for his needs, and finally to have him perform a meas- ured amount of work at the same time adding to his diet an amount of fats and carbohydrates just sufficient to furnish the extra energy required for the work performed. Such elaborate experiments have not yet been made, but we have sufficient data to show that they are not necessary for practical purposes. Many experiments have shown conclusively that increased work, when accompanied by a sufficient increase in the amount of fats and carbohydrates fed, does not necessarily increase the metab- olism of protein. The following data from Atwater {Report of the Storrs, Con- necticut, Agricultural Experiment Station for igo2-igoj, page 127) show the average results of a long series of rest and work experiments with men in the respiration calorimeter : 228 CHEMISTRY OF FOOD AND NUTRITION Muscular Work and Protein Metabolism (Atwater) Nature of Experiment Resl: Food generally sufficient for equilibrium ; 5 subjects, 27 experiments, covering 82 clays Work : 8 hours per day. Food generally not quite sufficient for equilibrium; 3 subjects, 24 experiments, covering 76 days Average Metabolism per Day Per Person Energy, Calories 2310 4556 Protein, Grams 103.8 Per Kilogram Body Weight Energy, Calories 33-5 62.9 Protein, Grams I-5I 1.49 Per Square Meter Surface Energy Calories 1116 2129 Protein, Grams SO. I 50.5 Comparing the figures either per unit of weight or of surface, it will be seen that muscular work sufficient to nearly double the energy metabolism had no appreciable effect upon the amount of protein metabolized. Considering the large amount of exceptionally accurate research represented in these figures, they seem to justify the conclusion that if muscular work has any tendency to increase the " wear and tear " of muscle substance, such effect is normally balanced by the tend- ency of the muscles to grow (and therefore store protein) when exercised. Moreover, it is certain that any effect which muscular work might possibly have in increasing protein metabolism would be incomparably less than its effect in increasing the total metab- oHsm. If, then, starting with a diet which maintains protein equiHbrium at rest, the total food is increased sufficiently to provide for the muscular work, and the increase in the diet is FACTORS DETERMINING PROTEIN REQUIREMENT 229 accomplished by adding any reasonable combination of food materials, we may feel sure that these will supply plenty of protein to meet any possible increase in the protein requirement. Hence, in planning the diet of a man at hard muscular work, any reasonable combination of foodstuffs given in sufficient abundance to meet the energy requirement will almost certainly supply an ample amount of protein. Shafi[er has studied the output of ammonia, creatinine, and uric acid as well as of total nitrogen during rest and work and finds no significant change in any of these. Lusk considers it fully proved that neither the amount nor the character of pro- tein metabolism is changed by muscular activity. Protein Requirement in Relation to Age and Growth If a man at moderately active work takes a diet which fur- nishes 3000 Calories and 75 grams of protein, he is taking 10 per cent of his calories in the form of protein. Of course the protein requirement cannot bear a fixed relation to the calorie requirement, since the latter is largely influenced by activity, while the former is not. Most men, when at complete rest, would require more than 10 per cent of their calories in the form of protein because the lack of exercise would not reduce the protein requirement to the same extent as the energy require- ment. On the other hand, most Americans are accustomed to take more than 10 per cent of their calories as protein regardless of whether they require it or not. If, then, the active man's need for protein is met by supplying him with 10 per cent of his needed calories in the form of protein, this will serve as a convenient starting point in considering the requirements of a child. Let this be compared with the normal dietary of an infant. Human milk averages about 1.6 per cent protein, 4.0 per cent fat, 7.0 per cent carbohydrate. Here about 9 per cent of the calories are taken in the form of protein, or about the same proportion as has been allowed for the full-grown active 230 CHEMISTRY OF FOOD AND XUTRmON man. Furthermore Hoobler has shown experimentally that this is as high a proportion of protein as the infant will utilize with the highest efficiency in growth of body tissue. During the suckling period the growth is relatively more rapid than at any other age. Mendel * gives the following figures : The Relative Daily Gain in Body Weight of Children In the first month is about i.oo per cent At the middle of the first year 0.30 per cent At the end of the first year 0.15 per cent At the fifth year 0.03 per cent Maximum in later years for boys 0.07 per cent for girls 0.04 per cent If, then, the full-grown man and the child at the time of most rapid growth each requires but 10 per cent of his calories in the form of protein, it seems probable that this proportion is also sufficient for any intermediate age, if the diet is of ample fuel value, and the protein is of the right kind. But the proper selection of the protein is of very great importance in the feeding of children, who differ from most other young mammals in that their period of growth is so many times longer than the suckling period. Even the child that is nursed for a year and attains three times his birth-weight before weaning will still have much the greater part (probably five sixths) of his growth to make on other food. By the time growth is complete he will prob- ably have about twenty times the body weight and more than twenty times the body protein with which he was born. Growth at the normal rapid rate of early childhood involves the conversion of a very considerable part, sometimes as much as one third, of the protein of the food into body protein. This can be accompHshed to the best advantage only when (i) the protein of the food is largely of the kind most efficient in sup- porting growth, /c. milk protein; (2) the protein is well " pro- * Childhood and Growth, p. 18. FACTORS DETERMINING PROTEIN REQUIREMENT 231 tected " by the protein-sparing action of liberal amounts of carbohydrate and fat. That the child needs a diet of high fuel value to meet the requirements of his energy metabolism has already been pointed out (Chapter VII). It is because the high protein requirement of childhood (for young children more than twice as much per unit of weight as for adults) is paralleled by an equally high energy requirement that the diet of the child need not contain a higher percentage of its calories in the form of protein than does the ordinary diet of the adult, if the protein for the child is well chosen. Usually, however, a well-planned dietary for a child will show a somewhat more than average proportion of its calories in the form of protein because after weaning the main feature of the child's diet should be cows' milk which furnishes about 19 per cent of its calories in the form of protein. A child, fed mainly upon cows' milk and taking enough food to amply cover his energy requirement, will therefore receive a safe surplus of pro- tein in the best available form. With a full quart of milk in the daily dietary of the growing child the other foods may be selected chiefly with reference to other quahties than their pro- tein content; without a liberal use of milk the proper feeding of a growing child becomes a very difficult problem. Having discussed the protein requirements of ordinary adult maintenance and of growth, the requirements of the aged should also be considered. This does not require extended discussion, since advancing age involves no new features but only a gradual modification of those pertaining to middle life. In general, elderly people show a somewhat diminished pro- tein requirement and likewise a diminished power of dealing with excess. Surplus protein taken in the food is not so rapidly absorbed and catabolized, and, while there appears to be no essential difference in the form in which the nitrogen is finally excreted, the susceptibility to excessive putrefaction of protein 232 CHEMISTRY OF FOOD AND NUTRITION appears to be increased. It would seem that in the dietary of the aged the protein should be reduced to at least as great an extent as are the calories. REFERENCES Atwater and Benedict. Comparison of Fats and Carbohydrates as Protectors of Body Material. Bulletin 136 (pages 176-187), Office of Experiment Stations, U. S. Dept. Agriculture. Benedict. The Influence of Inanition on Metabolism (Publication 77) and A Study of Prolonged Fasting (Publication 203). Carnegie Insti- tution of Washington. Catiicart. Physiology of Protein Metabolism. Chittenden. Physiological Economy in Nutrition. Chittenden. The Nutrition of Man. Hart and Humphrey. The Relation of the Quality of Proteins to Milk Production. Journal of Biological Chemistry, Vol. 21, page 239 (1915) ; Vol. 26, page 457 (1916) ; Vol. 31, page 445 (iQi?)- Hint)hede. Protein and Nutrition. Hindhede. Nutritive Value of the Proteins of Potatoes and of Bread. Skandinavisches Archivf. Physiologic, Vol. 30, page 97 (1913) ; Vol. 31, page 259 (1914). Hoobler. The Protein Need of Infants. American Journal of Diseases of Children, Vol. 10, page 153 (1915). Hoobler. The Effect on Human Milk Production of Diets Containing Various Forms and Quantities of Protein. American Journal of Dis- eases of Children, Vol. 14, page 105. See also Journal American Medical Association, Vol. 69, page 421 (August, 1917). LusK. Elements of the Science of Nutrition. McCoLLUM. The Nature of the Repair Processes in Protein Metabolism. American Journal of Physiology, Vol. 29, page 215 (1912). McCoLLUM. The Value of the Proteins of Cereal Grains and of Milk, for Growth in the Pig. Journal of Biological Chemistry, Vol. 19, page 323 (1914). McCoLLUM and Davis. Influence of the Plane of Protein Intake on Growth, Journal of Biological Chemistry, Vol. 20, page 415 (1915). McCoLLUM, SiMMONDS, ANT) PiTz. Effects of Feeding the Proteins of the Wheat Kernel at Different Planes of Intake. Journal of Biological Chemistry, Vol. 28, page 211 (1916). McKay. The Protein Element in Nutrition. FACTORS DETERMINING PROTEIN REQUIREMENT 233 Mendel. Nutrition and Growth. Harvey Society Lectures, I9i4-i9i5,and Journal of the American Medical Association, Vol. 64, page 1539 (1914). MuRLiN. The Nutritive Value of Gelatin. American Journal of Physi- ology, Vol. 19, page 285; Vol. 20, page 234 (1907-1908). MuRLiN AND Bailey. Protein Metabolism in Normal Pregnancy. Ar- chives of Internal Medicine, Vol. 12, page 288 (1913). Osborne and Mendel. (A Series of papers upon the nutritive functions and relative efficiency of individual proteins and amino acids in main- tenance and growth.) Journal of Biological Chemistry, Vol. 1 2, page 473 ; Vol. 13, page 233 (1912); Vol. 17, page 325; Vol. 18, page i (1914) ; Vol. 20, page 351 ; Vol. 22, page 241 (1915) ; Vol. 25, page i ; Vol. 26, pages I, 293 (1916). (Subsequent issues should also be consulted for papers appearing after the compilation of this list.) Rose and Cooper. The Biological Efficiency of Potato Nitrogen. Jour- nal of Biological Chemistry, Vol. 30, page 201 (191 7). SrvEN. (Experiments on Protein Requirement.) Skandinavisches Archiv f. Physiologic, Vol. 10, page 91; Vol. 11, page 308. VoN NooRDEN. Metabolism and Practical Medicine, Vol. i, pages 283-383. Wilson. Nitrogen IMetabolism during Pregnancy. Bjilletin of the Johns Hopkins Hospital, Vol. 27, page 121 (191 6). CHAPTER IX INORGANIC FOODSTUFFS AND THE MINERAL METABOLISM The Elementary Composition of the Body From various estimates by different writers the average ele- mentary composition of the human body may be presumed to be approximately as follows : Oxygen, about 65. per cent Carbon, about 18. pier cent Hydrogen, about 10. per cent Nitrogen, about 3. per cent Calcium, about 2. per cent Phosphorus, about i. per cent Potassium, about 0.35 per cent Sulphur, about 0.25 per cent Sodium, about 0.15 per cent Chlorine, about 0.15 per cent Magnesium, about 0.05 per cent Iron, about 0.004 per cent Iodine "j f Very Fluorine > { minute Silicon J [ quantities Traces of some other elements such as manganese and alumin- ium may perhaps be normal constituents of the body also, and even arsenic has been discussed as a possible essential element. In this book only those elements are discussed of which the amounts concerned in daily metaboHsm can be measured quan- titatively by present methods. Since all of the substances in the body are continually under- going disintegration and renewal, it follows that there must be 234 INORGANIC FOODSTUFFS AND MINER.AL METABOLISM 235 a constant metabolism or exchange of every element which enters into body structure. More or less of each element must each day be metabolized and eHminated ; and, if equilibrium is to be maintained, an equal amount must be supplied. Simple proteins furnish only five of the fifteen chemical ele- ments which are known to be essential to human nutrition, while fats and carbohydrates are composed of but three of these five. Ten of the fifteen essential elements, or seven of the twelve which are essential in amounts sufficiently large to be measurable by present methods, must therefore be furnished by some ingredients of the intake other than simple proteins, fats, and carbohydrates. These same elements are found to remain either wholly or largely in the ash of food materials when the latter are burned in tne air ; and when the food is metab- olized in the body they are excreted chiefly in the form of mineral matter. These elements are therefore grouped as " ash constituents," " minerals," " mineral salts," " inorganic ele- ments," or " the inorganic foodstuffs " ; and their metabohsm is commonly designated as " the mineral metabohsm." None of these terms is entirely appropriate. To designate the ele- ments which remain in the ash when food is burned as ash con- stituents is accurate but not very instructive, since the materials of which a food ash is composed may have existed in quite dif- ferent forms of combination in the food before it was burned. The terms " mineral " and " inorganic " are likely to be some- what misleading. Some of the elements (as sodium and chlo- rine) do exist in the food and enter and leave the body in in- organic forms ; others (as iron and sulphur) exist in the food and function in nutrition as essential constituents of organic matter and become inorganic only as the organic matter is oxi- dized, i.e. only in the late stages of their metabolism ; still others (as phosphorus) are supplied to the body by the food in both organic and inorganic forms. The elements concerned in " the mineral metabolism " may 236 CHEMISTRY OF FOOD AND NUTRITION exist in the body and take part in its functions in at least three kinds of ways : (i) As bone constituents, giving rigidity and relative per- manence to the skeletal tissues. (2) As essential elements of the organic compounds which are the chief solid constituents of the soft tissues (muscles, blood cells, etc.). (3) As soluble salts (electrolytes) held in solution in the fluids of the body, giving these fluids their characteristic influence upon the elasticity and irritability of muscle and nerve, supply- ing the material for the acidity or alkalinity of the digestive juices and other secretions, and yet maintaining the neutrality or slight alkalescence of the internal fluids as well as their osmotic pressure and solvent power. A man under average conditions of diet, activity, and health usually excretes daily from 20 to 30 grams of mineral salts, consisting essentially of chlorides, sulphates, and phosphates of sodium, potassium, magnesium, and calcium (as well as am- monium salts from the protein metabolism). The purpose of this chapter and the one following is to sketch briefly the metaboHsm of these substances, with a more detailed quantitative study of the three elements (cal- cium, phosphorus, and iron) which assume an especial promi- nence in the practical problems of nutrition. Metabolism of Chlorides — Use of Common Salt Except for the hydrochloric acid of the gastric juice, prac- tically all the chlorine involved in metabolism enters, exists in, and leaves the body in the form of chlorides — much the greater part as sodium chloride. The amount of sodium chloride which is ordinarily added to food as a condiment is so large that the amounts of sodium and chlorine present in the various foods in the fresh state become of little practical consequence. Among animals the herbivora require salt while the carnivora do not, INORGANIC FOODSTUFFS AND MINERAL METABOLISM 237 the latter obtaining sufficient salt for their needs from the flesh, and more especially from the blood, of their prey. Sodium occurs, chiefly as chloride, abundantly in the blood and other fluids of the animal body and in much lower concen- tration in the tissues. Potassium, on the other hand, occurs to a greater extent as phosphate than as chloride. It is most abundant in the soft solid tissues — in the corpuscles of the blood, the protoplasm of the muscles, and other organs, and also in the highly speciaHzed fluids which some of the glandular organs secrete, notably in milk. Since the cells arc in constant contact with the circulating fluids, the abundance of potassium in the cells and of sodium in the fluids makes it evident that the taking up of salts by the cells is an active or " selective " process. A conspicuous function of the salts in the tissues is the maintenance of the normal osmotic pressure, but solu- tions of different salts of equal osmotic pressure are by no means interchangeable, and it is not possible to replace suc- cessfully the potassium in the cell by an equivalent amount of sodium. There ^eems to be a relation between the taking up of salt and the retention of water in the tissues. The effect of decreas- ing the salt in the diet is to decrease the quantity of salt in the tissues, and at the same time their water content. An explana- tion of this Ues in the fact that, since body tissues and fluids must maintain a constant concentration of sodium chloride, a reduction in the absolute quantity of salt must result in a cor- responding reduction in the quantity of water present. Attention is frequently called to the fact that sodium chloride is the only salt which we seem to crave in greater quantities than occur naturally in our food, and that we share this appetite with the herbivorous animals. Bunge holds that this is because a high intake of potassium (as in most vegetable foods) tends to increase sodium elimination. Bunge tested this theory upon his own person by taking 18 grams of potash (as phosphate and 238 CHEMISTRY OF FOOD AND NUTRITION citrate) in one day. This increased the eUmination of sodium chloride by 6 grams. In his Physiological and Pathological Chemistry (Chapter VII), Bunge records extended and interesting observations and discussion upon the relation of diet to the craving for salt, and concludes that while one might Hve without the addition of salt to the food even on a diet largely vegetarian, yet without salt we should have a strong disinclination to eat much of the vegetables rich in potassium, such as potatoes. " The use of salt enables us to employ a greater variety of the earth's prod- ucts as food than we could do without it." But also, accord- ing to Bunge : " We are accustomed to take far too much salt with our viands. Salt is not only an aliment, it is also a condi- ment, and easily lends itself, as all such things do, to abuse." While Bunge's explanations may not be entirely adequate in detail, there seems to be little doubt as to the correctness of his main deductions. Since the sodium chloride taken with the food passes through the body and is excreted by the kidneys without undergoing any chemica' change, the rate of excretion quickly adapts itself to the rate of intake within wide variations. When no chloride is taken, the rate of excretion falls rapidly to a point where the daily loss is only a very small fraction of the amount ordinarily consumed and excreted. Thus in an experiment by Goodall and Joslin * in which a healthy man was placed upon a diet adequate in protein and energy value but practically free from salt, the excretion of chlorine on each of 13 successive days was respectively: 4.60, 2.52, 1.88, 0.87, o.6g, 0.48, 0.46, 0.40, 0.26, 0.22, 0.22, 0.17, 0.17 grams. Cetti in ten days of fasting excreted all together 13.13 grams, and BelH in ten days on a diet poor in salt lost 11.8 grams of sodium chloride. In Benedict's recent study of prolonged fast- * Goodall and Joslin, Transactions oj the Association of American Physicians, Vol. 23, page 92 (1908). INORGANIC FOODSTUFFS AND MINER.\L METABOLISM 239 ing * his subject lost 8.44 grams of chlorine (equivalent to 13.93 grams sodium chloride) during the first ten days, 2.13 grams chlorine during the second ten days, and 1.57 grams chlorine during the third ten days of the fast. (The detailed data may be found on a later page.) Since the body is supposed to con- tain about 100 grams of sodium chloride, it will be seen that even when there was complete deprivation of salt for ten to thirty days, the total losses did not exceed 10 to 20 per cent of the amount estimated as usually present in the body. The salt thus readily given off by the body has been regarded by some as a measure of the excess which the body has been forced to carry in consequence of the extravagant amounts of salt which are commonly taken with the food. Magnus-Levy, how- ever, thinks that the reduced amount of sodium chloride left in the body after such a loss is " not a physiological optimum, but rather a physiological minimum." Moderate variations in the amount of salt taken have no significant effect upon metabolism. Large amounts increase the quantity of protein cataboHzed, and, through overstimulating the digestive tract, may also interfere with the absorption and utilization of the food. Metabolism of Sulphur Plants absorb sulphates from the soil and use the sulphur in the synthesis of proteins. Minute quantities of inorganic sul- phates may be taken by man in food and drink, but by far the greater part of the sulphur concerned in metabolism enters the body in organic combination and, so far as known, chiefly as protein. The metabolism of sulphur is therefore a part of the protein metaboHsm, and in many respects the metabolism of sulphur tends to run parallel with that of nitrogen. In a series of ten experiments (each of 3 to 5 days' duration) upon * Benedict, Publication No. 203, Carnegie Institution of Washington. 240 CHEMISTRY OF FOOD AND NUTRITION man,* in which the food consisted of bread and milk in varying amounts and proportions, the percentage absorption from the digestive tract was nearly the same for the sulphur as for the nitrogen of the food, and the excretion of the end products ran so closely parallel that in every case in which the body stored nitrogen it also stored sulphur, and vice versa. f It is well known that individual proteins show relatively much greater differences in sulphur than in nitrogen content, so the ratio of nitrogen to sulphur varies widely, as is shown by the following examples selected from the data for pure proteins compiled by Osborne : Kind of Protein Legumiri . . Zein . . . . Edestin . . . Gliadin . . . Leucosin . . Casein . . . Myosin . . . Serum globulin Egg albumin . Nitrogen Per Cent 18.04 16.13 18.69 17.66 16.80 15-78 16.67 15-85 15-51 SULPHDR Per Cent 0.385 0.600 0.88 1.027 1.280 0.80 1.27 I. II 1.616 Ratio of Nitro- gen TO Sulphur 46.9 26.9 21.2 17.2 13-1 19.7 I3-I 14-3 9.6 Thus, while many proteins approximate the usually assumed average of 16 per cent nitrogen and i per cent sulphur, there are considerable deviations from this ratio in both directions. Under ordinary conditions, however, no protein is eaten in a pure state, but only as the material containing it is used as an article of food. It is therefore the proportion of sulphur to the total protein of the food which determines the ratio of sulphur to nitrogen available for nutrition. ♦Bulletin 121, Office of Experiment Stations, U. S. Department of Agriculture. t Exceptions to such parallelism of nitrogen and sulphur balances have, however, been reported in certain pathological conditions. INORGANIC FOODSTUFFS AND MINERAL METABOLISM 241 The proportion of sulphur to total protein has been deter- mined in most staple foods, of which the following are repre- sentative : * Food Material SuLPHUK IN Percentage of Total Protein Lean beef 0.95-1.00 1.4 0.95-1.09 I.15-1.29 1.30 1-55 0.69-1.00 0.80-0.94 1.07 Eggs Milk Wheat flour, crackers Entire wheat Oatmeal Beans Peas Potatoes Taking these figures as typical, it would appear that in those staple foods which contribute the greater part of the protein of the diet, the ratio of protein to sulphur does not differ greatly, and that in most cases of ordinary mixed diet there would be consumed not far from i gram of sulphur in each 100 grams of protein. We may therefore expect that in health and on an ordinary diet the sulphur requirement will usually be covered when the protein supply is adequate. When proteins (or their cleavage products) are oxidized in the body, the sulphur becomes converted for the most part into sulphuric acid, which, of course, must be neutralized as rapidly as it is formed. The greater part of the sulphuric acid formed in metabolism appears in the urine as inorganic sulphates ; a smaller part is found combined with organic radicles in the form commonly known as " ethereal " or " conjugated " sulphates. The amount of ethereal sulphate or the ratio of ethereal to in- organic sulphate is quite variable, depending mainly upon the * In the data here given, nitrogen and sulphur were determined in the same specimens. Average percentages of protein and sulphur in nearly all important food materials may be found in Tables I and II, respectively, of the Appendix. R 242 CHEMISTRY OF FOOD AND NUTRITION amount and character of the intestinal putrefaction, which in turn is apt to be considerably influenced by the food. On ordi- nary mixed diet about one tenth or one twelfth of the sulphate sulphur in the urine ordinarily appears as ethereal sulphates; but when the meat in the diet is replaced by milk, the putre- faction is usually lessened and the proportion of ethereal sul- phates lowered. In one case of a healthy man who had been on a bread and milk diet for a week, only one thirtieth of the sulphate sulphur was in the form of ethereal sulphates. Not all of the metabolized sulphur is eliminated as mineral or " ethereal " sulphate ; a part is given off in less completely oxidized forms. This " unoxidized " or " neutral " sulphur usually constitutes in healthy persons on full diet from 5 to 15 per cent of the total sulphur eliminated. In Folin's experiment upon very low protein diet, although the total sulphur metab- olism was markedly decreased, the quantity of neutral sulphur excreted remained about constant, so that the relative proportion of sulphur appearing in this form was increased. Metabolism of Phosphorus Phosphorus compounds are as widely distributed in the body and as strictly essential to every living cell as are proteins. Phosphates are constantly excreted from the body even after long fasting. During a fast the rate of excretion of phosphates does not fall off rapidly like that of chlorides, but tends to run more nearly parallel with the nitrogen excretion, as would be expected in view of the fact that the phosphates of the urine represent not only an excretion of preexistent salts, but also the result of the metabolism of body tissue. Some of the relations of the phosphorus compounds to nutri- tional functions are outlined by Forbes and Keith as follows : " Among the several inorganic elements involved in animal life phosphorus is of especial interest. No other one enters into such a diversity of compounds and plays an important INORGANIC FOODSTUFFS AND MINERAL METABOLISM 243 pari in so many functions. Structurally, it is important as a constituent of every cell nucleus and so of all cellular structures ; it is also prominent in the skeleton, in milk, in sexual elements, glandular tissue, and the nervous system. Functionally, it is involved in all cell multiplication, in the activation and control of enzyme actions, in the maintenance of neutrality in the organism, in the conduct of nerve stimuli, and through its rela- tion to osmotic pressure, surface tension, and imbibation of water by colloids it has to do with the movement of liquids, with the maintenance of proper liquid contents of the tissues, with cell movements, and with absorption and secretion " (Ohio Agricultural Experiment Station, Technical Bulletin No. 5, page 11). While the phosphorus compounds of the body and of the food are very^numerous and might be classified differently according to the standpoint from which they are being considered, it will be convenient for our present purposes to divide them into four main groups : 1. Inorganic phosphates, of which potassium phosphate is probably the most abundant in food and in the fluids and soft tissues of the body, while calcium phosphate is the chief inorganic constituent of bones. 2. Phosphorus-containing proteins, including the nucleo- proteins of cell nuclei, the lecitho-proteins, and the true phos- phoproteins such as casein or caseinogen of milk and ovovitellin of egg yolk. 3. Phosphatids, phospholipins or phosphorized fats — includ- ing lecithins, lecithans, kephalins, etc. — which occur in large quantity in brain and nerve tissue and in smaller concentra- tion (but probably as essential components) in all the cells and tissues of the body, not only of man, but of plants and animals generally. The phosphatids are therefore widely distributed in food materials, but are found in extremely varying proportions in foods of different types. Egg yolks are conspicuously rich 244 CHEMISTRY OF FOOD AND NUTRITION in phosphatids, about two thirds of the phosphorus of the egg being present in this form. 4. Phosphoric acid esters of carbohydrates and related sub- stances such as inositol (" inosite ") and the natural salts of such esters. The calcium, magnesium, and potassium salts of " phytic acid," * collectively known as phytates, phytins, or phytin, have for some years been regarded as the most abundant phosphorus compounds of the wheat kernel and probably of the grains and legumes generally, if not of all vegetable foods. Recent investigations indicate, however, that not all the phos- phorus compounds which were supposed to be phytins are really salts of phytic acid. As has been explained in Chapter I, the recent work of Northrup and Nelson indicates that starch con- tains phosphorus as an essential constituent, and there are other indications of phosphorus-containing carbohydrates or carbo- hydrate-phosphoric acid esters in food materials and also of the formation of hexose-phosphoric acid esters in the body in the course of the carbohydrate metaboHsm. Thus we may think of the phosphorus with which we have to deal in food and nutrition as being partly in the form of in- organic phosphates and partly in combination with (or present as a constituent of) each of the three groups of organic food- stuffs — proteins, fats, and carbohydrates, or closely related substances. In the course of digestion and metabolism the phosphoric acid radicles are split off from the organic radicles and ulti- mately nearly all of the phosphorus leaves the body as inorganic phosphate. To what extent the cleavage of the organic phos- phorus compounds occurs in the digestive tract under ordinary conditions and to what extent, if at all, the phosphorus of phos- phoproteins or phosphatids, for example, is absorbed in organic form is still a subject of investigation. * Phytic acid is probably inositol-hexa-orthophosphoric acid, CsHziOjiPe (Rob- inson and Mueller). INORGANIC FOODSTUFFS AND MINERAL METABOLISM 245 Interrelations of Phosphates, Phosphoproteins, and Phosphatids Phosphates, nucleoproteins, and phosphatids are all promi- nent as body constituents. The insoluble phosphates constitute the chief mineral matter of bone ; while soluble phosphates are essential constituents of the blood and protoplasm. It is largely to the presence of the phosphates that the blood and protoplasm owe their ability to remain neutral or faintly alkaline, notwithstanding the constant production of acid in metabolism, as will be seen in connection with the discussion of the maintenance of neutrality below. The nucleoproteins as constituents of cell nuclei and the phos- phatids as prominent constituents of brain and nerve tissue and as less prominent but doubtless essential components of the tissues generally have functions distinct from each other and from the phosphates. On the assumption of a more active metaboHsm in the cell nuclei or in the brain and nerve tissue than in the bones, there has sometimes been a tendency to regard fluctuations of phosphorus output as indicative of increased or decreased metabolism of nucleoproteins or phosphatids. It is probable, however, that the eliminated phosphorus represents more largely material which has functioned as phosphate. One reason for this is that the bones contain so large a share of the total phosphorus of the body. According to Voit's estimate, a man's skeleton contains about 600 grams of phosphorus; his muscles, about 56 grams ; his brain and nerves, about 5 grams. With the bones in possession of such a predominant share of the body phosphorus, it would seem that the metabolism of bone tissue, even though relatively inactive, must exert a con- siderable influence upon the phosphorus output. Moreover, the soluble phosphates of the blood and protoplasm are con- stantly tending to be eliminated from the body (through the kidneys or the intestinal walls or both) and perhaps increasingly 246 CHEMISTRY OF FOOD AND NUTRITION so in proportion as ihcy become changed into acid phosphates in the performance of their function of maintaining neutrality by reacting with the acids produced in metai)olism. Before taking up the quantitative study of the phosphorus requirement we must consider the nutritive relations of the diflerent t^q^es of phosphorus compounds, and whether these are sufBciently interchangeable in nutritive function so that one may properly speak of phosphorus requirement, simply, \\dthout discriminating between phosphates, phytates, phosphoproteins, and phosphatids. Such experimental evidence as is cited here will be given in general in chronological order, to indicate, if possible, how pres- ent \'iews have actually developed, and to suggest that they may at any time require modification as a result of further research. Meischer studied the formation of complex from simpler phos- phorus compounds in the adult animal body by observations upon the Rhine salmon, which during the breeding season re- main a long time in fresh water, taking no food, but developing large masses of roe and milt at the expense of muscular tissue. This process evidently involves the formation of considerable amounts of nucleoproteins and phosphatids from simpler pro- teins, fats, and phosphorus compounds of the muscles. Paton * has studied the salmon of Scotland with similar results. Is there then any advantage in feeding phosphorus in organic forms? Marcuse,t followed by Steinitz,J Zadik, § and Leipziger, || studied, by metabolism experiments on dogs, the nutritive value of phosphoproteins, when fed to the exclusion of phosphates and when contrasted with equivalent amounts of phosphorus and nitrogen fed in the form of mixtures of inorganic phosphates and simple proteins. Casein and ovovitellin were taken as * Journal of Physiology, Vol. 22, page 333. t Archiv Jiir die gesammle Physiologic (I^fluger), Vol. 67, page 373. X Ibid., Vol. 72, page 75. § Ibid., V'ol. 77, page i. II Ibid., Vol. 78, page 402. INORGANIC FOODSTUFFS AND illNERAL METABOLISM 247 typical phosphoproteins and compared with either myosin or edestin fed with inorganic phosphates. Rohmann * summarized the results as a whole and found a striking difference in the phos- phorus balances in favor of the phosphoproteins as against the mixtures of simple proteins with inorganic phosphates. The storage of nitrogen was also more pronounced in the periods in which the phosphorized proteins were fed. The results appear to justify Rohmann's conclusion that the nutritive values of phosphorized and phosphorus-free proteins are not entirely the same, the former being especially adapted to furnish the material for tissue growth. In experiments upon men, Ehrstrom f and Gumpert J have found that a smaller amount of phosphorus will maintain phos- phorus equilibrium when taken in the form of casein than when taken largely as dicalcium phosphate or as meat, the phosphorus of which is largely in the form of potassium phosphate. On the other hand Keller § in a study of the phosphorus metabolism of young children found evidence that storage of phosphorus was favored by food (like milk) which contained a liberal supply of phosphates in addition to the organic phosphorus compounds; and Von Wendt found that the loss of phosphorus occurring on a diet very poor in ash could be greatly reduced by the addition of dicalcium phosphate to the food. In cow's milk the greater part of the phosphorus appears to exist as phosphate, but there can be no doubt that the milk phosphorus as a whole is available for the needs of the young of the species, especially in view of the parallelism pointed out by Bunge and Abderhalden between the phosphorus and cal- cium content of milk and the rate of growth of the young. (See accompanying table.) * Berlin klinische Wochensckri/l, Vol. 35, page 78g. "I" Skandinavisches Archiv fiir Physiologic, \o\. 14, page 82. X Medische Klinik, Vol. i, page 1037. § Archiv J Ur KinderheUkunde, Vol. 29, page i. 248 CHEMISTRY OF FOOD AND NUTRITION No. OF Days Required to Double the Birth Weight Percentage Composition of Milk (Partial) Spectes Protein Ash Calcium Phosphorus Human Horse Cow Goat Sheep Swine Dog Rabbit 1 80 60 47 22 15 14 9 6 1.6 2.0 3-5 3-7 4-9 5-2 7-4 14.4 0.2 0.4 0.7 0.78 0.84 0.80 1-33 2.50 0.02 0.09 0.12 0.14 0.18 0.18 0.32 0.65 0.02 0.06 0.09 0.18 O.II 0.14 0.22 0-43 It is, however, not without possible significance that the phos- phorus of human milk is mainly in organic forms (Soldner) and that, notwithstanding its much lower content of total phos- phorus, human milk contains as high a percentage of lecithin as does cow's milk (Stoklasa). An infant fed on diluted cow's milk must therefore receive less lecithin than the breast-fed infant while it may receive more total phosphorus. In general the more recent investigations favor the view that the body can use inorganic phosphates to meet all its phosphorus requirements. Hart, McCollum, and Fuller showed in 1909 that with young pigs on a ration too poor in phosphorus to support normal growth the deficit could be made good by feeding phosphates as well as by feeding foods containing organic phosphorus compounds. The following year (1910) McCollum reported that, other things being satisfactory, all the phosphorus requirements of an animal can be met by feeding inorganic phosphates. In one of these experiments McCollum kept a rat for 104 days on diets of purified food materials in which phosphorus was given only as phosphate. It maintained good condition but suffered some loss of weight as it would not eat enough of the artificial food to meet the energy requirement. In another case in which an amino acid mixture from the hydrolysis of beef muscle was INORGANIC FOODSTUFFS AND MINERAL METABOLISM 249 added to the diet the food was eaten more readily and one rat increased in weight from 153 to 176 grams while receiving only inorganic phosphorus. As young rats eat unpalatable food more readily than do adults, McCollum fed the ration containing phosphate as sole source of phosphorus to young growing rats, one of which ate the ration for 127 days, during which time he doubled in weight. At the end of this experiment the rat was killed and analyzed and found to be of normal composition. There was therefore no reason to doubt that the rat synthesized the nucleoproteins and phosphatids of his growing tissues from the inorganic phos- phorus of his food. Subsequent experiments by McCollum and Davis, as well as those of Osborne and Mendel described in connection with the discussion of proteins (Chapter III), afford many instances of long-continued growth of rats on rations made up of " isolated " foodstuffs in which all or nearly all of the phosphorus was in the form of simple phosphates. In order to determine whether the synthesis of lecithin in the animal body can be demonstrated experimentally, McCollum, Halpin, and Drescher (191 2) fed 3 hens for 10 weeks a ration consisting of 30 per cent skim milk powder and 70 per cent polished rice, both of which were freed from phosphatids. This diet it will be noted contained phosphoprotein as well as phos- phate, but very little fat, and it was believed no phosphatid. The hens produced eggs in normal number and of normal com- position. The phosphatid in the eggs produced was 27.65 grams per hen, and this was behaved to have been synthesized rather than to have come from material previously stored. Fingerhng (191 2) kept ducks for 8 months on a diet of pota- toes, blood albumin, starch, and lime salts. The ducks laid normally and the phosphatid content of the eggs produced was determined. Since the phosphatid content of the food must have been small and the feces always contained some lecithin- 2 50 CHEMISTRY OF FOOD AND NUTRITION like substances, and since the ducks did not lose weight, Finger- ling concludes that the organic phosj^horus compounds in the eggs were synthesized from inorganic phosphorus obtained in the food. Later he fed the same ducks on food richer in organic phos- phorus ; and as they produced about the same number of eggs of similar phosphatid content he concluded that the egg-phos- phatids were synthesized as readily from inorganic as from organic phosphorus compounds. The evidence seems sufficient to warrant the statement that animal organisms are able to synthesize nucleoproteins, phospho- proteins, and phosphatids from inorganic phosphate. It may, however, still be questioned whether the nutritive conditions are as favorable when the body is forced to do this as when a part at least of the phosphorus reciuirement is met by feeding phosphoproteins and phosphatids. The above-mentioned experiments of Rohmann and his pupils on dogs and of Ehrstrom and Gumpert on men seemed to demonstrate that the phosphoproteins have a higher food value than a corresponding mixture of simple proteins and simple phosphates ; and the recent feeding experiments, while showing the efficiency of phosphates in meeting the phosphorus require- ment, do not show conclusively that the phosphates are of fully equal value with the organic phosphorus compounds. Feeding experiments of long duration are well fitted to give convincing evidence on the former point, but are not so well suited for the purposes of exact quantitative comparisons because the very fact of their long duration gives opportunity for other factors to enter, such as differences in vitality among the experimental animals. Masslow, as the result of recent investigation of phosphorus metabolism during growth, holds (1913) that for the best results a considerable part of the phosphorus should preferably be supplied in organic forms. Some writers have argued that the presence in extracts of intestinal mucosa of enzymes capable of splitting off phosphoric INORGANIC FOODSTUFFS AND MINERAL METABOLISM 25 1 acid from the organic phosphorus compounds of the food may be taken as evidence that phosphorus is absorbed as phosphoric acid or phosphate whatever the form in which it occurs in the food ; but in view of the reversibihty of enzyme action and the great extent to which it is influenced by conditions, it seems pref- erable to form our impressions regarding the equivalence or relative values of the different phosphorus compounds from observations or experiments upon animals rather than from tests for enzymes in tissue extracts. Forbes holds that even though the phosphorus be absorbed as inorganic phosphate there is advantage in having it supplied largely in organic . forms since " much larger amounts of phos- phorus may be utilized in a normal manner if they are gradually liberated in the usual way by the digestive cleavage of the organic complexes with which they are combined." * Forbes and Keith (1914) after reviewing most thoroughly the whole literature of phosphorus compounds in animal metab- olism, draw, among others, the following conclusion : " That organic phosphorus is absolutely essential to any animal has not been demonstrated. The proof that inorganic phosphorus can serve all of the purposes for which any animal needs phosphorus is incomplete.! There is much evidence to imply that, with some species at least, some organic phosphorus compounds are more useful than is inorganic phosphorus in the sense of being more readily and economically utihzed, and of maintaining a higher state of vitahty as revealed by tissue enzyme estimations, the difference probably depending, in part at least, on the fact of the partial absorption and utilization of organic phosphorus compounds as such, without complete diges- tive cleavage " (Ohio Agricultural Experiment Station, Tech- nical Bulletin No. 5, pages 364-365). ^= Ohio Agricultural Experiment Station, Technical Bulletin No. 5, page 357. t (Some of the experiments of Osborne and Mendel and of McCollum and Davis have appeared since the above was written by Forbes and Keith. H. C. S.) 252 CHEMISTRY OF FOOD AND NUTRITION On the other hand Marshall * considers the evidence fully sufficient to warrant the conclusion that organic phosphorus compounds are of no more value as food than are the inorganic phosphates. In the present state of our knowledge there is at least no quantitative measure of differences in nutritive value as between different forms of phosphorus. If differences in nutritive value between the different groups of phosphorus compounds exist, they are doubtless in favor of the phosphoproteins and phos- phatids and are more significant for the growing than for the full-grown organism. For the reasons explained in Chapters VIII, XIII, and XIV the diet of growing children should always contain a Hberal allowance of milk. The milk will pro- vide, in addition to the best form of protein, a high proportion of phosphoprotein and also significant quantities of phosphatids. Hence it seems justifiable to assume that, if the food is properly selected, one may compute its total phosphorus content and compare it with the total phosphorus requirement of the body without separate computation of the different forms of phos- phorus. Estimation of the Phosphorus Requirement Since phosphorus compounds are essential to all the tissues of the body, the growth of new tissue requires a storage of phosphorus along with that of protein, but aside from this it is evident that the phosphorus metabolism presents a separate problem from the metabolism of protein. The phosphorus of the tissues exists largely in the form of nucleoproteins — the characteristic substances of cell nuclei — and, as these are important in metabolism, there was a tendency for a number of years to regard the phosphorus elim- ination as largely a measure of the metabolism of nucleo- proteins somewhat as the nitrogen is taken as a measure of the * Journal of the American Medical Association, Vol. 64, page 573 (1915). INORGANIC FOODSTUFFS AND MINERAL METABOLISM 253 metabolism of proteins in general. It is probable, however, that such a view of the phosphorus metabolism is of only very Hmited application, because of the influence of other factors. Voit showed that the material metabolized in fasting comes largely from the bones. Undoubtedly the bones take part in the daily metabolism, and while they may undergo a less active exchange of material than the soft tissues, they possess such a large proportion of the phosphorus in the body that they prob- ably contribute a considerable part of what is metabolized from day to day. Moreover, recent investigations upon the func- tion of the soluble phosphates of the blood in maintaining neu- trahty in the body indicate that the neutralization of acid by conversion of di- into mono-phosphates may be followed by an increased excretion of the acid phosphate in the urine. Finally, it is evident that the amount of phosphorus metaboHzed is very directly influenced by the amount taken in the food. The phosphorus which has been metabohzed is excreted from the body almost entirely in the form of inorganic phosphates, the organic phosphorus of the urine constituting as a rule only I to 3 per cent of the total.* Carnivorous animals excrete phos- phates mainly through the kidneys, but in the herbivora the excretion occurs almost entirely through the intestinal wall, whether the phosphate be taken by the mouth, or injected sub- cutaneously, or be formed by metabolism of organic phosphorus compounds in the body. In man, the ehmination of metab- olized phosphorus is partly through the kidneys and partly through the intestinal wall, the relative quantities in urine and feces varying within rather wide hmits. As a rule, foods rich in calcium, or which yield an alkaline ash, tend to increase the proportion of phosphorus excreted by way of the intestine. Attempts have sometimes been made to estimate the phos- phorus requirement from the amount excreted in the urine. * Some investigators have doubted the occurrence of organic phosphorus in urine while others have estimated it as high as 6 per cent of the total urinary phosphorus. 254 CHEMISTRY OF FOOD AND NUTRITION The results thus obtained are always too low (usually very much so), and are largely responsible for the fact that the amount of phosphorus required for the normal nutrition of man is seriously underestimated in many of the standard textbooks. Since the excretion of metabolized phosphorus through the intestine is in man too large to be neglected and too variable to be allowed for by calculation, we can expect reliable data on phosphorus requirements from those experiments only in which the amounts of phosphorus are actually determined in food, in feces, and in urine. In such experiments it is found (as in the case of nitrogen) that the output obtained upon the experi- mental days is influenced not only by the food taken at the time, but also by the rate of metabolism to which the body had been accustomed on the preceding days. This is shown by the following results obtained in a 12-day series of experiments upon a healthy man: Phosphorus Metabolism ^\^TH Different Amounts of Phosphorus IN THE Food Experimental Period Phosphorus per Day No. Duration In Food Grams In Feces Grams In Urine Grams Output Grams Balance Grams I II III 3 days 6 days 3 days 0.40 0.77 I-5I 0.45 O.IQ 0.50 0.70 0.72 0.99 I-I5 0.91 1.49 - 0-75 - 0.14 -}- 0.02 Here the output of phosphorus was greater in the first period with 0.40 gram in the food than in the second when the food furnished 0.77 gram, probably because the first period followed and was influenced by a preceding diet fairly rich in phosphorus, whereas the output in Period II was influenced by the low- phosphorus diet of Period I. For the same reason Period II offered favorable conditions for the establishment of equilibrium INORGANIC FOODSTUFFS AND MINERAL METABOLISM 255 on a minimum diet, and the results show that in this case the subject was unable to reach equilibrium on 0.77 gram per day, the output averaging 0.91 gram. When the intake was in- creased to 1. 5 1 grams, the output rose rapidly and averaged 1.49 grams. In this case the amount which would have been just sufficient for equilibrium evidently lay between 0.91 and 1.49 grams per day. By means of well-planned experiments or series of experiments it is possible to fix for a given individual much narrower limits within which the exact amount required for equilibrium must lie, and when it is known that the intake approximates this required amount, it is justifiable to regard the output as an indication of the normal nutritive requirement. Study of the data of 93 such phosphorus balance experiments upon 27 subjects, 21 men and 6 women, has shown a range of 0.52 to 1.75 grams with an average of 0.96 gram phosphorus (2.20 grams PoOs) per 70 kilograms of body weight per day. This corresponds with the average requirement of 50 grams protein per day per man of 70 kilograms as estimated on page 220. Allowing 50 per cent above the bare minimum would give a phosphorus " standard " of 1.44 grams (3.30 grams P2O5) corresponding to a protein " standard " of 75 grams. Phosphorus in Food Materials and Typical Dietaries A comparison of the amounts of phosphorus contained in the food of typical American famihes with the amounts metabohzed in the experiments above mentioned indicates that a freely chosen diet does not always furnish an abundance of phosphorus compounds. In 150 American dietaries of families or larger groups believed to be fairly representative, the estimated amount of phosphorus furnished per man per day was below 0.96 gram in 7 cases, while in no case was there less than 50 grams of pro- tein per man per day. If we allow a margin of 50 per cent for safety in both protein and phosphorus, we find 8 per cent of the dietaries below the protein standard of 75 grams and 41 per 256 CHEMISTRY OF FOOD AND NUTRITION Approximate Amounts of Phosphorus in Food Materials Food Phosphorus PER 100 Grams Edible Substance Phosphorus per 100 Graus Protein Phosphorus per 3000 Calories Beef, all lean Eggs Egg yolk Milk Cheese Wheat, entire grain . . . White flour Rice, polished .... Oatmeal Beans, dried Beets Carrots Potatoes Turnips Apples Bananas Oranges Prunes, dried Almonds Peanuts Walnuts 0.218 .180 •524 •093 .683 •423 .092 .096 •392 .471 •039 .046 .058 .046 .012 ■031 .021 •105 •465 •399 •357 0.96 1-35 2.73 2.82 2.58 3-25 .81 1. 19 2.36 2.20 2.42 4.17 2.60 3-55 3-iS 2.35 2.58 5.00 2.25 1-55 1.96 5-2 3-66 3-54 4.02 4.68 3-54 •78 .81 2.97 4.ir 2.52 303 2.07 3.51 0.60 0.93 1.20 1-05 2.16 2.19 1-53 cent below the phosphorus standard of 1.44 grams. These results indicate plainly that present food habits are more hkely to lead to a deficiency of phosphorus compounds than to a deficiency of protein in the diet, and it is not improbable that many cases of malnutrition are really due to an inadequate supply of phosphorus compounds. INORGANIC FOODSTUFFS AND MINERAL METABOLISM 257 That the cases of low phosphorus dietaries are not to be ascribed simply to inadequacy of the total food supply of these families was shown by computing the amounts of phosphorus which would have been furnished in each case had the total amount of food been so increased or decreased as to furnish just 3000 Calories per man per day. On this basis only one of the 150 dietaries shows less than 0.96 gram, but 49 of them or 2,1, per cent show less than 1.44 grams of phosphorus, as against only 2 per cent with less than 75 grams of protein, per 3000 Calories. The table on the preceding page compares some staple foods as sources of phosphorus. It will be seen that, whether compared on the basis of weight, or of protein content or energy value, the different staple foods vary greatly in phosphorus content. In the planning of dietaries this fact should be kept in mind and care taken that foods fairly rich in phosphorus be adequately represented in each day's food. REFERENCES (See also the references at the end of the next chapter.) Abderhalden. Lehrbuch der Physiologische Chemie, 3 Aufl., Vorlesungen 34-37- Anderson. The Organic Phosphoric Acid Compound of Wheat Bran, Journal of Biological Chemistry, Vol. 20, pages 463, 475, 483, 493 (1915). Babcock. Metabolic Water. Wisconsin Agricultural Experiment Station, Research Bulletin 22 (191 2). Bayliss. Principles of General Physiology, Chapters 7 and 8. Benedict. A Study of Prolonged Fasting. Carnegie Institution of Washington, Publication No. 203, pages 247-291. BoUTWELL. The Phytic Acid of the Wheat Kernel and Some of Its Salts. Journal of the American Chemical Society, Vol. 39, page 491 (191 7). BuNGE. Physiological and Pathological Chemistry, Chapters 7 and 8. Ehrstrom. Phosphorus Metabolism in Adult Man. Skandinavisches Archivfur Physiologie, Vol. 14, pages 82-1 11 (1903). Emmett and Grindley. a Study of the Phosphorus Content of Flesh. Journal of the American Chemical Society, Vol. 28, pages 25-63 (1906). S 258 CHEMISTRY OF FOOD AND NUTRITION Eppler. Investigations of Phosphatids, especially those of the Egg Yolk. Zeitschrift fur physiologisclie Cliemie, Vol. 87, pages 233-254 (1913). FiNGERLlNG. Formation of Organic from Inorganic Phosphorus Compounds in the Animal Body. Zeitschrift fiir Biologic, Vol. 38, page 448 ; Vol. 39, page 239 (191 2). Forbes. The Mineral Elements in Animal Nutrition. Ohio Agricultural Experiment Station, Bulletin 201 (1909). Forbes. Specific Effects of Rations upon the Development of Swine. Ohio Agricultural Experiment Station, Bulletins 213 and 283. Forbes and Keith. A Review of the Literature of Phosphorus Compounds in Animal Metabolism. Ohio Agriculture Experiment Station ; Technical Bulletin No. 5 (19 14). Hart, McCollum, and Humphrey. Rdle of the Ash Constituents of Wheat Bran in the Metabolism of Herbivora. American Journal of Physiol- ogy, Vol. 24, pages 86-103 (1910). Hawk. The Relation of Water to Certain Life Processes and more es- pecially to Nutrition. Biochemical Bulletin. Vol. 3, page 420 (1914). GuMPERT. Metabolism of Nitrogen, Phosphorus, Calcium, and Magnesium in Man. Medizinische Klinik, Vol. i, page 1037 (1905). Hart, McCollum, and Fuller. The R61e of Inorganic Phosphorus in the Nutrition of Animals. Wisconsin Agricultural Experiment Station, Research Bulletin No. i; American Journal of Physiology, Vol. 23, page 246 ( I 908-1909). Herbst. Calcium and Phosphorus in Growth at the End of Childhood. Zeitschrift der Kinderheilkunde, Vol. 7, page 161 (1913). Jordan, Hart, and Patten. ^Metabolism and Physiological Effects of Phosphorus Compounds of Wheat Bran. New York State .Agricultural Experiment Station, Technical Bulletin No. i ; and American Journal of Physiology, Vol. 16, page 268 (1906). McCollum. Nuclein Synthesis in the Animal Body. Wisconsin Agri- cultural Experiment Station, Research Bulletin No. 8 (1910). McCollum, Halpin, and Drescher. Synthesis of Lecithin in the Hen and the Character of the Lecithin Produced. Journal of Biological Chemistry, Vol. 13, page 219 (1912). McCri^dden and Fales. Complete Balance Studies of Nitrogen, Sulphur, Phosphorus, Calcium and Magnesium in Intestinal Infantilism. Jour- nal of Experimental Medicine, Vol. 15, page 450 (1912). McLean. On the Occurrence of a Mon-amino-diphosphatid Lecithin-like Body in Egg Yolk. Biochemical Journal, Vol. 4, page 168 (1909). Marshall. Comparison of Value of Organic and Inorganic Phosphorus. Journal of the American Medical Association, Vol. 64, page 573 (1915). INORGANIC FOODSTUFFS AND MINERAL METABOLISM 259 Masslow. Significance of Phosphorus for the Growing Organism. Bio- chcmisches Zeitschrifl, Vol. 55, page 45 ; Vol. 56, page 174 (1913). Meischer. Biochemical Studies on the Rhine Salmon. Archiv fiir Ex- perimental Pathologic mid Pharmacologie, Vol. 37, page 100 (1896). Plimmer. The Metabolism of Organic Phosphorus Compounds. Their Hydrolysis by the Action of Enzymes. Biochemical Journal, Vol. 7, page 48 (1913)- Schlossmann. On the Kind and Amount of Phosphorus in Milk and its Significance in Infant Nutrition. Archiv fiir Kinderhcilkundc, Vol. 40, page I (1905)- Sherman, Mettler, ant) Sinclair. Calcium, Magnesium, and Phos- phorus in Food and Nutrition. U. S. Department of Agriculture, Office of Experiment Stations, Bulletin 227 (1910). CHAPTER X INORGANIC FOODSTUFFS AND THE MINERAL METABOLISM (Continued) Metabolism of Sodium, Potassium, Calcium, Magnesium The distribution of sodium and potassium in the body and some of their mutual relations in metabolism have been referred to in the section on the chlorides. The distribution and func- tions of calcium have been studied in greater detail than -those of magnesium. It is estimated that about 85 per cent of the mineral matter of bone, or at least three fourths of the entire ash of the body, consists of calcium phosphate. Probably over 99 per cent of the calcium in the body belongs to the bones, the remainder occurring as an essential constituent of the soft tissues and body fluids. Of the magnesium in the body about 71 per cent is contained in the bones (Lusk). The muscles contain considerably more magnesium than calcium ; the blood contains more calcium than magnesium. That calcium salts are necessary to the coagulation of the blood has long been known and frequently cited as an example of the great importance of calcium salts to the animal economy. Equally striking is the function of these salts in regulating the action of heart muscle. It is well known that heart muscle may be kept beating nor- mally for hours after removal from the body when supplied, under proper conditions, with an artificial circulation of blood or lymph or a water solution of blood ash. Howell, Loeb, and others have studied the parts played by the several ash con- 260 INORGANIC FOODSTUFFS AND MINERAL METABOLISM 261 stituents. The sodium salts take the chief part in the main- tenance of normal osmotic pressure and have also a specific influence. Contractility and irritabihty disappear if they are absent, but when present alone they produce relaxation of the muscle tissue. Calcium salts also, although occurring in blood in very much smaller quantity, are absolutely necessary to the normal action of the heart muscle ; while if present in quantities above normal, they cause a condition of tonic contraction (" cal- cium rigor "). There is a balance which must be maintained between calcium on the one hand and sodium (and potassium) on the other. Thus it is found that the alternate contractions and relaxations which constitute the normal beating of the heart are dependent in part upon the presence of a sufficient but not excessive concentration of calcium salts, and in part upon the quantitative relationship of calcium to sodium and potassium, in the fluid which bathes the heart muscle. Other active tissues of the body doubtless have analogous requirements as to in- organic salts. Regarding the adequacy of the ordinary intake to meet the specific requirements for sodium, potassium, calcium, and mag- nesium, it would seem that only in the case of calcium is it ordi- narily necessary to take thought in the selection of food materials or the arrangement of dietaries. The amount of sodium chlo- ride usually added to food is much more than sufficient to meet the sodium requirement of the body, even if the natural sodium content of the food be entirely disregarded. Potassium and magnesium are relatively abundant in meat (muscle) and also in most plant tissues, so that an ordinary mixed diet, unless it consist too largely of highly refined food materials, wall usually furnish a safe surplus of these elements. Dietaries entirely adequate in energy value and protein content may, however, contain too little calcium. Calcium requirement is therefore a question of much practical importance in human nutrition, and requires quantitative study. 262 CHEMISTRY OF FOOD AND NUTRITION The Calcium Requirement Calcium constitutes a larger proportion of the body weight (about 2 per cent) than does any other of the " inorganic " ele- ments. It is very unevenly distributed in the body, over 99 per cent of the total amount being in the bones. It is also very irregularly distributed among the staple articles of food, many of which are extremely poor in calcium, while milk contains it in abundance. The " ordinary mixed diet " of Americans and Europeans, at least among dwellers in cities and towns, is prob- ably more often deficient in calcium than in any other chemi- cal element. In studying the effects of insufficient calcium, Voit kept a pigeon for a year on calcium-poor food without observing any effects attributable to the diet until the bird was killed and dis- sected, when it appeared that, although the bones concerned in locomotion were still sound, there was a marked wasting of calcium salts from other bones such as the skull and sternum, which in places were even perforated. Thus in adults there may be a continued loss of calcium without the appearance of any distinct symptoms because the losses from the blood and soft tissues may be replaced by calcium withdrawn from the bones. The injurious effect of an insufficient intake of calcium is of course more noticeable with growing than with full-grown animals. Abnormal weakness and flexibility of the bones (^re- sembling the condition of rickets in children) has been produced experimentally by feeding puppies with lean and fat meat only, while others of the same litter, receiving the same food, but with the addition of bones to gnaw, developed normally. In this con- nection it should be remembered that no animal is literally car- nivorous in nature, that is, none lives on flesh alone ; the animals called carnivora always eat more or less of the bones of their prey. According to Herter * many cases of arrested development in * On Injanlilism jrom Chronic Intestinal Injection, New York, 1908. INORGANIC FOODSTUFFS AND jNIINERAL METABOLISM 263 infancy may be due to an insufficient assimilation of calcium from the food. Such a deficiency in the amount assimilated may be due to defective digestion or to a diet inadequate in calcium content. Many medical writers have attributed different diseases to inadequate calcium supply or disturbance of calcium metabo- lism. Conclusive proof or disproof of such theories would how- ever require more detailed and exact quantitative studies of the intake and output of calcium in health, and the amounts re- quired in normal nutrition at different ages and under different conditions, than have yet been made. The fact that normal urine has a low calcium content while the feces usually contain much the greater part of the calcium which has been taken in the food has often been interpreted as meaning that the absorption of food calcium is poor or that the calcium requirement of the body is low. It is now known, how- ever, from experimental evidence, that most of the calcium which has been absorbed and carried through the metabolic processes is normally excreted through the intestinal wall and thus leave? the body in the feces instead of the urine. When the diet is very poor in calcium and the output of this element materially exceeds the intake, the feces often contain a larger amount of calcium than was present in the food. Observations upon Breithaupt and Cetti showed a consider- able elimination of calcium in the feces during fasting. On the other hand, Benedict reports the result of a 31-day fast during which no feces were passed, but considerable quantities of calcium continued to be lost through the urine throughout the entire period. On account of the fluctuating distribution of the calcium be- tween urine and feces, conclusions regarding the calcium re- quirement can properly be drawn only from those experiments in which the amounts of this element in the food, in the feces, and in the urine have been directly determined. A compilation of 264 CHEMISTRY OF FOOD AND NUTRITION such experiments has been made, and the reported results cal- culated to a uniform basis of 70 kilograms of body weight. On this basis, 63 experiments on 10 subjects (6 men and 4 women) show calcium outputs ranging from 0.27 to 0.78 gram and averaging 0.45 gram of calcium " per man per day." This includes the experiments which appear most rehable as indicat- ing the actual (minimum) requirement in that the food did not furnish an excess of calcium over the needs of the subject, and the calcium balance showed a reasonable approach toward equihbrium. It will be noted that this average of 0.45 gram calcium (equivalent to 0.63 gram CaO) represents the expendi- ture under conditions of closely restricted calcium intake. It corresponds to the average of 49.2 grams of protein per man per day reached on page 220, and approximates the minimum of actual need rather than a normal allowance. The margin for safety should probably be larger for calcium than for protein because of the likehhood of relatively greater losses in cooking and in digestion, while there is much less danger of any injurious result from surplus calcium than from surplus protein. Nelson and Williams have recently found the calcium output of four healthy men on normal unrestricted diet to range from 0.68 to 1.02 grams of calcium (0.95 to 1.43 grams of CaO) per day. Here as in the case of protein the rate of metabolism to be ex- pected in a normal man on unrestricted diet and well fed, ac- cording to American standards, runs from 50 to 100 per cent above the amount which would probably suffice to meet the actual requirement. The calcium requirements of women are greatly increased by maternity. The need of an abundance of calcium for the rapidly growing skeleton of an infant is obvious. Before birth, and normally for several months after, this demand of the child is satisfied through the mother, whose calcium requirement is thus greatly increased. The weakening of the bones and teeth which is said to be a common accompaniment of pregnancy and INORGANIC FOODSTUFFS AND MINERAL METABOLISM 265 lactation is held by Bunge to be largely due to a withdrawal of calcium from these structures to meet the mitritive require- ments of the embryo or the nursling. Lusk also emphasizes the importance of a diet rich in calcium for pregnant women, especially during the last ten weeks of pregnancy, when the fetus is storing calcium at a rapid rate. He cites * the data of Hoffstrom,t who computed in consider- able detail the demands of the fetus upon the mother for nitro- gen, phosphorus, calcium, and magnesium at different stages of intrauterine hfe. Strong confirmation of this has recently been obtained from investigation of farm animals. The experiments of Steenbock and Hart show that the production of milk in cows and goats causes a heavy drain upon the calcium of the skeleton unless the amount of calcium contained in the food be very abundant. They also point out that the mammary glands likewise make large demands upon the phosphorus supply and suggest that if the food be not rich in phosphorus the destruction of bone tissue to furnish phosphorus for milk production may result in still further loss of calcium from the body. Forbes and Beegle in studying the mineral metabolism of the milch cow found a heavy loss of body calcium, notwithstand- ing the fact that the food was beheved to supply liberal amounts of all essential elements and was eaten in sufficient quantity to induce storage of nitrogen. That calcium may be lost from the body while nitrogen is being stored has also been emphasized by several other investigators (Steenbock and Hart, Weiser, and others) . According to Forbes it may be necessary to continue high calcium feeding for some time after the cessation of lactation, in order to replace the calcium which the maternal organism has lost. In children after weaning and throughout early childhood there are apt to be frequent disturbances of the absorption and metab- * Lusk. Science of Nutrition, 3d edition, pages 380-390. t HoSstrom. Skandinavisches Archiv Jiir Physiologic, Vol. 23, page 326 (1910). 266 CHEMISTRY OF FOOD AND NUTRITION olism of calcium, in some cases due to distinct disorders of digestion, in other cases to more obscure irregularities in nutri- tion. In order that these fluctuations shall not interfere with the steady growth of the child, it is obvious that the food must furnish a fairly Hberal surplus of calcium. Even under the most favorable conditions, a rapidly growing child will pre- sumably need more bone-making material in proportion to its total food than do adults, who alone have served as subjects for the metabolism experiments upon which our present estimate of calcium requirement is based. Camerer, in summarizing a long series of investigations upon the food requirements of children at different ages, concluded that the amount of calcium received by the average nursling is just about sufficient to main- tain a normal rate of growth, leaving little if any " margin of safety " ; and Bunge, from a comparison of the calcium contents of different staple foods, points out that calcium more than any other inorganic element is likely to be deficient as the result of the change of diet from mother's milk to other forms of food. Herter * estimates that in order to support normal growth of the skeleton there must be an average storage of about 37 grams of calcium (51.6 grams of calcium oxide) annually through- out the period from the third to the sixteenth year. This means an average daily storage of somewhat more than o.io gram of calcium during this thirteen-year period. In order to accom- plish such a storage it is plain that the daily food of the child must contain a surplus of more than o.io gram of calcium per day beyond the amount required for maintenance, which latter amount should provide for the frequent failures of complete utilization which have already been mentioned. Herbst f studied the calcium metabolism of 6 boys between the ages of 6 and 14 years and found that they were storing from o.oio to 0.016 gram of calcium per kilogram per day, or 0.21 * Infantilism. t Jahrh. Kinderheilkunde, Vol. 76. Ergdnzungshcjt, pages 40-130. INORGANIC FOODSTUFFS AND MINER.\L METABOLISM 267 to 0.39 gram per capita per day. If normal growth of boys of these ages involves such a large storage of calcium, it is plain that the food of such boys must be rich in calcium if they are to develop advantageously. These boys consumed about 3 to 4 times as much calcium in proportion to their weight as is required for the maintenance of men. From such considerations as these it is evident that one should be very liberal in calculating the amount of calcium to be sup- pHed to growing children. If 0.45 gram is the minimum on which an average man can maintain equihbrium, it would seem that the food of a family should furnish at least 0.67 gram* of calcium or 0.9 to i.ogram of calcium oxide per man per day. This is less than is advo- cated by such recent writers as Albu and Neuberg, Gautier, Obendoerffer, and Emmerich and Loew, or reported by Nelson and Williams ; yet about 50 per cent of the American dietaries which have so far been studied with respect to their ash con- stituents show less than 0.67 gram of calcium per man per day, and about 15 per cent of them show less than 0.45 gram calcium (0.63 granj CaO) per man per day. In some cases the deficiency in calcium is incidental to a general deficiency in the amount of food ; but if the food consumed in each dietary had been in- creased or decreased to just 3000 Calories there would have been less than 0.67 gram of calcium in 46 per cent, and less than 0.45 gram in 8 per cent of the cases. Since inorganic forms of cal- cium are utilized in nutrition, the lime of the drinking water may be added to that of the food in calculating the amount consumed, and to this extent the actual nutritive supply may be greater than the dietary studies show, but unless a very " hard " water be used for drinking, it is unhkely that the Ume from this source will cover more than a small part of the cal- cium requirement. It is probable too that losses of food cal- * This amounts to setting a tentative "standard " 50 per cent higher than the average minimum, as in the cases of protein and of phosphorus. 268 CHEMISTRY OF FOOD AND NUTRITION cium in cooking may fully offset the calcium obtained from the drinking water. Apparently the American dietary is more often deficient in calcium than in any other element ; certainly more attention should be paid to the choice of such foods as will increase the calcium content of the dietary. The use of more milk and vegetables with less meat and sugar will accomplish this and usually improve the diet in other directions as well. Calcium Content of Typical Foods The table on the following page shows the comparative richness in calcium of a number of staple articles of food. It will be seen that there are enormous differences in the cal- cium content of different foods, whether expressed in percentage of the food material or in relation to its protein content or energy value. Meat is exceedingly poor in calcium and is therefore, notwithstanding its high protein content, a very one- sided and inadequate source of " building material." Milk is so rich in calcium that one need take only 400 Calories of milk to obtain the entire day's supply of this element, while to get the same amount of calcium from round steak and white bread it would be necessary to take 10,000 Calories. Polished rice and new process corn meal are even poorer in calcium than white flour. The difference in calcium content between the whole grains and the " fine " mill products, while not so great as in the case of iron or phosphorus, is still considerable. In general the milling removes more than half of the calcium. The fruits and vegetables in general are fairly rich in calcium, while some of the green vegetables are strikingly so ; but in most cases the intake of calcium depends mainly upon the extent to which milk (and its products other than butter) enters into the dietary. A quart of milk contains rather more calcium than a quart of clear saturated lime water. By far the most practical means of insuring an abundance of calcium in the dietary is to use milk freely as a food. INORGANIC FOODSTUFFS AND MINERAL METABOLISM 269 Approximate Amounts of Calcium in Food Material Food Beef, all lean . . Eggs Egg yolk . . . Milk Cheese .... Wheat, entire grain White flour . . Rice, polished Oatmeal . . . Beans, dried . . Beets .... Cabbage . . . Carrots .... Potatoes . ' . . Turnips .... Apples .... Bananas . . . Oranges . . . Prunes, dried . . Almonds . . . Peanuts . . . Walnuts . . . Calcium Per 100 Grams Edible Substance grams 0.007 0.067 0.137 0.120 0.931 0.045 0.020 0.009 o.o6g 0.160 0.029 0.045 0.056 0.014 0.064 0.007 0.009 0.045 0.054 0.239 0.071 o.oSg Calcium Per too Grams Protein grams 0.03 0-5 0.9 3-7 3-5 0-33 0.18 0.06 0.4 0.7 1.9 2.8 5-1 0.6 5-0 1.9 0.7 5-7 2.6 1.2 0.3 o-S Calcium Per 3000 Calories grams 0.18 1-35 I.I 5-2 6.4 0.40 0.18 0.04 0-5 1.4 1.9 4-3 3-7 0-5 4.8 0.36 0.27 2.6 o-S I.I 0.4 0.4 Relations of the Inorganic Elements to Each Other It is evident from what has already been seen that the custom which has been more or less prevalent of referring to the ash or mineral matter of a food as if it were a substance is wholly 270 CHEMISTRY OF FOOD AND NUTRITION illogical and incorrect. Food ash is always a mixture of the com- pounds of several different elements, and each element has its own functions and significance in nutrition. Even elements so closely related chemically as are sodium and potassium, or calcium and magnesium, are not only not interchangeable, but are, in some of their functions, directly antagonistic in their action in the body. Bunge's experiment showing the effect of potassium upon sodium excretion has already been noted. Meltzer and his associates have shown that the injection of magnesium salts has a marked general inhibitory effect, and that this can be quickly overcome by the subsequent injection of calcium salt. Summarizing the results of extended series of in- vestigations by himself and others, Meltzer stated, in the Trans- actions of the Association 0} the American Physicians for igo8 : " Calcium is capable of correcting the disturbances of the inorganic equilibrium in the animal body, whatever the. direc- tions of the deviations from the normal may be. Any abnormal effect which sodium, potassium, or magnesium may produce, whether the abnormality be in the direction of increased irrita- bility or of decreased irritability, calcium is capable of reestab- lishing the normal equilibrium." More recently Hart and Steenbock have found that the ad- dition of magnesium salts to an otherwise well-balanced ration tends to cause a loss of calcium from the body. Several other observers have reported similar unfavorable effects of magne- sium upon the metabohsm of calcium, and some are inclined to regard this as a matter of much importance to the well-being of the body. On the other hand, calcium seems to exert a favor- able influence upon the economy of iron in metabolism, inas- much as it appears to be possible to maintain equilibrium upon a smaller amount of iron when the food contains an abundance of calcium. It would thus appear that an adequate study of the subject should take account of the relative, as well as the absolute, INORGANIC FOODSTUFFS AND MINERAL METABOLISM 27 1 amounts of the different inorganic elements of the food. Tables showing these elements for the different articles of food are in- cluded in the Appendix at the back of this book. Not only do the different food materials differ greatly in the absolute and relative abundance of the different elements, but the same is also true of the total food intake of different groups of people. Studies of 150 freely chosen American dietaries each covering the food of a group of people for a week or more show the follow- ing range and average intake, per man per day and per 3000 Calories. Inorganic Elements in 150 American Dietaries Per Man Per Day Per 3000 Calories Min. Max. Average Min. Max. Average Calcium Magnesium Potassium Sodium Phosphorus . Chlorine . Sulphur . Iron . . 0.24 0.14 1-43 0.19 0.60 0.88 0-51 0.0080 1.87 0.67 6.54 4.61 2.79 5.83 2.82 0.0307 0.73 0-34 3-39 1.94 1.58 2.83 1.28 0.0173 0.35 0.17 1.63 0.22 0.72 0.83 0.80 0.0090 1-47 0-53 5-27 4-83 2.30 7.26- 2.35 0.0234 0.73 0.34 340 1-95 1-59 2.88 1-30 0.0174 Since these dietary records did not show the quantities of salt used, the figures for sodium and chlorine in the table cover only the amounts in the food as purchased and are greatly below the actual intake of these elements. It will be seen that the intake of any given element may be widely different in the different dietaries, even though each represents the daily average for at least a week. To some extent this is due to the variable amounts of total food consumed, but even when the data are reduced to a uniform basis of 3000 Calories the differences be- tween minimum and maximum are still quite wide. 272 CHEMISTRY OF FOOD AND NUTRITION Output of Inorganic Elements during Fasting In view of the relationships discussed above it is of interest to examine the absolute and relative excretion of the different elements as recently reported by Benedict for a subject who fasted for thirty-one days. Urinary Excretion of Different Elements during a 31-DAY Fast (Benedict) Day Nitrogen gms. Chlo- rine gms. Phos- phorus gms. Sul- phur gms. Calcium gms. Magne- sium gms. Potas- sium gms. Sodium gms. 1 7.10 3-77 0.73 0.46 0.217 0.046 1.630 2.070 2 8.40 1.02 1.08 0.61 •243 .106 1.368 .926 3 11-34 0.79 1. 10 0.68 •243 .106 1.368 .926 4 11.87 0.59 1.27 0.67 ■243 .106 1.368 .926 5 10.41 0.41 I-I5 0.65 .274 .098 1-445 .276 6 10.18 0.40 1.02 0.65 .274 .098 1-445 .276 7 9-79 0.55 0.80 0.62 •253 .070 .883 •154 8 10.27 0.32 0.80 0.64 ■ •253 .070 .883 •154 9 10.74 0.31 0-93 0.66 •253 .070 .883 ■154 10 10.05 0.28 0.86 0.61 .220 .072 1.006 .100 II 10.25 0.36 0.85 0.62 .220 .072 1.006 .100 12 10.13 0.31 0.74 0.62 216 .065 — — 13 10.35 0.32 0.85 0.62 .216 .065 — — 14 10.43 0.26 0.81 0.60 .236 .071 .814 .109 15 8.46 0.16 0.64 0.50 .236 .071 .814 .109 16 9.58 0.14 0.89 0.59 .214 .078 — — 17 8.81 0.12 0.87 0.53 .214 .078 — — 18 8.27 0.15 0.81 0.54 •251 •059 .676 -051 19 8.37 0.16 0.77 0.55 •251 •059 .676 •051 20 7.69 0.15 0.64 0.51 • 237 ■053 .644 .066 21 7-93 0.18 0.70 0.51 • 237 •053 .644 .066 22 7-75 0.21 0.69 0.50 .179 •050 •643 .083 23 7.31 0.18 0.71 0.51 .179 .050 -643 .083 24 8.1S O.IO 0.68 0.49 .167 .056 -787 .065 25 7.81 0.18 0.67 0.49 .167 .056 -787 .065 26 7.88 0.16 0.65 0.54 •I S3 •051 .656 •055 27 8.07 0.16 0.62 0.52 •153 •051 .6s6 -055 28 7.62 0.14 0.59 0..53 •131 .047 -585 .036 29 7-54 0.12 0.64 0.52 •131 ■047 -585 .036 .SO 7.83 0.14 0.61 0.52 .138 .052 .606 .053 31 6.94 0.13 0.58 0.49 .138 .052 .606 •053 INORGANIC FOODSTUFFS AND MINERAL METABOLISM 273 It will be noted that the nitrogen output and the output of chlorine run entirely different courses, especially in the early days of the fast. Each of the other elements seems to run its own course except that the sulphur tends to remain relatively constant like the nitrogen (both being derived from protein metaboHsm), and the output of sodium tends to run parallel with that of chlorine, since these two elements are excreted mainly in combination with each other as common salt. The Maintenance of Neutrality in the Body One of the interesting relationships among the ash constit- uents of foods is that between the acid-forming and the base- forming elements, since this has a direct bearing upon the im- portant problem of the maintenance of neutrality in the body. Although the reaction of normal human blood is alkaline to litmus, the actual excess of hydroxyl over hydrogen ions is found by modern methods to be so slight that blood as well as protoplasm is commonly spoken of as neutral. Thus Henderson writes : " Neutrahty is a definite, fundamental, and important characteristic of the organism." The normal processes of metabolism, however, involve a contin- ual production of acid (chiefly carbonic, phosphoric, and sulphuric) which must be disposed of in order to maintain this neutrality. The factors generally recognized as concerned in the main- tenance of neutrality are: (i) carbonates, (2) phosphates, (3) ammonia, (4) proteins. As preliminary to even a brief mention of the function of these different mechanisms for maintaining neutrality, it may be well to recur for a moment to the fundamental conceptions which have recently been so well summarized by Henderson as follows : * " First, the product of the concentrations of hydrogen and hy- droxyl ions (at constant temperature) is approximately constant. (H+) • (0H-) = c * Science, Vol. 46, page 78 (July 27, 1917). T 274 CHEMISTRY OF FOOD AND NUTRITION Therefore the concentrations of these two ions always vary inversely ^ ^ (0H-) " Secondly, if for convenience, just as the histologist uses mi- crons instead of meters, we adopt as unit concentrations of hydrogen and hydroxyl ions a very small quantity, viz. the concentration of these ions in neutral solutions, the value of this constant becomes unity.* (H+) ■ (0H-) = I, It may be noted that, using this unit of concentration, an ordi- nary decinormal solution of hydrochloric acid has a concentra- tion of hydrogen ions of nearly 1,000,000; and a decinormal solution of sodium hydroxide, a corresponding concentration of hydroxyl ions. " Thirdly, upon this basis the definitions of neutrality, acid- ity, and alkahnity are as follows : For neutrality, (H-^) = I = (0H-) For acidity, For alkalinity, (H+) > I > (0H-) (H+) < I < (0H-) '' Finally, in any solution containing a weak acid and its salts with one or more bases, regardless of the other components of the solution, the concentration of hydrogen ions is appro.xi- mately proportional to the ratio of free acid to combined acid. * The more usual method of expressing hydrogen ion concentration has been referred to in an earlier chapter (page 77). INORGANIC FOODSTUFFS AND MINER.\L METABOLISM 275 This relation, however, holds only when the ratio of acid to salt is neither very large nor very small. " It is therefore evident that in the solution of any weak acid, when the quantities of free and combined acid are equal, the value of (H"'') is yfe ; if the ratio of acid to salt be 10 : i, (H"*") is 10 k, if the ratio be i : 10, (H"^) is o.i k." In the case of carbonic acid and of acid phosphates the value of k is near enough to unity so that solutions containing acid carbonate or a mixture of primary and secondary phosphates must always remain nearly neutral. Carbonic acid produced in metabolism is chiefly disposed of by elimination as carbon dioxide through the lungs. For description of the mechanism and regulation of carbon dioxide elimination the reader must be referred to discus- sions of the physiology of respiration. Its bearing upon the problem of neutrality is summarized by Henderson as follows : " This substance is the chief excretory product of the organism. As such it must be eliminated promptly and completely. More- over, in that it leaves the body not in aqueous solution and as an acid, but almost exclusively in the form of gaseous carbon dioxide, there is no possibility of any variation of the permanent effect produced upon the reaction of the body by the elimina- tion of a definite amount of it. In the final regulation by ex- cretion it is not, therefore, concerned. And yet it has, in the process of excretion, a very important role in regulating the reaction of the body. This depends upon the fact that carbonic acid is not only a waste product, but also a normal constituent of the blood, and, as such, a principal factor in the physico- chemical regulation. Thus, if the ratio of carbonic acid to bicarbonates in a normal individual were i : 15, a large produc- tion of acid might cause a destruction of a third part of all the bicarbonates, producing in its place an equivalent amount of free carbonic acid. This, if nothing else occurred, would reduce 276 CHEMISTRY OF FOOD AND NUTRITION the relative amount of bicarbonates from 15 to 10, and simul- taneously increase the free carbonic acid from i to 6. The ratio would now be 6 : 10, and since the hydrogen ion concentration is proportional to this ratio, this ion would suffer a nearly ten- fold increase of concentration. But at this point, or, more strictly speaking, continuously during the process, the excretory function intervenes. There is a tendency for the respiratory process to hold the tension of carbon dioxide in the blood nearly constant. This is the reason why carbonic acid has some- times been thought the respiratory hormone. Assuming that the exact quantity of carbonic acid set free by the reaction of neutralization were thus eliminated, the ratio would be reduced to 1 : 10, and the hydrogen ion concentration would rise but one third above its original value. More recent investigations, however, have shown that a tendency to acidity is accomplished by a lowering of the tension of carbon dioxide. Let us suppose that in this case the tension was lowered one third. The free carbonic acid of the blood would then become 0.67 instead of I. GO, and the ratio of acid to salt 0.67: 10, which is exactly equal to i : 15, the original ratio. Accordingly, the hydrogen ion concentration would be restored exactly to its original value, and the regulation by excretion would be quite perfect. Now there is abundant evidence to show that something very much like this is always occurring in the body, and, on the whole, I believe that the most delicate of all means to regulate the reac- tion of the body is to be found in this variation of the tension of carbonic acid during its excretion. Such considerations have strengthened the hypothesis that the hydrogen ion is the true respiratory hormone." (Henderson, he. cit.) Phosphates are regularly present in blood and urine in no- table amounts. From what has already been seen regarding the reaction of the blood, it may be inferred that in it the primary and secondary phosphates are normally present in such pro- portions as to produce a practically neutral mixture. In urine, INORGANIC FOODSTUFFS AND MINERAL METABOLISM 277 on the other hand, acid phosphate predominates, because the kidney usually removes from the blood a larger proportion of primary than of secondary phosphate. Thus by virtue of this ability of the kidney to secrete an acid urine from a neutral blood, the excess of phosphoric acid produced in metabolism is readily disposed of. The disposal of the sulphuric acid pro- duced in the metabolism of protein is a more complicated prob- lem. Sulphuric is so strong an acid that it would soon poison the body unless quickly neutraUzed. When a fairly strong acid such as the sulphuric acid produced in the metabolism of protein enters a neutral or slightly alkaline solution of phosphates and carbonates such as the blood, it reacts with secondary phosphate to form primary phosphate and with bicarbonate to form carbonic acid. Since secondary phosphate (K2HPO4 or Na2HP04) is but faintly basic, and pri- mary phosphate (KH2PO4 or NaH2P04) is but faintly acid, the ratio of these phosphates may be considerably changed (i.e. a considerable amount of strong acid may be received by the phosphate mixture) without appreciably diminishing the alka- linity of the solution. Thus the blood may neutralize a con- siderable amount of acid without appreciable change in its reac- tion, or as ordinarily expressed, without alteration of its own neutrality.* * This property is also referred to as the "buffer efifect" of phosphate solutions and is of course connected with the capacity for secondary ionization, readily re- versible according to the reaction of the medium : Acid , ^ U2VO,- - ^ HP04= Alkaline H3PO4 or ^^P04= HsPO. ± H2PO4- HP04= P04^ - + H+ = -|-H+ + H+ ■ For discussion of acid-base equilibria in phosph Henderson cited at the end of the chapter. late solutions see the works of 278 CHEMISTRY OF FOOD AND NUTRITION Ammonia, which is continually being formed in the body by deaminization of amino acids in the course of protein metabo- lism, constitutes another means of neutralization of acid. It will be remembered that, according as more or less acid is fojmed in, or introduced into, the body, a larger or smaller proportion of the nitrogen eliminated appears in the urine as ammonium salts.* Proteins, such as those of blood serum, are amphoteric sub- stances and can unite with acid by virtue of their amino, and perhaps other basic, groups. The constant presence of pro- teins in all parts of the body constitutes, therefore, a further mechanism for the immediate fixation of any strong acid pro- duced. This, however, is only a temporary and partial solution of the problem, since the acid thus fixed would remain to be dis- posed of when the protein is hydrolyzed to amino acids. The relations of these different factors in the maintenance of neutrality under normal conditions are summarized by Hen- derson as follows : f " The hydrogen ion concentration of the body has been seen to depend on the ratio H2CO3 NaHCOs Acid reacting with this system causes a diminution of the de- nominator and an increase in the numerator of the fraction, the value of the fraction increases, and with it the hydrogen ion concentration. Hereupon the lung reduces the value of the numerator by diminishing the concentration of carbon dioxide in blood and alveolar air, the value of the fraction is restored * Two facts should, however, be kept in mind as possibly limiting the utility of this means of disposing of acid. In the first place, ammonium salts are generally regarded as somewhat toxic, their accumulation in the body being normally pre- vented by conversion into urea. Secondly, there is no good reason to suppose that the deaminization processes which form ammonia will always go on in the same cells and at the same time with the o.xidation processes which produce sulphuric acid. t Loc. cit., page 81. , INORGANIC FOODSTUFFS AND MINERAL METABOLISM 279 more or less exactly to its original value and with it the concen- tration of the hydrogen ion. But the denominator is still below normal. To offset this, there occurs, on the one hand, a pro- duction of ammonia which takes the place in the urine of alkaH existing as salt in the blood. This alkali recombines with car- bonic acid, forming bicarbonate, and thus increasing the de- nominator. On the other hand the kidney removes less alkali in combination with phosphates than exist in this state in the blood. This alkah, too, helps to regenerate sodium bicarbonate, and thus to increase the denominator. Both of these processes are so regulated that the denominator is restored to normal. The concentration of carbonic acid responds through the ac- tivity of the respiratory mechanism, and the organism returns to its normal state. " These processes, of course, go on simultaneously and not in succession. They are, moreover, far less simple than such an analysis admits, for on the one hand the interaction of phos- phates and proteins has not been fully described, and, on the other hand, many of these variations influence other conditions and processes in the organism." The normal fluctuations of fixed acid production in healthy man on ordinary mixed diet are apparently taken care of in part by neutrahzation with ammonia and in part by the forma- tion and excretion of acid phosphate. In an experiment upon man by Gettler and the writer it was found that, of the extra acid formed in metabolism as the result of replacing the potato of a mixed diet by rice, about t,^ per cent was accounted for by the increased ammonia and about 40 per cent by the in- creased acidity of the urine, leaving a remainder which may have been ehminated, in part at least, through the skin, since no attempt was made to measure the amount or acidity of the per- spiration, or may have been neutraUzed by sodium or potassium carbonate in the blood or other fixed alkali from the body. In this experiment the intake and output of phosphorus was ap- 28o CHEMISTRY OF FOOD AND NUTRITION proximately the same on both diets. The increased acidity of the urine, therefore, impHed an increased ratio of primary to secondary phosphate in the urine but not necessarily any in- crease in the amount of fixed base leaving the body. In the neutralization of sulphuric acid by means of phosphate, each molecule of hydrogen sulphate (representing one atom of sul- phur oxidized in protein metabolism) changes two molecules of secondary into primary phosphate. In order that the orig- inal condition of equiUbrium may continue, the surplus acid phosphate thus formed must be excreted. Whether or not this results in an increased excretion of phosphates and there- fore of sodium or potassium (or only, as in the experiment just cited, an altered ratio of primary and secondary phosphates in the urine), apparently depends not only upon the balance of acid-forming and base-forming elements in the food, but also upon the quantities of fixed bases and of phosphates which are being metabolized and of ammonia available from the protein metabohsm. It would seem that in any case in which sulphuric acid produced in metabolism is neutralized by the sodium or potassium carbonate of the blood, the resulting sulphate must be eliminated with corresponding loss of sodium or potassium and decrease of the capacity of the blood for combining with carbon dioxide. This is an important feature of acidosis. It is diag- nosed by determining the carbon-dioxide-holding capacity of a sample of blood serum and the result is expressed as the " alkali reserve " or " reserve alkalinity " of the blood. Thus while the phosphates and carbonates of the blood and tissues serve for the immediate neutralization of acid without appreciable change in the normal reaction of the blood or tissue itself, yet when much strong acid such as the sulphuric acid from protein metabolism is neutralized in this way, there is apt to result an increased output of the base-forming elements, which if not made good by the intake must tend to diminish the " reserve alkaUnity " or " alkali reserve " of the body. INORGANIC FOODSTUFFS AND MINEIL\L METABOLISM 281 That an excess of acid-forming elements in food, even if long continued, does not necessarily lead to any apparent injury is shown by experiments of McCollum, in which rats were main- tained throughout a large part of their adult lives and produced healthy young on a diet of egg-yolk, in which there is a great predominance of acid-forming over base-forming elements. Yet in man an increase in the ammonia content and acidity of the urine is usually regarded (if pronounced and persistent) as indicating an unfavorable tendency. In this connection the decreased uric acid solvent power of the more acid urine is to be considered, especially in view of the present belief that the human organism does not destroy uric acid but must trans- port and excrete all that is produced in the body. Hindhede * found that the eating of vegetables, particularly potatoes, in- creases the capacity of the urine for dissolving uric acid. Fur- thermore, Hasselbalch f showed that the carbon dioxide tension of the alveolar (expired) air, which is indicative of the carbon- dioxide-carrying capacity and therefore of the reserve alka- linity of the blood, is influenced in a similar way by the food. On a diet rich in meat he found a tension of 37.8 mm. ; on an ordinary mixed diet, 38.3 mm. ; on a vegetarian diet, 43.3 mm. In an extended series of experiments, Blatherwick J Hkewise finds that foods which have a preponderance of base-forming elements lead to the formation of a urine which is less acid, both as regards hydrogen ion concentration and titration acidity, and which has an increased capacity for dissolving uric acid, while the ammonia content of the urine is diminished and the carbon dioxide tension of the alveolar air, indicative of reserve alkalinity, is increased. Conversely, foods with a predominance of acid-forming elements increase the urinary acidity and urinary ammonia, decrease the uric acid solvent * Skandinavisches ArchivfUr Physiologic, Vol. 26, pages 87, 384 (1912). t Biochemisches Zeitschrift, Vol. 46, page 403 (191 2). } Archives of Internal Medicine, Vol. 14, pages 409-50 (1914). 282 CHEMISTRY OF FOOD AND NUTRITION power, and show, through lowered carbon dioxide tension of the alveolar air, a tendency toward depletion of the reserve alkalinity of the blood. The benefit to health which so generally results from a free use of milk, vegetables, and fruits in the diet may be attributable in part to the fact that these foods yield alkahne residues when oxidized in the body ; but this point should not be too greatly emphasized, for there are several other respects in which the eating of liberal amounts of milk, vegetables, and fruits is certainly beneficial, notably in supplying calcium, iron, and vitamines, and in improving the intestinal conditions. REFERENCES (See also the references at the end of Chapter IX.) Aron. Calcium Requirement of Children (and the Relation of Calcium Metabolism to Rickets). Biochemisches Zeitschrift, Vol. 12, page 28 (1908). Aron and Frese. Utilization of Different Forms of Food-Calcium in the Growing Organism. Biochemisches Zeitschrift, Vol. 9, page 185 (1908). Aron and Sebauer. Importance of Calcium for the Growing Organism. Biochemisches Zeitschrift, Vol. 8, page i (1908). Benedict. A Study of Prolonged Fasting. Carnegie Institution of Wash- ington, Publication No. 203, page 247 (191 5). Blather WICK. Foods in Relation to the Composition of the Urine. Ar- chives of Internal Medicine, Vol. 14, page 409 (1914). Blauberg. Mineral Metabolism of Infants. Zeitschrift fiir Biologic, Vol. 40 (N. S. 22), pages i, 36 (igoo). Camerer and Soldner. Ash Constituents of the New Born Infant and of Human Milk. Zeitschrift fur Biologic, Vol. 44 (N. S. 26), page 61 (1903). DiBBELT. Significance of Calcium Salts during Pregnancy and Lactation and the Influence of a Loss of Calcium upon Mother and Offspring. Beitrdge pathologische Andtomie (Zeigler), Vol. 48, page 147 (1910). Evvard, Dox, and Guernsey. Effect of Calcium and Protein Fed Preg- nant Swine upon the Size, Vigor, Bone, Coat, and Condition of the Offspring. American Journal of Physiology, Vol. 34, page 312 (1914)- FiTZ, Alsberg, and Henderson. Concerning the E.xcretion of Phosphoric Acid during Experimental Acidosis in Rabbits. Anr.rican Journal of Physiology, Vol. 18, page 113 (1907). INORGANIC FOODSTUFFS AND MINERAL METABOLISM 283 Forbes. The Balance between Inorganic Acids and Bases in Animal Nutrition. Ohio Agricultural Experiment Station, Bulletin 207 (1909). Forbes. The Mineral Nutrients in Practical Human Dietetics. Scientific Monthly, Vol. 2, page 282 (1916). Forbes and Beegle. The Mineral Metabolism of the Milch Cow. Ohio Agricultural E.xperiment Station, Bulletin 295. GiVENS AND Mendel. Studies in Calcium and Magnesium Metabolism. Journal of Biological Chemistry, Vol. 31, pages 421, 435, 441 (1917). Hart and Steenbock. The Effect of High Magnesium Intake on Calcium Retention by Swine. Journal of Biological Chemistry, Vol. 14, page 75 (1913)- Henderson. The Fitness of the Environment. Henderson. Equilibrium in Solutions of Phosphates. American Journal of Physiology, Vol. 15, page 257 (1906). Henderson. A Critical Study of the Process of Acid Excretion. Journal of Biological Chemistry, Vol. 9, page 403 (191 1). Henderson. The Regulation of Neutrality in the Animal Body. Science, Vol. 37, page 389 (March 14, 1913). Henderson. The Excretion of Acid in Health and Disease. Harvey Society Lectures for 1914-1915. Henderson. Acidosis. Science, Vol. 46, page 73 (1917). Kastle. On the Available Alkali in the Ash of Human and Cow's Milk and its Relation to Infant Nutrition. American Journal of Physiology, Vol. 23, page 284 (1908). LusK. Science of Nutrition, 3d edition, pages 215-222, 358-361. Mathews. Physiological Chemistry. McCoLLUM ANTJ HoAGLANTD. The Effect of Acid and Basic Salts and of Free Mineral Acids on the Endogenous Nitrogen Metabolism. Journal of Biological Chemistry, Vol. 16, page 299 (1913). MiCHAELis. Die Wasserstaffion-concentration. Nelson and Williams. The Urinary and Fecal Output of Calcium in Normal Men. Journal of Biological Chemistry, Vol. 28, page 231 (1916). Osborne. Sulphur in Proteins. Journal of the American Chemical Society, Vol. 24, page 140 (1902). Robertson. On the Nature of the Chemical Mechanism which ISIaintains the Neutrality of the Tissues and Tissue Fluids. Journal of Biological Chemistry, Vol. 6, page 313 (1909). Sherman antd Gettler. The Balance of Acid-forming and Base-forming Elements in Foods and its Relation to Ammonia Metabolism. Journal of Biological Chemistry, Vol. 11, page 323 (1912), 284 CHEMISTRY OF FOOD AND NUTRITION Steenbock, Nelson, axd Hart. Acidosis in Omnivora and Herbivora and its Relation to Protein Storage. Journal of Biological Chemistry, Vol. 19, page 399 (1914)- Steexbock axd Hart. Influence of Function on the Lime Requirement of Animals. Journal of Biological Chemistry, Vol. 14, page 59 (1913). Stoeltzner. The Two-fold Significance of Calcium in the Growth of Bone. Archiv fur die gesamlc Physiologic {Pjliigcr), Vol. 122, page 599 (1908). Taxgl. The Metabolism of an .\rtif1ciall3' Fed Child. Ibid., Vol. 104, page 453 (1904)- TiGERSTEDT. Ash Content of the Ordinary Dietary of Man. Skandina- visches Archiv fiir Physiologic, Vol. 24, page 97 (191 1). Ukderhill. Studies on the Metabolism of Ammonium Salts. Journal of Biological Chemistry, Vol. 15, pages 327, 337, 341 (1913). Van Slyke, Cullex, Stillmax, ant) Fitz. (.-Vcid Excretion and the .\lka- line Reserve.) Proceedings of the Society of Experimental Biology and Medicine, Vol. 12, pages 165, 184 (1915); Journal of Biological Chem- istry, Vol. 30, pages 289, 347, 369, 389, 401, 405 (1917)- VoiT (E.). Significance of Calcium in Animal Nutrition. Zeitschrifl fiir Biologic, Vol. 16, page 55 (1880). CHAPTER XI IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION The amount of iron contained in the body is small, but its functions are of the highest importance. As previously noted, the iron content of the adult man or woman is estimated at only 0.004 per cent, or i part in 25,000 parts of the body weight, or rather less than 3 grams (hardly one tenth of an ounce) in the entire body. Much the greater part of this iron exists as a constituent of the hemoglobin of red blood corpuscles and is constantly functioning in the general metabolism as the carrier of the oxygen upon which all of the oxidative (energy-yielding) processes of nutrition depend. There is no considerable reserve store of relatively inactive iron in the body corresponding to the store of calcium and phosphorus in the bones. Hence if the intake of iron fails to equal the output there must soon result a diminution of hemoglobin, which if continued must mean a greater or less degree of anemia. The investigation of iron metabolism has therefore been largely connected with the study of anemia and of hemoglobin formation. Important changes of view in regard to the metabolism of iron have followed so closely and have depended so directly upon the progress of experimental methods that it seems desirable, in this case, to review in chronological order some of the more important steps in the development of our present knowledge. ' Development of Modern Views It has long been known that iron is essential to the nutrition of both plants and animals, and that small amounts of the oxide 28s 286 CHEMISTRY OF FOOD AND NUTRITION or phosphate of iron occur in the ash of all natural food materials. A few decades ago it was assumed that the iron exists in the food as oxide or phosphate, and that hemoglobin is formed in the body by the combination of protein with inorganic iron. This view was hardly consistent with the ideas of animal metabolism taught by Liebig and generally held at the time, but appeared to be supported by the successful use of inorganic iron in the treatment of anemia. The results obtained in a number of investigations published between 1854 and 1884 threw doubt upon the utilization of in- organic iron for the production of hemoglobin, since they indi- cated that iron salts when injected act as poisons and are quickly eliminated from the blood, and when given by the mouth reappear almost quantitatively in the feces, little, if any, evidence of absorption being obtained except when the doses were so large or long continued as to cause irritation of the intestine. In the attempt to harmonize this result with clinical ex- perience it was suggested that the inorganic iron might act by absorbing the hydrogen sulphide of the intestine, thus protecting the food iron from waste. The view that medicinal iron acts by stimulation of the absorbing membrane was also advocated at about this time. It was held that the amount of iron in the ordinary food is always sufficient for the needs of the body, but that sometimes the intestinal mucous membrane becomes so blood- less that it cannot properly perform its functions of absorp- tion. Under such conditions inorganic iron was believed to stimulate and tone up the membrane so that in a short time the increased absorption of food iron makes good the defi- ciency in the blood. A very suggestive discussion of the metabolism of iron, the effects of a lack of iron in the food, and the amounts of iron required for the maintenance of the body in health was IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 287 published by Von Hosslin in 1882, and long before this some attention had been given to the iron content of food materials by Boussingault. Boussingault's figures, however, are not sufficiently accurate to be of value at the present time, and httle attention was given to the subject discussed by Von Hosslin until it was reopened by Bunge about two years later. Bunge, in 1884, doubting the ability of the animal body to form hemoglobin from inorganic iron, undertook the study of the iron compounds of food materials in order to find in what form iron is normally absorbed and from what sort of iron com- pounds the growing organism ordinarily forms its hemoglobin. Practically all of the iron of eggs was found to be in the yolk. Yolk of egg does not contain any hemoglobin, but it must con- tain substances from which hemoglobin can be formed, since the incubation of the egg results in the development of hemo- globin without the introduction of anything from without. Bunge found no inorganic iron in egg yolk, but isolated con- siderable amounts of the precursor of hemoglobin, which he called " hematogen," and which exhibited the properties of a phosphoprotein containing about 0.3 per cent of iron in such firm " organic " combination that it gives none of the ordinary reactions of iron salts. In milk, cereals, and legumes similar organic compounds of iron and only traces of inorganic iron were found. At this time Bunge distinctly stated that iron occurs in food solely in the form of comphcated organic com- pounds which have been built up by the life processes of plants. In this form, said Bunge, is the iron absorbed and assimilated, and from these compounds hemoglobin is produced. In 1890 and subsequently, the absorption and assimi- lation of iron was studied by several experimenters, usually with particular reference to the question whether inorganic or synthetic organic compounds of iron are absorbed and assimilated, and especially whether such preparations contribute directly to the formation of hemoglobin. This question is, of 288 CHEMISTRY OF FOOD AND NUTRITION course, extremely important, not only in connection with the therapeutic use of medicinal iron, but also in its bearing upon the iron requirements in health ; for if inorganic iron could be utilized in the body in exactly the same way as the complex organic iron compounds of the food, it would follow that the iron of drinking water could replace that of food, and the supply- ing of food iron would be a matter of indifference to a man whose drinking water suppHed a few milligrams of iron per day. In opposition to this view, Bunge held that little if any inorganic iron is assimilated, and that any effect of medicinal iron should be attributed to its action in protecting the food iron from loss in digestion, principally by absorbing the sulphur liberated as sulphide through intestinal putrefaction. Socin demonstrated the superiority of the iron of egg yolk over iron chloride by dividing a number of mice into groups, some of which were fed on a mixture of iron-free food and iron chloride, while others received the same iron-free food with the addition of egg yolk. None of the mice fed without organic iron lived for more than thirty-two days, while some of those receiving egg yolk lived as long as the experiments were con- tinued (sixty to ninety-nine days), and gained in weight. Gottlieb, recognizing the fact that iron might be absorbed and used by the body, yet finally excreted with the feces, determined the intestinal elimination of iron in dogs before and after subcutaneous and intravenous injections of known amounts of iron salts. From the results obtained it was esti- mated that practically all of the injected iron was eliminated by the intestines. Voit studied the metabolism of iron in dogs by direct ob- servations of absorption and elimination in isolated sections of the small intestine. Opening the peritoneal cavity, he separated the desired section, removed the contents, closed the ends, and left the sac thus formed in its normal position after having reunited the remainder of the intestine. Under IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 289 these conditions the isolated section of intestine, while not coming in direct contact with anything taken by the mouth, would still receive its proportional share of anything elim- inated from the body through the intestinal wall. By kill- ing and examining animals which had been kept for some time after such an operation, Voit was able to compare the amount of iron eliminated through the intestinal wall with the amounts contained in food and feces, and thus to infer the extent to which the iron taken by the mouth was ab- sorbed and returned to the intestine for elimination. In fasting, the daily elimination found for each square meter of intestinal surface was 6 milligrams in the feces and the same amount (per square meter of surface) in the isolated loop of intestine. On food poor in iron the feces contained in each of two cases 10 milligrams, the isolated loops 6 and 9 milligrams, of iron per square meter of intestinal surface; while on food rich in iron the corresponding figures for two experiments were 43 and 78 milligrams in the feces, and 8 and 6 milligrams in the isolated portion of the intestine. Hence it appears that the iron eliminated in the feces during fasting or on food poor in iron came from the body through the intestinal wall, while most of the extra iron given with the food in the last two experiments passed through the al- imentary canal without being absorbed and metaboHzed. Stockman, in a paper upon the metabolism of iron, pub- Hshed in 1893, while discussing mainly the therapeutics of chlorosis (a type of anemia occurring in girls and young women) undertook to solve the question of the absorption of inorganic iron. He reasoned as follows : If inorganic iron preparations given hypodermically will cure chlorosis, there can in such cases be no possibiUty of the iron exerting its effect by the stimulation of the alimentary canal or by combining with hydrogen sulphide in the intestine. If iron sulphide given by the mouth cures chlorosis, it must 290 CHEMISTRY OF FOOD AND NUTRITION be through absorption of the iron, since ferrous sulphide has no stimulating effect and cannot take up more sulphur. If bismuth, manganese, etc., take up hydrogen sulphide as readily as iron, but are inert in chlorosis, a further indirect evidence of absorption of iron is obtained. Stockman made experiments and observations upon hos- pital patients (of which he cites nine cases) which appeared to substantiate each of the three propositions, and thus to establish the fact that inorganic iron preparations cure chlorosis through being absorbed and utilized in the formation of hemo- globin. During the years 1 894-1 897 several investigators studied the absorption of different forms of iron by microchemical methods. Suitable stains having been found for the iden- tification of iron in the microscopic sections of tissue, it was possible by examination of the intestinal wall and the various organs and tissues of the body to follow the absorption, storage, and ehmination of the iron given medicinally or occurring in the food. Macallum investigated in this manner the behavior of inorganic salts of iron, iron albuminates, and the iron com- pound of the egg yolk, and found that iron taken in any of these forms may be absorbed from the small intestine. Woltering compared microchemically and by quantitative determination the amounts of iron in the livers of mice, rabbits, and dogs, fed with and without sulphate of iron, and reported an increase in the iron content of the liver and in the hemoglobin and red corpuscles of the blood as the result of feeding the iron salt. Gaule, using principally microchemical methods, found no reaction for iron in the chyle under normal conditions ; but a distinct reaction appeared in the lymph nodes, and extended to the spleen soon after the feeding of iron salt to rabbits. This absorption of inorganic iron was followed by an increase in the number of red corpuscles and percentage of hemoglobin in the blood. IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 29 1 In the meantime, Kunkel and Egers studied especially the influence of iron salts upon the regeneration of blood after hemorrhage. Kunkel kept two dogs on a hmited milk diet, but gave one of them, in addition to the milk, iron in the form of albuminate. Each of the animals was bled every seven days, about one third of the total blood being taken each time. The iron in the drawn blood was deter- mined and ascertained to be greater than the amount sup- plied by the milk, but less than the total iron received by the dog which was fed with albuminate. The experiment was continued seven weeks, at the end of which time the blood and organs of the dog which had been kept on milk alone were poorer in iron than those of the dog which had received the iron albuminate. Only one animal was fed in each way, and no determinations of hemoglobin are recorded. According to Egers, the regeneration of blood after severe losses (one third of the estimated total) is very slow on food poor in iron, unless medicinal iron is also given, when the rate of regeneration becomes better, but not so good as on a diet supplying an abundance of food iron alone. Even when the diet was rich in food iron, however, Egers found that medicinal iron appeared to aid the regeneration of blood after hemorrhage. These investigations having shown that inorganic iron is at least to some extent absorbed and carried to organs which take part in the production of hemoglobin, it became of especial im- portance to determine by long-continued feeding experiments whether the inorganic iron thus absorbed can take the place of food iron in the production of hemoglobin under normal condi- tions. This question was studied by Hausermann in an extended series of experiments in Bunge's laboratory. The general plan of these experiments was to feed young animals from the end of the normal suckling period upon food poor in iron, 292 CHEMISTRY OF FOOD AND NUTRITION usually milk and rice. One half of the animals, however, received ferric chloride in addition to this food. After the animals had been thus fed for from one to three months and had usually doubled in weight, they were killed, and the amount of hemoglobin in the entire body was estimated ; also, in the case of small animals, the total amount of iron. Ex- periments were carried out in this way upon 24 rats, 17 rabbits, and 14 dogs. The results are summarized essentially as follows by Bunge : * The rats all became highly anemic, for at the end of the experiment the percentage of hemoglobin was diminished to about half that of animals from the same litter which had received their normal food, namely, meat, flies, yolk of egg, fruit, and vegetables. The rats which had taken ferric chloride in addition to the milk and rice contained no more hemoglobin than those which had received milk and rice only. Moreover, the amount of iron was in each case the same. In one experiment alone, in which the addition of ferric chloride was continued for three months, was the iron found to be double as much in the animals which had received it as in those which had only milk and rice. But here again the proportion of hemoglobin remained the same in both instances. We thus see that some iron is absorbed if small doses of iron are persisted in for a long time, as well as if large amounts be suddenly administered. But this inorganic iron, when absorbed, is not utilized in the for- mation of hemoglobin to any appreciable extent, but remains unused in the tissues. Whether inorganic iron was absorbed in the experiments which lasted only from one to two months can- not be decided ; it is possible that some of it was absorbed and was again eliminated in the same degree. Certainly no storing up nor increase of iron could be detected in the whole organism. * Physiological and Palholofiical Chemistry, Blakiston's edition. Philadelphia, 1902, page 379. IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 293 The experiments on rabbits gave less decisive results. The average proportion of hemoglobin in the animals that received inorganic iron was somewhat higher than that in the animals which were fed on milk and rice only. But when the great individual differences between various animals are taken into consideration, too much importance must not be ascribed to this slight divergence. At any rate, the amount of hemoglobin in the control animal, which received its normal diet — fresh green cabbage, bran, etc. — was nearly twice as high as in the animal which received the inorganic iron. The experiments upon dogs were not attended with decisive results, as dogs are not suitable animals for these experiments, owing to the variation in individuals. Moreover, the growth of these animals after the period of lactation is at a much slower rate, and their appetite is so enormous that they might readily be able to assimilate sufificient iron for hemoglobin formation even from a material so poor in iron as milk. In fact, Hauser- mann found the largest proportion of hemoglobin in a dog which had been fed exclusively upon milk. The animals which received ,ferric chloride in addition to a milk diet certainly con- tained no more hemoglobin than animals from the same litter which were fed on meat and bones. Abderhalden, following Hausermann, studied the subject even more exhaustively. In order to ascertain whether and to what extent sulphides normally exist in the alimentary canal, — a question of special importance in connection with one view of the mode of action of inorganic iron, — Abder- halden killed and examined rats, mice, cats, dogs, guinea pigs, and rabbits in the following way: Immediately upon kilHng the animal, the abdomen was opened and the intestinal tract from the esophagus to the rectum was ligated in sections. The contents of each section were then removed and tested qualitatively for sulphides. Hydrogen sulphide was obtained from the contents of the large intestine, but not from those 294 CHEMISTRY OF FOOD AND NUTRITION of the small inlcstine nor of the stomach. Hence, if in- organic iron acts by improving the absorption of food iron, it must do so in some other way than by simply preventing its precipitation as sulphide, since this would not occur in the small intestine, where the principal absorption of iron takes place. The next step in the investigation was to study by microchemical methods the absorption of inorganic iron, its behavior in the body, and its elimination. Experiments were made upon 49 rats from 7 litters, 14 guinea pigs from 6 litters, 12 rabbits from 2 Htters, 10 dogs from 3 Utters, and 6 cats from 2 litters. From all of these experiments, Abderhalden concluded that the complicated iron compounds of the normal food, the iron in the form of hemoglobin, and hematin, and the inorganic iron, were all absorbed in the same general way, stored in the same organs, and eliminated by the same paths. In studying the utilization by the body of the different forms of iron, Abderhalden fed animals from the end of the suckling period, or, in the case of guinea pig, from birth, on food poor in iron, and divided each litter into two groups, one of which was given inorganic iron in addition. After a sufficient time the animals were killed, and the total hemo- globin in the body of each was estimated. Experiments of this kind were made upon 48 rats, 44 rabbits, 14 guinea pigs, 17 cats, and 11 dogs. The animals fed with food poor in iron plus an addition of inorganic iron were unable to produce as much hemoglobin as those receiving normal food. In these experiments, Abderhalden had noticed some facts which indicated that the favorable influence of inorganic iron upon metabolism and blood formation was greater on a diet rich in food iron than when the amount of food iron was kept small. In order to test this, experiments were made with 66 rats, 10 rabbits, and 14 guinea pigs, in the manner already described, but with diets arranged to bring out this IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 295 particular point. These experiments led to the conclusion that the greater the cjuantity of food iron present, the greater the influence of the inorganic iron upon the hemoglobin formation. Abderhalden's experiments also showed that the production of hemoglobin was not stimulated indefinitely by inorganic iron, but only for a short time, and he concluded that, while inorganic iron may be absorbed and may favorably influence blood formation, it is not used as material for the production of hemoglobin. It has also been found clinically that medicinal iron gives better results when used intermittently than when used continuously, which indicates that the action is due to stimulation rather than to the inorganic iron actually going to form hemoglobin. The results obtained by Tartakowsky * were more favorable to the view that hemoglobin may be formed from inorganic iron. He found that young growing animals fed on rice and milk gradually became anemic and finally ceased to grow ; but that when inorganic iron was added to the rice-milk diet the blood regained its normal iron content and the animal soon began to grow again. From such experiments together with a large number of microchemical observations, Tartakowsky concludes that medicinal (inorganic) iron is assimilated hke food iron and serves in the same way for the production of hemoglobin and the other organic iron compounds of the body. He further insists that Abderhalden's experiments should also be interpreted in the same way, since in many cases the animals which received inorganic iron in addition to their food formed more hemoglobin than the control animals. More recently, Schmidt f has described some interesting experiments upon mice with a similar iron-poor rice-and-milk diet. According to Schmidt this diet did not cause anemia * Archiv Jiir die gesamte Physiologic, Vol. 100, page 586; Vol. loi, page 423 (1903, 1904). t Verhandlungen der Dcutsches Palhologisdies Gescllschafl,Vo\. 15, page gi (1912). 296 CHEMISTRY OF FOOD AND NUTRITION in adult mice ; but the oflspring of mice which had been kept on such diet seemed to lack the normal reserve store of iron, and by continuing the milk-rice diet to the third generation there were obtained what this investigator describes as " iron-free families " of mice. In these the red blood cells were very poor in hemoglobin. From such a family of mice two sisters seven months old were selected ; one was continued on the milk-rice diet alone while the other was fed medicinal iron (Ferrum oxyda- tum saccharatum) in addition for eleven days ; then both were killed and examined. The first showed the typical anemic condition of these " iron-free families," the hemoglobin number and number of red blood cells being both less than half of the normal ; while in the second mouse, which had received medicinal iron for eleven days, the hemoglobin number and number of red blood cells were both about twice as high as in the first. This is held by Schmidt to show that medicinal iron does not merely stimulate the blood-forming organs to greater activity but does itself enter into hemoglobin formation. It is difficult to determine how much weight should be given to the findings of Tartakowsky and of Schmidt as opposed to the more extended and more quantitative experiments of Hauser- mann and of Abderhalden. While it cannot yet be stated positively that inorganic iron is or is not used by the animal body as material for the pro- duction of hemoglobin, the best medical opinion appears to support the conclusion reached by Abderhalden, that hemo- globin is derived essentially from the organic iron compounds of the food, while inorganic iron acts mainly if not entirely as a stimulus. This view is strongly supported by Von Noorden in his treatise on chlorosis in Nothnagel's Encyclopedia of Practical Medicine, and Ehrlich and Lazarus, writing on anemia in the same work, state : " It is not very probable that the (medicinal) iron stored by the liver and spleen is directly employed in the formation of hemo- IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 297 globin ; on the contrary, the assumption first suggested by Von Noorden seems much more plausible, namely, that the iron exer- cises a direct irritative action on the function of the blood-making organs." The Iron Requirement of the Body * A very brief summary of the leading facts regarding the normal nutritive relations of iron may well precede the dis- cussion of the amount required. Iron is an essential element of hemoglobin and of the chro- matin substances, i.e. of the body constituents most directly concerned with the processes of oxidation, secretion, reproduc- tion, and development. The substances thus fundamentally connected with metabolism processes contain their iron in firm organic combination, as a constituent of their charac- teristic proteins; and the normal materials for the production of these body constituents are the similar iron-protein com- pounds of the food. The iron of the food is absorbed from the small intestine, enters the circulation by way of the lymph, and is deposited mainly in the liver, spleen, and bone marrow. Its final elim- ination takes place mainly through the walls of the intestines. Both inorganic and synthetically prepared organic forms of iron are absorbed from the same part of the digestive tract, stored in the same organs, and eliminated by the same paths as the iron of the food. These medicinal forms of iron often stimulate the production of hemoglobin and red blood corpuscles. Whether medicinal iron actually serves as material for the construction of hemoglobin is not positively known, but we have what appears to be good evidence that food iron is assimilated and used for growth and for the regeneration of hemoglobin to much better advantage than are inorganic or synthetic forms, and that when medicinal iron increases the 298 CHEMISTRY OF FOOD AND NUTRITION production of hemoglobin, its effect is more beneficial in pro- portion as the food iron is more abundant — a strong indica- tion that the medicinal iron acts by stimulation rather than as material for the construction of hemoglobin. Evidently, then, we should look to the food rather than to medicines or mineral waters for the supply of iron needed in normal nutrition. Comparatively few experiments upon the amount of food iron required for the maintenance of equilibrium in man have been made. Cetti and Breithaupt eliminated 0.0073 ^^'^ 0.0077 gram per day, respectively, when fasting. Three men observed by Stockman while receiving in the food about 0.006 gram each per day eliminated 0.0063, o-oo9,3' ^^^d 0.0115 gram, respectively. Von Wendt found his requirements to range in a number of experiments on different diets from 0.008 to 0.016 gram per day, the largest amount being required in a case where the diet was deficient in calcium. In three experi- ments by Sherman in which the food contained 0.0057 to 0.0071 gram of iron there was metabolized 0.0055, 0.0087, ^^^ 0.0126 gram per day, respectively, and here also the amount of iron which sufficed for equilibrium when taken in the form of bread and milk (a diet rich in calcium) was insufficient when taken in the form of a diet (poor in calcium) consisting of bread and egg white, or bread alone. In this case, however, the difference in the economy of the metaboHsm of the iron may have been due not simply to the change in the calcium content of the food, but also to a superior nutritive value of the iron compounds of milk over those of bread and to the fact that the general conditions of digestion and nutrition were better when milk was included in the diet than when it was excluded. The nitrogen, phosphorus, calcium, and iron balances for two of these experiments per- formed upon the same man and with diets practically alike in energy value and protein content, are shown in the following table : IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 299 Comparison of Balances of Different Elements Nature of Element Amount in Grams per Day Nature of Diet In food In feces In urine Balance Bread and milk . . Bread and egg white . Nitrogen Nitrogen 10.10 10.69 0.46 0.75 13-09 13.21 - 3-45 - 3-27 Bread and milk . . Bread and egg white . Phosphorus Phosphorus 1-55 0.38 0.57 0.22 1-03 0.75 — 0.05 - 0-59 Bread and milk . . Bread and egg white Calcium Calcium 1.89 o.io 1-34 0.34 0.15 0.07 + 0-A° - 0.31 Bread and milk . . Bread and egg white . Iron Iron 0.0057 0.0065 •0053 .0085 .0002 .0002 + .OC02 — .0022 Here, although the nitrogen balance was practically alike on the two diets, there was on the bread and milk diet prac- tical equilibrium of phosphorus and iron and a storage of calcium, while on the diet of bread and egg white there were noteworthy losses of all three of these elements. Returning to the problem of the quantitative determination of the iron requirement it will be seen that in the cases in which the intake and output of iron have been determined, the require- ment appears to have varied with individuals and with the nature of the diet from 0.006 to 0.016 gram (6 to 16 milligrams) of iron per man per day. We might conclude from these results that a daily allow- ance of 10 to 12 milligrams of food iron should suffice for the maintenance of iron equilibrium in an average man under favorable conditions, but until the conditions which deter- mine a larger metabolism of iron are more clearly defined, it would seem desirable to set a higher standard, perhaps 15 milligrams of food iron per man per day. In calculating the iron requirement for a family dietary, it 300 CHEMISTRY OF FOOD AND NUTRITION is well to make the allowance for women and children more liberal than would be indicated by their total food require- ment. A woman requiring eight tenths as much food as a man will probably require more than eight tenths as much iron, and a child requiring half as much food may easily re- quire more than half as much iron; for the influence of menstruation, pregnancy, and lactation in women and of growth and development in children may reasonably be ex- pected to affect the demand for iron to an even greater extent than they affect the requirement for total food. It is probable that pregnancy and lactation increase the iron requirement of the mother by at least 3 milligrams per day, and at other times the losses of blood in menstruation must call for a greater intake of iron than would be needed by a healthy man of equal energy and protein requirement. Since milk is the sole food of young mammals during a considerable period of rapid growth, Bunge was surprised to find only small amounts of iron in milk ash. Comparing the composition of the ash of milk with that of the newborn animals of the same species, it was found that, while other constituents occurred in nearly the same relative proportions, the iron was six times as abundant in the ash of the young animal as in that of the milk on which it was nourished. That the suckling animal grows rapidly and increases its blood supply in spite of this apparent deficiency of iron in its food is due to the fact that the body contains a reserve supply of iron at birth. In confirmation of this statement Bunge and his pupils have published many analyses showing that the percentage of iron in the entire organism is highest at birth, and that during the suckling period the amount of iron in the body remains about constant, notwithstanding the increase in body weight. In all cases in which the young depend entirely upon the milk of the mother during the suckling period the body con- stituents of the young must evidently be derived entirely IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 301 from the maternal organism either before birth through the placenta or after birth through the milk glands of the mother and the digestive tract of the young. Since disordered diges- tion may readily lead to defective absorption of the iron of the food, nature apparently takes the precaution of conveying the necessary iron from mother to offspring mainly by the safer method, i.e. through the placenta. Hence in the case of animals which feed solely upon milk for some time after birth, a relatively large amount of iron is stored before birth for use in the formation of hemoglobin during the suckling period. This has been shown by analysis to be true of children, puppies, kittens, and rabbits. On the other hand, guinea pigs, which feed on green leaves or other food rich in iron from the first day of life, are born without this reserve store of iron (Bunge). From recent analyses it appears that the percentage of iron in the human body is about three times as high at birth as at maturity. If it be assumed, as indicated by Bunge's work, that during the milk feeding of infancy the amount of iron in the body remains about constant, it would follow that the percentage of iron in the child's body would be reduced to that in the adult when the body weight becomes about three times what it was at birth — usu- ally when a little over one year old, — and that from this time on throughout the period of growth, care should be taken that the food is sufficiently rich in iron to provide not only for equihbrium, but also for the constantly increasing blood supply. Iron in Foods Little weight can be attached to such statements regarding the iron content of foods as are based upon the data obtain- able from the ordinary tables of ash analyses, since these have usually been obtained by methods which are Hkely to greatly overestimate the amount of iron. In the following table are shown the approximate amounts of iron now believed to be present in the average edible portion of typical food materials 302 CHEMISTRY OF FOOD AND NUTRITION expressed (i) in milligrams per loo grams of edible material, (2) in milligrams per 100 grams of protein, (3) in milligrams per 3000 Calories: Iron in Typical Food Materials Food Beef, all lean . . . Beefsteak, medium fat Eggs Egg yolk .... Milk, whole . . . Milk, skimmed . . Cheese Oatmeal .... Rice, polished . . White flour . . . Wheat, entire grain . Beans, dried . . . Beans, string, fresh . Beets Cabbage . . . . Carrots Corn, sweet . . . Peas, dried . . . Potatoes .... Spinach Turnips .... Apples Bananas .... Oranges .... Prunes, dried . . . Almonds .... Peanuts Walnuts .... Iron per ioo Grams Fresh Sub- stance, Milli- grams 3.85 2.2 3-0 8.6 0.24 0.25 1-3 3-8 0.9 i.o S-o 7.0 I.I 0.6 I.I 0.6 0.8 5-7 1-3 3.6 0-5 0.3 0.6 0.2 30 3-9 2.0 2.1 Iron per ioo Grams Protein, Milligrams 16 16 22 53 7 7 5 7 37 40 48 38 69 55 26 23 55 135 39 78 47 25 143 19 Iron per 3000 Calories, Milli- grams 97 47 57 69 10 20 9 26 7 7 42 60 80 39 104 40 23 46 42 450 38 15 18 12 30 18 Percentages of iron in some other foods will be found in the tables of ash constituents in the Appendix. Using these recent data for iron in food materials, approximate estimates of the IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 303 amounts of iron contained in 150 American dietaries have been made. The majority of these were found to furnish 14 to 20 milligrams of iron per man per day. Apparently therefore the typical American dietary does not contain any such sur- plus of iron as would justify the practice of leaving the supply of this element entirely to chance. The available data rather indicate that foods should be selected with some reference to the kinds and amounts of iron compounds which they contain. Meats. — In meat as ordinarily eaten the iron exists largely as hemoglobin, due to the blood contained in the muscular tissue as usually sold and prepared for the table. Muscular tissue washed free from blood contains iron, but the amount is comparatively small. Since fatty tissue contains much less iron, the iron content of fat meat is much lower than that of lean, and in order to establish any useful estimate of the amount of iron in meat it is practically necessary to consider the lean tissue alone or to refer the iron to the protein content rather than to the gross weight of the meat. When expressed on the former basis, the results will still be influenced by the extent to which the' blood has been either accidentally or intentionally removed from the muscle. For fresh lean beef containing the full proportion of blood, the results obtained by most investigators are in satisfactory agreement, and the average figure, 0.00375 per cent iron in the fresh meat free from visible fat, can be accepted with Httle danger of serious error. This corresponds to about 15 to 16 milligrams of iron per 100 grams of protein in beef, and since no certain differences in iron content in the flesh of dif- ferent species have been shown, it is assumed for the present that approximately the same ratio of iron to protein will hold for meats in general. The iron of meat, as already mentioned, is largely due to the blood retained in the muscular tissue. The nutritive value of blood is often questioned. So far as the iron compounds 304 CHEMISTRY OF FOOD AND NUTRITION of the blood arc concerned, it seems to be established that hemoglobin and hematin may be absorbed and assimilated to some extent, but probably not to such good advantage as the iron compounds of eggs, milk, and vegetable foods. Eggs. — The edible portion of hens' eggs has shown as the average of several analyses 0.00303 per cent of iron. Whether the iron content of eggs can be increased by giving to poultry food rich in iron, is a disputed question. There can be no doubt regarding the assimilation and utiliza- tion of the iron compounds of eggs, since they serve for the production of all the iron-holding substances of the blood and tissues of the chick, there being no possibility of the introduction of iron from without during incubation. Milk. — Analyses of samples of cow's milk of various origin have given results varying from 0.0002 to 0.0003 per cent, and averaging 0.00024 per cent of iron in the fresh substance. It cannot be doubted that the iron of milk is readily absorbed and assimilated, since this constitutes the sole natural source of iron for all young mammals during a period of rapid growth. Moreover, metaboHsm experiments indicate that the iron of milk is likely to be utilized to especially good advantage, perhaps on account of its association with a high proportion of calcium. The question of the iron supply of infants fed upon diluted or modified cow's milk may, however, be considered at this point. It is now generally recognized that the best substitute for mother's milk is obtained by diluting whole cow's milk or top milk with a solution of lactose or maltose. By varying the richness of the milk or top milk used and the amounts of water and sugar added, the composition of the modified milk can be controlled at will. In order to ascertain whether the iron compounds of milk tend to condense upon the fat globules or for any other reason are altered in their distribution by the rising of the cream, a sample of milk was allowed to stand, and after the cream had risen, the iron and nitrogen contents were IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 305 determined separately in the upper half, containing all of the cream, and in the lower half, which consisted of skimmed milk. These analyses showed in the upper half 0.000277 P^^ cent of iron and 0.54 per cent of nitrogen; in the lower half 0.000293 per cent of iron and 0.59 per cent of nitrogen. It is evident, therefore, that the ratio of iron to nitrogen was practically the same in the cream as in the milk. It is therefore important to recognize that the iron content of cow's milk is little if any higher than that of human milk, while the protein content is at least twice as high ; that any modification of cow's milk which reduces its protein content will reduce the iron content in practically the same proportion, and that an infant fed upon cow's milk, modified or diluted to contain less than 3 per cent of protein, is probably receiving food poorer in iron than human milk. According to present estimates an infant fed on any modification of cow's milk must consume the equivalent of nearly a quart of undiluted milk or cream in order to obtain as much iron as is supplied daily in the milk of the average healthy nursing mother. Since no such quantity of cow's milk can safely be fed in early infancy, it is to be expected that during the first months of life the artificially fed infant will use up the surplus store of iron with which it was born more rapidly than will the child of the same age which receives the milk of a healthy mother. Grain products. — Iron in combination with protein matter is found in considerable quantity in the cereal grains, but the greater part of it is in the germ and outer layers, and so is rejected in the making of the " finer " mill products, such as patent flour, polished rice, and newrprocess corn meal. In view of the part which the iron of the germ takes in the sprouting of the seed and the nutrition of the young plant, there is little room for doubt that it is of value also in the animal economy. To test the value of the iron in the outer layers of the grain Bunge * carried out the following experiment : * Zeitschrift fur physiologische Chemie, Vol. 25, page 36 (1898). X 3o6 CHEMISTRY OF FOOD AND NUTRITION A litter of eight rats was divided into two groups of four each, one group fed upon bread from ftne flour, the other upon bread made from flour including the bran. At the end of the fifth, si.xth, eighth, and ninth weeks, respectively, one rat of each group was killed, and the gain in weight, the total amount of hemoglobin, and the percentage of hemoglobin in the entire body were determined. The average results were as follows : Effect of Feeding Different Kinds of Bre/Vd on Growth ant) Iron Content of Body in Experiments wtth R.ats Kind of Ration Gain in Weight OF Body Total Hemoglobin IN Body Proportion of Hemoglobin in Body White bread Bran bread Grams 4.81 20.76 Grams 0.2395 0.3492 Per cent 0.613 0.714 Here the bran-fed rats not only made a much greater general growth, but developed both a greater amount and a higher percentage of hemoglobin. There can be no doubt that the iron and other ash constituents of the outer layers of the wheat were well utilized in these cases. Vegetables and fruits. — Not many direct studies upon the iron compounds of the fruits and vegetables have been made, but Stoklasa has separated from onions an iron-protein com- pound very similar to the hematogen obtained by Bunge from egg yolk, but containing a considerably higher proportion of iron. Preparations similar in properties were also obtained from peas and from mushrooms. In view of the fact that the herbivorous animals, which are less liable to anemia than the carnivora, obtain their normal food iron entirely from vegetable sources there is every reason to suppose that man makes good use of the iron of the fruits and vegetables in his diet. Moreover, since (as Herter has IRON IN FOOD AND ITS FUNCTIONS IN NUTRITION 307 shown) anemic conditions and excessive intestinal putrefaction often go together, the bulkiness and laxative tendency of fruits and vegetables, along with their relatively high iron content, are advantageous in combating the conditions which give rise to excessive putrefaction, and at the same time increasing the supply of food iron. Among typical food materials omitted from the above table because of containing little if any iron, may be men- tioned fat pork, bacon, lard and suet, butter, salad oil, sugars, starches, and confectionery. All of these foods have high fuel value, and many are economical and highly important ele- ments in a normal dietary. Excessive use of these foods, however, would tend to satisfy the appetite and supply the body with the needed fuel without furnishing the desirable amount of iron. On the other hand, the fruits and fresh vege- tables are often regarded as of low nutritive value because of their high water content and low proportions of protein and fat. But it is largely this property which makes them especially important as sources of food iron, because they can be added to the diet without replacing the staple foods of high calorific and protein value, and without making the total food consump- tion excessive. Thus the above table shows plainly that the ratio of iron both to protein and to fuel value is high in nearly all of the typical fruits and vegetables, so that in most cases it would be necessary to increase only slightly the amount of protein and fuel value derived from these sources, in order to effect a material increase in the iron content of the dietary. The iron content of eggs is also high, but the cost of these is often such as to restrict their use in families of limited means, while present methods of drying and preserving tend to equalize the cost and increase the available variety of fruits and vegetables throughout the year. The ratio of iron to fuel value is also high in lean meat, but here, as has already been pointed out, the iron exists largely in the form of hemoglobin, 3o8 CHEMISTRY OF FOOD AND NUTRITION which appears to be of distinctly lower nutritive value than the iron compounds of milk, eggs, and foods of vegetable origin. Especially in families where there are young children itlwould be a mistake to rely too largely upon meat as a source of iron. Von Noorden, who is one of the strongest advocates of a liberal use of meat in the adult dietary, says in regard to the feeding of children : " The necessity of a generous supply of vegetables and fruits must be particularly emphasized. They are of the greatest im- portance for the normal development of the body and of all its functions. As far as children are concerned, we believe we could do better by following the dietary of the most rigid vegetarians than by feeding the children as though they were carnivora, ac- cording to the bad custom which is still quite prevalent. . . . If we limit the most important sources of iron, — the vegetables and the fruits, — we cause a certain sluggishness of blood forma- tion and an entire lack of reserve iron, such as is normally found in the liver, spleen, and bone marrow of healthy, well-nourished individuals." In an experimental dietary study made in New York City it was found that a free use of vegetables, whole wheat bread, and the cheaper sorts of fruits, with milk but without meat, resulted in a gain of 30 per cent in the iron content of the diet, while the protein, fuel value, and cost remained practically the same as in the ordinary mixed diet obtained under the same market conditions. REFERENCES Abderhalden. Phj'siological Chemistry, English Edition, Chapter 17; Third German Edition, Chapter 35. BuNGE. Physiological and Pathological Chemistry, Chapter 25. Gaule. Resorption von Eisen und Synthese von Haemoglobin. Zeil- schrift fiir Biologie, Vol. 35, page 377 (1897). Gottlieb. Ueber die Ausscheidungsverhiiltnisse des Eisens. Zeilschrift fiir physiologische Chemie, Vol. 15, page 371 (1891). IRON L\ FOOD AND ITS FUNCTIONS IN NUTRITION 309 Macallum. On the Absorption of Iron in the Animal Body. Journal of Physiology, Vol. 16, page 268 (1894) ; also Proceedings Royal Society (London), Vol. 50, page 277 (1891-1892); Quarterly Journal of Micro- scopical Science (London), Vol. 38, page 175 (1896). NoTHNAGEL. Encyclopedia of Practical IMedicine. Diseases of the Blood, pages 17, 339 (1905)- Sherman. Iron in Food and its Functions in Nutrition. Bull. 185, Office of Experiment Station, U. S. Dept. Agriculture (1907). SociN. In welcher Form wird das Eisen resorbirt ? Zeitschrift fiir physio- logische C hemic, Vol. 15, pages 93-139 (1891). Tartakowsky. Ueber de Resorption und Assimilation des Eisens. Pfi'i- gers Archiv ftir die gesammte Physiologic, Vol. 100, page 586; Vol. loi, page 423 (1903, 1904). Von Wexdt. Untersuchungen ueber den Eiweiss und Salz-Stoffwechsel beim JNIenschen. Skandinavisches Archiv fiir Physiologic, Vol. 17, pages 211-289 (1905)- Woltering. Ueber die Resorbirbarkeit der Eisen-salze. Zeitschrift fiir physiologische Chemie, Vol. 21, page 186 (1895). CHAPTER XII ANTISCORBUTIC AND ANTINEURITIC PROPERTIES OF FOOD Recent investigations have shown that food furnishing suf- ficient amounts of proteins, fats, carbohydrates, and inorganic foodstuffs may not always prove permanently adequate. Some at least of the food materials which go to make up a completely adequate diet must have properties beyond those which have been considered in the preceding chapters. For the present these additional properties are best expressed in terms of their physiological effects. The term " deficiency diseases " has been introduced as a designation for those disorders which are thought to be due to dietary deficiencies of this sort, and the nature of the disorder arising from the use of any given diet serves to designate the property which has to do with the cause or prevention of the disease. Scurvy and beriberi have in recent years been considered the typical deficiency diseases. In nor- mal nutrition the occurrence of scurvy is prevented by the antiscorbutic properties of the food. Beriberi is primarily a disease of the nerves, a neuritis, and can be prevented by the use of food adequate in antineuritic substances or properties. Similarly some foods have growth-promoting properties beyond what can be accounted for by the proteins, fats, carbohydrates, and salts which they contain. As our knowledge in this field is not yet sufficiently developed to permit a satisfactory chemical classification of the subject matter, the antiscorbutic and antineuritic properties of foods will be considered in this chapter, and the growth-promoting 310 PROPERTIES OF FOOD 311 properties in the next. The reader should keep clearly in mind the fact that these are matters of active investigation at the present time so that even while this is being printed, new re- sults tending to modify our views on these subjects may appear. The present text is written chiefly in the light of such investi- gations as were available in May, 1917- Scurvy and the Antiscorbutic Property of Food For centuries scurvy was one of the most common diseases in Europe and at times among people of European races in North America. It was most frequent and most severe in the more northern regions, where the people were often confined to a limited and monotonous diet of bread or other grain products and meat or fish through a large part of the year. As a rule fruits and vegetables were eaten only during their short natural season. On the long voyages which followed the discovery of America, sailors were often obliged to subsist for many months at a time on food even more restricted in variety than that of the winter diet of Europe because they were cut off not only from supplies of fresh fruits and vegetables but also from fresh meat. Their food supplies thus often consisted essentially of breadstuffs and salted meats. On such voyages there were many ex- ceedingly severe outbreaks of scurvy and it gradually came to be recognized that scurvy might be expected when men were kept for a long time on diets which lack fresh food. The European sailors whose experiences on their long voy- ages to America did so much to establish the relationship be- tween diet and scurvy and the fact that fresh foods, particularly fresh fruits and vegetables, have antiscorbutic properties, were also instrumental in bringing about a great diminution of the disease. They introduced into Europe from America the cul- tivation of the potato and since that time, as potato culture and the use of potatoes as food throughout the year have 312 CHEMISTRY OF FOOD AND NUTRITION become more common in Europe, scurvy has become less common. For the past two or three generations serious epidemics of scurvy among adults have not often occurred except as the result of crop failure, imprisonment with inadequate food sup- ply, or siege. In all such cases of wliich we have accurate accounts the common feature appears to be the lack of potatoes or other fresh vegetable or fruit in the diet. Scurvy on shipboard is nov/ avoided by carrying more liberal quantities of potatoes among the rations, and, in case of long voyages, the juice of lemons or limes is taken specifically for its antiscorbutic prop- erties. Garrod called attention to the fact that foods shown by ex- perience to have good antiscorbutic properties (potatoes, lemon and lime juices, fruits and vegetables generally) are rich in potassium; and suggested that the cause of scurvy may be too small an intake of potassium — particularly of " acid veg- etable potassium " convertible into potassium carbonate on oxidation. However, the tendency of scurvy to occur epidemically (as well as some other pathological features) has also seemed sug- gestive of a bacterial origin and Litten after weighing the evi- dence available in the early years of this century wrote : * " However fascinating the potassium theory may be, it is by no means absolutely proven, and it does not contradict the view that scurvy may, in spite of this, be an infectious disease. Scurvy may perhaps be assumed to be an infectious disease of a non-contagious nature produced by a microdrganism which finds in a body deficient in potassium a favorable culture medium for its development." Wright, impressed with the fact that experience has shown scurvy to develop in cases in which the diet contains a pre- * Cabot's Diseases oj Metabolism (translation from Die Deutsche Kliiiik), p. 399- PROPERTIES OF FOOD 313 ponderance of " acid forming " foods such as bread and meat, while foods of high antiscorbutic value, i.e. fruits and vegetables, are such as yield alkaline ash, was led to advocate the view (held also by Gautier) that the cause of scurvy is a sort of acidosis due to the constant production of a relative excess of acid in metabolism. An outbreak of the disease among the English soldiers besieged in Ladysmith during the Boer War gave Wright an opportunity to test his views and he found that in the scurvy patients the " titration alkalinity " (" alkali reserve ") of the blood was considerably below normal and that by feeding sodium or potassium salts of organic acids such as acetate, citrate, or lactate he was able to effect a rapid improvement both in the scurvy symptoms and in the blood alkalinity. Hoist and Frohlich, studying experimental scurvy in guinea pigs, find that some foods such as cabbage show a marked loss of antiscorbutic power as the result of simple heating or slow drying, while others (grains) develop antiscorbutic value in sprouting. In neither of these cases is the relation of acid-form- ing to base-forming elements altered and these authors there- fore consider that they have entirely disproven the acidosis theory of Wright and that antiscorbutic properties of foods have no connection with their ash constituents but are due to the presence of small quantities of a specific organic substance or substances, of undetermined chemical nature, and (in most cases at least) very readily destroyed by heat. Guinea pigs fed exclusively on bread or grain developed symptoms which Hoist and Frohlich considered to be " identical in all essentials with those of human scurvy." Since one of these symptoms is loss in weight and since animals may fail to eat enough of a one-sided diet to meet the energy require- ment, special experiments were made to establish the distinc- tion between the effects of scurvy and those of starvation or undernutrition. It was found that guinea pigs kept on an ex- clusive diet of fresh raw cabbage, dandelion greens, or even 314 CHEIMISTRY OF FOOD AND NUTRITION carrots may die of starvation ; but they do not become scor- butic. Those kept on grain alone regularly became scorbutic. Those fed grain plus a moderate allowance of cabbage, dande- lion, carrot, potato, or other fresh vegetable remained normal. The antiscorbutic properties of other foods were then tested by adding them to a bread or grain diet and observing whether the guinea pigs developed symptoms of scurvy or not. Raw cabbage, dandelion greens, lettuce, endive, sorrel, potatoes, carrots, bananas, apples, and cloudberries all showed antiscorbutic properties — apparently in varying degrees. Apples and bananas were thought to be somewhat less effective than the potatoes, lettuce, greens, and berries. Cabbage and dandelion juices seem to lose their antiscorbutic properties more rapidly than the vegetables themselves. Fruit juices and sorrel juice on the other hand retain their efficacy as antiscorbutics remarkably well. Raspberry juice seemed but little injured by heating for i hour at ioo° or even iio°. Acidulated cab- bage or dandelion juice retained its antiscorbutic property much better than the natural juice of these vegetables. If, as these experiments indicate, the antiscorbutic property is due to the presence of some unstable substance, the latter would appear to be much more stable in an acid than in a neutral or alkaline medium. The effect of cooking was studied in the case of several dif- ferent foods with the following results : Cabbage cooked at ioo° for I to I hour was still a good antiscorbutic. Carrots cooked at ioo° for i hour showed a great diminution in antiscor- butic power. Cooking for ^ hour at the same temperature showed a less serious injury to the antiscorbutic property. Cauliflower was much injured by cooking for i hour at ioo°; when cooked only | hour it was a much better antiscorbutic. Dandelion leaves lost much of their antiscorbutic property when cooked for i hour at ioo°. Potatoes cooked at ioo° " in the usual way " (| hour) had excellent antiscorbutic properties. PROPERTIES OF FOOD 315 Turnips and kohlrabi cooked at 100° had antiscorbutic power similar to cooked potatoes. Cloudberries retained their effi- ciency after cooking to a very marked degree. When cooked at 100° as usual they were still excellent antiscorbutics and were shown to retain this property when kept for at least 3 months after cooking. From this it would appear that canned fruit which has been sterilized at temperature of boiling water and then kept in a cool place ought to be a good antiscorbutic even after many months, and in general that ordinary cooking of vegetables (or low temperature pasteurization of milk) de- stroys only a part of the antiscorbutic substance, and so the food still possesses antiscorbutic properties though not in as high degree as when raw. The results of several recent investigations are, however, not entirely consistent with the findings of Hoist and Frohlich. Funk, who had been a prominent advocate of the theory that scurvy is due to deficiency of a specific unidentified substance, has recently concluded that the disease produced in guinea pigs by a diet of oats (Hoist and Frohhch's experimental scurvy) may be due to acidosis. It has also been found independently by Jackson and by McCollum that guinea pigs are so suscep- tible to nutritive disorders with scurvy symptoms when placed upon experimental diets as to make the interpretation of such experiments exceedingly difficult. Jackson finds in the scor- butic tissues of the experimental animals bacteria of the Dip- lococcus type which appear to be specific to the scurvy lesions and pathogenic when inoculated into other guinea pigs. The results of such inoculation depend largely upon the diet ; guinea pigs fed on carrots, cabbage, and hay appear relatively im- mune, while those fed on grain or bread diet are much more susceptible. According to McCollum the physical character of the diet and of the resultant intestinal residues is responsible for guinea pig scurvy. His examinations of guinea pigs dying with scurvy symptoms reveal characteristically an abnormal 3l6 CHEMISTRY OF FOOD AND NUTRITION accumulation of fecal material in the caecum. McCollum holds that the guinea pig will have scurvy on any diet which does not contain a succulent vegetable and that this is due to the anatomical character of the digestive tract, the caecum being relatively large and deHcate in this species and especially liable to the accumulation of fecal residues when the food is not of suitable physical character. His guinea pigs showing typical scurvy symptoms recovered after liberal doses of petro- leum oil. He therefore holds that guinea pig scurvy, although " referable to faulty diet, " is not a deficiency disease, the fault lying rather in the unsatisfactory physical character of the diet which leads to an injurious accumulation of material in the cae- cum. The immediate cause of the pathological symptoms of scurvy is not known. It may perhaps be due to absorption of toxic substances resulting from bacterial action in the caecum or to invasion of bacteria through an injured intestinal wall. In view of these results so recently reported by McCollum it becomes extremely difficult to interpret the work of Hoist and FrohHch, who apparently failed to reahze the part played by such digestive disorders. It also remains an open question whether guinea pig scur\y and human scurvy are referable to the same causes. Recently, as a result of war conditions, there has been re- newed interest in human scurvy and a tendency toward the view that this may be a disease in which two factors, a nutri- tional condition and an infection, may both be involved. It also seems probable that the terra " scurvy " may have been applied to more than one disease in man. Infantile Scurvy (Barlow's Disease) An investigation conducted by the American Pediatric Society in 1898 showed that infants developing scurvy had in nearly all cases been fed with heated milk or with proprietary foods. PROPERTIES OF FOOD 317 Infantile scur\y is usually quickly cured by feeding either raw milk, or milk which has been pasteurized at a low tem- perature supplemented by some fresh fruit juice (usually orange juice). Investigations to determine whether children are subject to scurvy when fed exclusively upon pasteurized milk have given conflicting results, probably for two reasons: (i) pasteurization of the milk at difEerent temperatures or for different lengths of time in different cases, (2) differences in susceptibility to scurvy among infants.* Aging of the milk may also be a factor (Hess). Hess and Fish report that they have had a considerable number of cases of infantile scurvy among hospital children fed on milk pasteurized at 145° F. for 30 minutes or 165° F. for 20 minutes. Orange juice was efficient as a preventive or cure and did not lose its antiscorbutic property when boiled for 10 minutes. It was found that the juice of the orange peel could be substituted for that of the orange as an antiscorbutic. Potato was found to be an excellent antiscorbutic for children, and the authors propose that potato water (made by mixing a tablespoonful of boiled potato in a pint of water) be used as a diluent instead of the barley water now commonly used in modifying cow's milk for infants. They held that if this is done, no other antiscorbutic will be necessary. In his later papers (1915, 1916), Hess reports that when milk which has been heated for 30 minutes at 145° F. is fed with sugar and cereal, but without orange juice or other antiscorbutic food, for from two to eight months there is usually a develop- ment of mild scorbutic symptoms, or a subacute scurvy such * Differences in susceptibility to scuny are to be expected in view of the well- known fact that when groups of men, as sailors and prisoners, are subjected to the same conditions and partake of the same rations, some become scorbutic while others do not. Physicians have also found that some infants show signs of scurvy when receiving an amount of antiscorbutic food which is amply sufficient for most infants and recover when a diet still richer in antiscorbutics is given. 31 8 CHEMISTRY OF FOOD AND XUTRTTTON as might pass unrecognized. Such cases are apt to show some but not all of the classical symptoms of infantile scurvy and usually involve retardation of growth. Under these conditions the addition of an antiscorbutic food such as orange juice to the diet induces an increased rate of growth as well as rehef of such other scorbutic symptoms as may have developed. Even if, as some critics have suggested, the symptoms reported by Hess are somewhat different from those shown by well-de- veloped and clearly marked cases of infantile scurvy, the in- fluence which the presence or absence of antiscorbutic food in the diet was shown to exert upon the nutrition and rate of growth of the infant is a matter of considerable interest from the standpoint of food chemistry. More recently still (191 7), Hess finds that infantile scurvy is possibly not a single disease, and probably not a simple dietary disease. Use of pasteurized milk is a contributing cause, but the aging of such milk is quite as much a factor as the heating. The diet is held to be at fault in allowing the intestinal bacteria to elaborate toxins, while antiscorbutic foods improve intestinal conditions and are also beneficial as diuretics. Antineuritic Properties of Food Our knowledge of the antineuritic properties of foods has been obtained through the study of beriberi in man or of ex- perimental beriberi in fowls or pigeons. While the symptoms of beriberi are variable the disease is chiefly characterized by degeneration of the nerves beginning with those of the ex- tremities (" polyneuritis," " multiple peripheral neuritis "). For a long time beriberi was very common in the Orient (Malay States, Siam, parts of Japan and the Philippines) and in recent years beriberi has also been found in Newfoundland and Labrador. Cases are also occasionally reported from the southern and western parts of the United States. PROPERTIES OF FOOD 319 Takaki, while Inspector General in the Japanese Navy, was much disturbed at the large proportion of men who suffered from beriberi, and in 1880 began a systematic investigation which indicated that the frequency of the disease was more closely con- nected with the nature of the food than with any other probable factor since climate was found to be without influence and the sanitary conditions on the Japanese ships were as good as those in the European navies which were not troubled with the disease. A Japanese naval vessel with 276 men on a g months' cruise from Japan to New Zealand, Valparaiso, and Honolulu had 169 cases with 25 deaths. Another vessel with a similar crew was sent by Takaki over the same route with a ration in which the rice was decreased, the barley increased, and vegetables, meat, and condensed milk added. In this case only 14 men had beriberi and each of these had failed to eat his full allow- ance of the new foods. As the result of this experiment Takaki secured the adoption of his new ration for the entire Japanese navy with the result that the number of cases of beriberi soon became practically negligible. Takaki attributed this to the fact that the new diet was richer in protein, having a ratio of i part nitrogen to 16 parts carbon, whereas the old ration had only i part nitrogen to 28 parts carbon. The great reduction in beriberi was undoubtedly due to the change of diet, but not primarily to the increased protein intake. Apparently because the explanation available at the time was not sufficiently convincing, Takaki's great achievement was not fully appreciated, and medical opinion continued for several years to regard beriberi as possibly an infectious dis- ease. But no success attended the attempts to check the dis- ease by sanitation, while indications that the cause might be nutritional continued to be found. In 1907 Braddon published in his book, " The Cause and Prevention of Beriberi," a large amount of evidence connecting the disease with the eating of polished rice. 320 CHEMISTRY OF FOOD AND NUTRITION At about the same time Fletcher, by experimenting with the diet in a lunatic asylum, showed that when 28 oz. of rice was fed daily with only small amounts of other food, the use of polished or unpolished rice was alone sufficient to determine the occurrence or non-occurrence of beriberi. During 1 907-1908 Fraser and Stanton took 300 laborers from Java into new and sanitary quarters in a virgin jungle and demonstrated in striking fashion that with rice as the main part of the diet, beriberi followed the use of polished but not of unpolished rice. Many other observations to the same effect were also published at about this time. In 1909, convinced that beriberi was related to diet, the U. S. Army Medical Commission in the Phihppines initiated changes in the rations of the " PhiHppine Scouts " and in 191 1 Chamberlain was able to announce the eradication of the dis- ease from these troops by the substitution of unpolished rice (and a small quantity of beans) for the polished rice previously used. Until the year 1910, the number of hospital cases of beriberi ranged from 115 to 618 (the force numbering about 5000 men). During 1910 changes in the dietary were begun and that year the cases dropped to 50. In 191 1 there were 3; in 1912, 2; in 1913, none; in 1914 up to June 30 (date of latest available report) there was i. In 1908-1909, when beriberi was at its worst among the Scouts, the diet consisted essentially of 12 oz. of beef, 8 oz. of white flour, 8 oz. of potatoes, and 20 oz. of rice (ordinarily polished). The change in the ration, as finally decided upon after some months of experimentation, consisted in giving, in place of the 20 oz. of polished rice, 16 oz. of unpolished rice and 1.6 oz. of dried beans. Experiments, made largely upon fowls as ex- plained below, have shown that while meat has some effect in preventing beriberi, an equal weight of beans, peas, or peanuts is much more efficacious. A further improvement could have been made by substituting PROPERTIES OF FOOD 32 1 some whole grain product for the white flour since it is now known that a diet consisting too largely of white flour or bread may in itself be a cause of beriberi ; * but this appeared un- necessary inasmuch as the changes already noted sufficed to eradicate the disease. Chamberlain states, in fact, that the disease had disappeared as the result of adding the beans to the ration, before the sub- stitution of unpohshed for polished rice had been completed. He believes " that the consumption of beans to the daily amount of 1.6 ounces would, unaided, have prevented a recurrence of beriberi, but it would obviously be diflBicult to make sure that all the men ate their share of this article over long periods, and it is therefore much safer that the largest component of the diet, the rice, should be of the unpolished variety and by itself sufficient to prevent neuritis." Several other investigations gave similar results. These re- peated demonstrations of a close connection between a diet consisting too largely of polished rice and the occurrence of beriberi naturally gave a great impetus to experiments de- signed to find what constituents of the rice are directly con- cerned in the disease. Attempts to Isolate an Antineuritic Substance Such experiments were greatly facihtated by the fact, dis- covered by Eijkmann in 1897, that fowls develop a diseased condition closely resembhng beriberi in man, when they are fed exclusively upon polished rice for 3 or 4 weeks. Ohler (1914) finds that an exclusive diet of white bread, especially when made without yeast, has the same effect as the polished rice diet. This experimental beriberi of fowls (or " polyneuritis gallinarum ") does not occur when whole rice or even rice which has been partially milled so as to retain the inner bran * Little (1914). The prevalence of beriberi in Newfoundland and Labrador appears to be due to a diet too largely restricted to white bread. Y 32 2 CHEMISTRY OF FOOD AND NUTRITION coat (pericarp or " silvcrskin ") is fed. It was soon found that rice polishings (bran) when added to the poUshed rice diet not only protected the fowls but also cured those which had already developed the disease. Aqueous and alcoholic extracts of the rice polishings also served to prevent or cure the disease. The same was found true of many ordinary foods such as meat, po- tatoes, beans, peas, and peanuts, the legumes being especially efficient. Aron working on Oriental beriberi in the Philippines, and Schaumann in Europe, centering his interest more particularly in ship beriberi, were both impressed with the fact that diets which cause beriberi are poor in phosphorus and that foods of good curative and preventive properties are rich in phosphorus. They were therefore inclined to regard beriberi as connected with a deficiency of phosphorus in the diet. Their attempts to prevent or cure the disease by adding definitely known phos- phorus compounds to the polished rice diet gave, however, for the most part negative results. In Aron's experiments the deleterious effects seemed to be reduced though not excluded when phyLin was fed. In Schaumann's experiments yeast lecithin and yeast nucleic acid seemed effective, but egg lecithin, . phytin, simple phosphates, and glycerophosphate showed no beneficial results. The direct evidence for the " phosphorus theory " is therefore weak and somewhat conflicting. (This does not exclude the possibility that deficiency of phosphatids may be at least a factor in the nerve degeneration as argued by Schaumann.) Furthermore, Fraser and Stanton showed that an extract of rice polishings which contained only 15 per cent of its total phosphorus was capable of preventing the neuritis, while the residue containing the other 85 per cent of the phosphorus was ineffective; and soon afterward Chamberlain and Vedder showed that an alcoholic extract of rice polishings which was highly protective contained only 0.0007 per cent of phosphorus PROPERTIES OF FOOD 323 or less than one part in one thousand of the phosphorus originally present in the poUshings. Chamberlain and his associates also tried the effects of various inorganic salts, of sugar, phytin, lecithin, allantoin, choUne, and many of the amino acids, all of which proved in- sufficient to prevent the development of polyneuritis in fowls kept on a pohshed rice diet. On the other hand, they added to the knowledge of the properties of the antineuritic substance by a study of the effectiveness of rice bran extracts after dif- ferent treatment. The antineuritic substance was found to be insoluble in ether but soluble in alcohol or in water and dialyzable. It was not volatile but was destroyed by heating or by alkah ; in the presence of acid, it was more stable. It was not precipitated by lead acetate. They held the curative substance to be an organic base but not an alkaloid. Bean extracts were found to contain one or more substances having similar properties. Fresh milk, meat, and potatoes were also found to have antineuritic properties. Later in 191 1 and early in 191 2, several investigators inde- pendently and almost simultaneously succeeded in isolating what appeared to be specific antineuritic substances. Funk's experiments, begun about the middle of 1911, have attracted special attention since he was the first to announce (December, 191 1) the isolation of a definite chemical substance possessing the antineuritic property. Pigeons paralyzed by neuritis induced by a polished rice diet were able to run and fly within a few hours after administration of 2 to 8 milligrams of this substance, which appeared to be an organic nitrogenous base related to the pyrimidines and to which Funk gave the name vitamine. He described the preparation of such substances from rice bran and from yeast, and inferred the existence of the same or a similar vitamine in all foods which have anti- neuritic properties. Funk's view of the relation of vitamine to the phenomena of beriberi is as follows : The lack of vitamine 324 CHEMISTRY OF FOOD AND NUTRITION in the food forces the animal to get this substance from its own tissues (with the result that there is wasting of the muscles causing emaciation unless accompanied by oedema). After the stock of vitamines available in the muscles begins to be scarce, there results a breaking down of the nerve tissue and the appear- ance of nervous symptoms such as are observed in beriberi. Funk called this beriberi vilamine. It constituted only 0.05 per cent of the rice polishings corresponding to about o.oi per cent in the whole grain. In March, 191 2, Edie, Moore, Simpson, and Webster (working independ- ently of Funk) described the isolation from yeast of a base which promptly cured pigeons suffering from polyneuritis. This base they described as having composition corresponding to the formula C7H17N2O6. They called it toniline. Schaumann (June, 191 2) reported the preparation of a phosphorus-free nitrogenous crystallizable base corresponding in general to the description given by Funk and exerting a marked restorative action upon polyneuritic pigeons. This base he considers the "activator" in the cure of polyneuritis, holding that it "mobilizes" the phosphatid substances which must be re- built into the degenerated nerve tissue in order to effect a permanent cure. In July, 191 2, Suzuki, Shimamura, and Odake, reported an extended investigation of experimental beriberi in which they had prepared from rice polishings by an independent method a base of high curati\e power which they called oryzanine. In preparing oryzanine they precipitated an alcoholic extract of rice polishings with tannin, decomposed the tannate by baryta, removed the barium by sulphuric acid, and precipitated the base as a picrate. Only 0.005 to 0.01 gram of oryzanine was required to make the dail}' diet of polished rice adequate for a pigeon. Since the pigeons ate 25 to 30 grams of rice per day this means that the oryzanine was only ^-^g^ to 55^5 ^^ the (dry) weight of the food eaten. Feeding 0.3 gram cured a dog that was already paralyzed by experimental beriberi.* It will be seen that these independent investigations all indi- cate that the antineuritic property shown by rice polishings, yeast, and other natural food materials is due to some basic nitrogenous substance or substances. Much work published * .\propos of the small quantities of vitamine or oryzanine necessary for pro- nounceci effects, Lusk calls attention to the fact that epinephrine (adrenaline), an essential of life, is present in the blood to the extent of only i part in 100,000,000. PROPERTIES OF FOOD 325 sinfce 191 2 confirms this general view without estabHshing the chemical identity of either " Funk's base," or " toruline " or " oryzanine." Pending chemical identification of the naturally occurring antineuritic base or bases the term " vitamines " is commonly applied to them. While the antineuritic property of such " vitamine " has been demonstrated usually by experiments upon animals, WiUiams and Saleeby have used a vitamine preparation, made from rice polishings, in a case of human beriberi with good results. In connection with this work it was found that acid hydrolysis renders the antineuritic substance of rice polishings more active, or more rapid in its action. It is possible that in natural food materials or simple water extracts the vitamine may exist, either wholly or in part, in combination. This would account for the greater activity and also for the instability of the free " purified " vitamine as compared with the natural form. Seidell * has devised a method for obtaining a stable prepara- tion of the antineuritic vitamine by precipitating it with hy- drous aluminum silicate (Lloyd's reagent). While the general view has been that a given organism re- quires a given amount of vitamine to maintain health (pre- sumably a larger amount to effect recovery from disease in- duced by a previous deficiency), it was suggested by Braddon and Cooper (1914), and a few simultaneous experiments by Funk, that there is a connection between the metabolism of carbohy- drate and of vitamine, so that the amount of antineuritic sub- stance required by the organism increases with the quantity of carbohydrate metabolized. It has also been suggested that the neuritis of beriberi is due to a toxic effect, upon the nerves, of some substance formed in, or absorbed into, the system and that the vitamine, when present in normal amounts, acts as a protection or antidote against such toxicity. * Reprint No. 325 from the Public Health Reports, U. S. Public Health Service. 326 CHEMISTRY OI- FOOD AND XUTRITION This hypothesis is difficult to lest and does not seem to have been much studied. Investigations designed to connect the physiological property with some definite chemical substance or type of molecular structure have, however, been continued and are yielding most interesting results. Relation of Chemical Structure to Antineuritic Action Williams has attacked this problem by synthesizing substances of known structure and testing them for curative action upon polyneuritic pigeons. Since such chemical examinations as had been made in connection with previous work upon active preparations from natural foods had suggested the presence of pyridine-Hke substances and also of hydroxyl groups in a benzene ring, Williams began by synthesizing a series of hy- droxy pyridines and other pyridine derivatives. Of these a-hydroxy pyridine, 2-, 4-, 6-trihydroxy and 2-, 3-, 4-trihydroxy pyridine were found to have curative power when tested upon polyneuritic pigeons. " The first of the curative substances tested was a-hydroxy pyridine. Three birds were treated with excellent results. However, three others later showed little or no improvement. On proceeding with the series of polyhydroxy compounds, a rapid striking cure was obtained with a preparation of 2-, 4-, 6-trihydroxy pyridine, followed by several partial or complete failures. A second and third fresh preparation, how- ever, produced two and three rapid cures respectively. . . . In each case all the cures obtained were of those pigeons which were first treated with a given preparation, while those treated with the same preparation a few days or weeks later invariably received no benefit. It was obvious that the substances had changed in some manner so as to lose the curative power. As there was no evidence of decomposition it seemed probable that it was due to isomerization." This suggested to Williams that an isomerism may be at least partially responsible for the instability of the natural " vita- PROPERTIES OF FOOD 327 mines " of foods and in conjunction with Seidell he reinvesti- gated the antineuritic properties of yeast extracts from this standpoint and obtained results indicating that the antineuritic vitamine of yeast is an isomer of adenine. Voegtlin and White report that they were unable to confirm these observations on attempting to repeat the work of WilUams and Seidell. Continuing his work on the relation of chemical structure to antineuritic activity Williams finds that )8-hydroxy pyridine, nicotinic acid, trigonelline, and betaine are also capable of ex- istence in forms which are curative in the sense of being " able promptly to dissipate the acute symptoms of polyneuritis galli- narum." " On the basis of these results it may be concluded with reasonable certainty that the relief of the paralysis by such substances is intimately connected with a betaine-Uke ring." WiUiams calls attention * to the fact that, on theoretical H (CH)3 = N-CH2-CO C Betaine HCl iC HN-0 Probable active form of a-hydroxy pyridine (Williams) grounds, the existence of betaine-like tautomeric modifications of the oxy- and amino-pyrimidines and purines is not less probable than in the case of the corresponding derivatives of pyridine, and proposes to search for active isomers in the pyri- midine series. REFERENCES Baumann and Hovard. Mineral Metabolism of Experimental Scur\'y of Guinea Pig. American Journal of the Medical Sciences, Vol. 153, page 650 (1917). Br addon. The Cause and Prevention of Beriberi. * Proceedings oj the Society Jar Experimental Biology andMedicinc, Vol 14, page 25. 328 CHEMISTRY OF FOOD AND NUTRITION Braddon antj Cooper. The Influence of Metabolic Factors in Beriberi. Journal of Hygiene, Vol. 14, page 331 (1914). Chamberlain. The Eradication of Beriberi from the Philippine (Nativ-e) Scouts by means of a Simple Chanjic in their Dietary. Philippine Journal of Science, Vol. 63, pages 133-146 (191 1). Also Journal Ameri- can Medical Association, Vol. 64, page 1215 (1915). Chamberlain and Vedder. Etiology of Beriberi. Philippine Journal of Science, Vol. 6 B, pages 251-258, 395-404; Vol. 7 B, pages 39-52 (1911-1912). Chick and Hume. Distribution Among Foodstuffs of the Substances Required for the Prevention of Beriberi and Scurvy. Journal of the Royal Army Medical Corps, Vol. 29, page 121 (1917). Darling. The Pathological Affinities of Beriberi and Scurvy. Journal of the American Medical Association, Vol. 63, pages 1290-1294 (1914). Edie, Evans, Moore, Simpson, and Webster. Anti-neuritic Bases of Vegetable Origin in Relation to Beriberi. Biochemical Journal, Vol. 6, pages 234-242 (191 2). Emmett and McKim. The Value of the Yeast Mtamine Fraction as a Supplement to a Rice Diet. Journal of Biological Chemistry, \'ol. 32, page 409 (19 1 7). Frohlich. Experimental Investigation of Infantile Scur\-y. Zcilschrift fiir Hygiene und Infeclionskrankhcitcn, Vol. 72, pages 155-180 (191 2). Funk. Chemical Nature of the Substance which Cures Polyneuritis in Birds Induced by a Diet of Polished Rice. Journal of Physiology, \'ol. 43, pages 395-400, and Vol. 45, page 75 (iQ", 1912). Funk. Die Vitamine und ihre Bedeutung fiir die Physiologic und Pathologic mit besonderer Beriicksichtigung der Avitaminoses (Beriberi, Skorbut, Pellagra, Rachitis) — Weisbaden, 19 14. Funk. Etiology of Deficiency Diseases (Beriberi, Scur\'y, etc.). Journal of State Medicine, Vol. 20, pages 341-368 (1913). Funk. Nature of the Disease due to an Exclusive Oat Diet in Guinea Pigs and Rabbits. Journal of Biological Chemistry, \o\. 25, page 409 (July, 1916). Funk and Schonborn. Influence of Vitamine-free Diet upon Carbohy- drate Metabolism. Journal of Physiology, Vol. 48, pages328-33i (1914). FuRST. Experimental Scurvy. Zeilschrift fiir Hygiene und Infectionskrank- heiteti, Vol. 72, pages 1 21-154 (191 2). Harden and Zilva. The Alleged Antineuritic Properties of a-Hydroxy- pyridine and Adenine. Biochemical Journal, \'o\. 11, page 172 (1917). Hart and Lessing. Der Skorbut der Kleiner Kinder. Hart, Miller, ant) McCollum. Further Studies of the Nutritive De- PROPERTIES OF FOOD 329 ficiencies of Wheat and Grain Mixtures and the Pathological Condi- tions Produced in Swine by their Use. Journal of Biological Chemistry, Vol. 25, page 239 (June, 1916). Hess anb Fish. Infantile Scurvy. American Journal of Diseases of Chil- dren, Vol. 8, pages 385-405 (1914). Hess. Infantile Scurvy. Journal of American Medical Association, Vol. 65, page 1003 (1915). American Journal of Diseases of Children, November, 1917. HoLST AND Feohlich. Experimental Studies relating to Ship-Beriberi and Scurvy. Journal of Hygiene, Vol. 7, page 634 (1907). HoLST AND Frohlich. Experimental Scurvy. Zeitschrift fiir Hygiene und Infcctionskrankheitcn, Vol. 72, pages 1-120 (1912). HoLST AND Frohlich. Experimental Scurvy, II. Zeitschrift fur Hygiene und Infectionskrankheiten, Vol. 75, pages 334-344 (1913). Jackson et al. Experimental Scurvy in Guinea Pigs. Journal of Infec- tious Diseases, Vol. 19, pages 478-510, 511-514 (September, 1916). Little. Beriberi caused by Fine White Flour. Journal of the American Medical Association, Vol. 58, page 202g; Vol. 63, page 1287 (1912-1914). LusK. Science of Nutrition, 3d edition. Chapter 13. McCoLLUM. Supplementary Dietary Relationships among our Natural Foodstuffs. Harvey Society Lectures, 1916-1917, and Journal of the American Medical Association, Vol. 68, pages 1379-1386 (1917). McCoLLUM ANT) KENNEDY. The Dietary Factors operating in the Pro- duction of Polyneuritis. Journal of Biological Chemistry, Vol. 24, page 491 (1916). McCoLLLTM ANT) PiTZ. The Vitamiue Hypothesis and Deficiency Diseases. A Study of Experimental Scurvy. Journal of Biological Chemistry, Vol. 31, page 229 (191 7). Ohler. Experimental Pol3'neuritis. Effect of an Exclusive Diet of White Bread on Fowls. Journal of Medical Research, Vol. 31, pages 239-246 (1914). Osborne and Mendel. The Role of Vitamines in the Diet. Journal of Biological Chemistry, Vol. 31, page 149 (1917). Schaumann. Preparation and Mode of Action of a Substance from Rice Bran which counteracts Experimental Neuritis. Archiv fiir Schijfs- mid Tropen-Hygiene, Vol. 16, pages 349-361, 825-837 (1912). Seidell. Vitamines and Nutritional Diseases. A Stable Form of Vitamine. Public Health Reports, Vol. 31, page 364 (February 18, 1916). Suzuki, Shimamura, ant) Odake. Oryzanine, a Component of Rice Bran and its Physiological Significance. Biochemische Zeitschrift, Vol. 43, pages 89-153 (1912). 330 CHEMISTRY OF I'OOIJ AND NUTRITION Vedder. Beriberi. Vedder. The Relation of Diet to Beriberi and the Present Status of Our Knowledge of the Vitamines. Journal of the American Medical As- sociation, Vol. 67, page 1494 (November 18, 1916). VoEGTLiN. The Importance of Vitamines in Relation to Nutrition in Health and Disease. Journal of the Washington Academy of Sciences, Vol. 6, page 575 (1916). VoEGTLiN AND White. Can Adenine .Vcquire Antineuritic Properties? Journal of Pharmacology and Experimental Therapeutics, Vol. 9, page 155 (December, 1916). Williams. The Chemical Nature of the "Vitamines." Journal of Biologi- cal Chemistry, Vol. 25, page 437 (July, 1916) ; Vol. 29, page 495 (1917). Williams. The Chemistry of the Vitamines. Philippine Journal of Science, Vol. 11 A, page 49 (191 6). Williams and S.\leeby. E.xperimental Treatment of Human Beriberi with Constituents of Rice Polishings. Philippine Journal of Science, Vol. 10 B, page 99 (1915). Williams and Seidell. The Chemical Nature of the "\'itamines," II. Isomerism in Natural Antineuritic Substances. Journal of Biological Chemistry, Vol. 26, page 431 (September, 1916). YoSHiKAWA, Yana, AND Menals. Studies of the Blood in Beriberi. Ar- chives of Internal Medicine, Vol. 20, page 103 (191 7). CHAPTER XIII FOOD IN RELATION TO GROWTH AND DEVELOP- MENT AND THE DIETARY DEFICIENCIES OF SOME INDIVIDUAL ARTICLES OF FOOD Nutritive Requirements of the Growing Organism " The upper limit of the size of an animal is determined by heredity. The stature to which an animal may actually attain, within this definitely fixed limit, is directly related to the way in which it is nourished during its growing period " (Waters). While feeding experiments upon growing animals and the influence of growth upon food requirements have been discussed to some extent in previous chapters, the great importance of adequate nutrition during the growing period demands special consideration. Recent investigations upon nutrition in growth are also of added interest in that the study of " growth-pro- moting properties " of food materials has broadened our con- ceptions of food values and of nutritive requirements in general. It is a familiar fact that the growing organism needs more energy, protein, and inorganic foodstuffs in proportion to weight than does one which is full-grown. But even a liberal diet made up of purified proteins, fats, carbohydrates, and salts does not suffice to support normal growth and complete de- velopment. Growth-Promoting Substances in Food Hopkins * found that the addition of very small amounts of milk to diets otherwise composed of purified foodstuffs sufficed * As early as igo6, Hopkins had found experimentally and published in brief {The Analyst, Vol. 31, page 395) the fact that an ani.Tial cannot live " upon a mixture 331 332 CHEMISTRY OF FOOD AND NUTRITION to induce growth in young rals (Fig. 12), and Osborne and Mendel demonstrated that a similar growth-promoting effect was obtained when they introduced into their rations of isolated foodstuffs a moderate amount of " protein- free milk" — a powder made by removing the fat, the casein, and the albumin from cow's milk and evaporating the clear filtrate to dryness. Since in both these investigations it was found that milk ash does not show this property, it follows that milk must contain some water-soluble or- ganic substance which exerts a distinctly fa- vorable effect upon growth. A little later it was found both by McCollum and Davis and by Osborne and Mendel that the fat of milk (butter fat) also exerts a growth- promoting influence, which, as it is shared by only certain other fats, is probably not due to the glycerides themselves, but rather of pure protein, fat, and carbohydrate, and even when the necessarj' inorganic mate- rial is carefully supplied the animal still cannot flourish." Seeking further light upon the chemical nature of the essential substance contained in milk and some other natural foods but not in the purified foodstuffs, he deferred publication of the details of the experiments until igi2 {Journal oj Physiology, Vol. 44, page 425). -iU ifO Fig. 12. — Growth curves of rats. Lower curve six rats on artificial diet alone. Upper curve six similar rats receiving in addition 2 cc. of milk each per day. Abscissae time in days : ordinates aver- age weight in grams. Courtesy of Dr. F. Gowland Hopkins. FOOD IN RELATION TO GROWTH 333 to a fat-soluble substance carried by butter-fat and the fat of egg yolk and in much smaller quantities if at all by most vegetable and meat fats. This fat-soluble substance (or something show- ing the same growth-promoting property) has also been found by McCollum to occur in certain plant tissues not rich in fat, notably in alfalfa and cabbage leaves and presumably in leaves generally. Normal growth and full development, as shown by ability to produce and nourish healthy young, demands, there- fore, in addition to adequate and appropriate supplies of proteins, fats, carbohydrates, and salts, at least two substances or kinds of substances which are distinguished by the solubility of one in water and of the other in fat. These substances, neither of which has yet been chemiically identified, are variously desig- nated by different writers. Hopkins used the term " accessory factors." Funk calls them "growth vitamines." McCollum criticizes the use of the term " vitamine " and proposes that until chemically identified the substances be known as " fat soluble A " and " water soluble B." The fats of milk, eggs, and cer- tain organs, and also the leaves of certain plants, are particu- larly rich in " fat soluble A " whereas many staple foods are very poor in this constituent. " Water soluble B " is more widely distributed, being found in the foods which have anti- neuritic properties, and it probably is the same as the sub- stance whose absence or insufficiency induces polyneuritis. Thus the feeding experiments with isolated foodstuffs have resulted in estabhshing the fact (until recently unsuspected and doubtless responsible for many of the failures met in earUer experiments) that there are required for normal nutrition, and most conspicuously during growth and development, these two factors A and B in addition to the previously known fac- tors of ample energy and adequate and appropriate supplies of protein and of inorganic foodstuffs. This has made it possible to proceed much more intelligently and effectively in the study of the relations of ordinary food materials to growth and de- 334 CHEMISTRY OF FOOD AND NUTRITION velopment. In ihis conncctioh it is important, as McCnllum has emphasized, that growth and development be considered not only in terms of gain in weight at a normal rate, but also in reference to the capacity to produce and nourish healthy young at intervals normal for the species. A diet lacking in growth-promoting properties is apt to have an unfavorable effect upon reproduction and lactation. In some cases a de- ficiency may become manifest in connection with reproduction, even when it has not appreciably retarded growth. In a recent summary,* McCollum points out that the de- ficiency of wheat as a sole food has been found to be associated with the nature of its proteins, of its ash constituents, its lack of the " fat soluble A," and possibly a toxic factor. He states that when wheat and a good salt mixture are fed there is im- provement in the condition of the experimental animals for a Hmited time. A rat will grow for a month f on this com- bination and then stop, whereas he could not grow at all on wheat alone. On feeding wheat and casein only there is also a marked improvement for a time, and the same is true for a mixture of wheat and butter fat, " but in no case does the beneficial eflfect extend beyond the first month. These results we interpret to mean that there were two at least of the dietary factors involved, unless the trouble was all the result of toxicity in the wheat kernel. The next step was to feed wheat together with two purified additions as wheat, salts, and casein ; wheat, salts, and butter fat . . . combinations (which) will make a young rat grow to practically the normal adult size and at nearly the normal rate, but rats so fed will never produce young, and will never live much beyond a third of the usual length of life of a well-nourished animal. When we feed wheat * McCoUum. The Present Situation in Nutrition, Hoard's Dairyman, July- August, 1916. t In a month a rat makes as large a fraction of his total growth as is made by a child in from one to two years. FOOD IN RELATION TO GROWTH 335 with all three of the purified additions, salts, protein, and but- ter fat, the animals are perfectly nourished and not only grow up at the regular rate but they are able to reproduce at fre- quent intervals and to successfully rear their young, and these young can complete the life cycle with no other food than that on which their parents Uved." Thus it now appears that the diet in order to be fully and permanently satisfactory must furnish (i) adequate energy value, (2) proteins sufficient in quantity and suitable in their amino acid make-up, (3) ash constituents each in sufficient quantity and all in well-balanced proportions, (4) " fat soluble A," and (5) " water soluble B." All of these factors are doubt- less necessary in order to make the diet really adequate at any time, but it is through studies of growth that the last-men- tioned factors were found, and all of the requirements are plainly more prominent in connection with growth, development, and reproduction than in the simple maintenance of healthy adults. Recognition of some of the factors just mentioned is too recent to have influenced the arrangement of many of the feeding experiments which have been made for the purpose of stud>ang the relation of diet to growth, so that it is not always possible to interpret the experimental data in terms of these five cate- gories. This can, however, be done to some extent. Influence of Restricted Food Supply (i) Energy When a diet of such character as would ordinarily meet all requirements is fed to a growing animal in amounts too small to meet the growth requirement, it is plain that such restriction may result in a deficiency of one, several, or all of the essential factors. If the diet is so selected as to be relatively rich in proteins, ash constituents, and the factors A and B, then re- striction of the amount of food will result primarily in an energy 336 CHEMISTRY OF FOOD AND NUTRITION deficit. Waters has described experiments which appear to have been of this character. He reports numerous cases of young cattle kept on restricted amounts of food of suitable kinds, the restriction being such as to materially retard the increase in weight as compared with that of a full fed animal of the same age, or even to hold the young animal at stationary weight at an age when it should have been growing rapidly. In such cases of insufficiency of the total food (energy) intake the skeleton continues to grow, in height at least, while adipose tissue steadily disappears, and the muscles become more or less depleted. In a young animal subjected to this type of under- nourishment the skeleton grows in height to a much greater extent than in width. Thus in a full-fed steer the increase in length of foreleg and in width of chest were about equal, while in one whose rate of growth was retarded by sparse rations the width of chest increased only one third as much as the length of foreleg, and in another animal of the same age whose food was so restricted as to permit no increase in weight the increase of chest-width was only one eighth as much as the increase in foreleg. The ratios actually measured in typical cases were as follows: Condition of Animal Width . Length of OF Chest * Foreleg I — full fed I : 0.97 I : 3-n I : 8.00 II — • retarded Ill — maintenance * Along with the narrower skeleton the underfeeding resulted in muscles of smaller diameter, absence of subcutaneous fat, and a general emaciated appearance. Young animals thus held at constant weight when they should be growing are in reality undergoing starvation. To quote from Waters' paper: * Just enough food to maintain constant weight in an animal which should have been growing rapidly had he been more Uberally fed. FOOD IN RELATION TO GROWTH 337 " Apparently the animal organism is capable of drawing upon its reserve for the purpose of sustaining the growth process for a considerable time and to a considerable extent. Our experi- ments indicate that after the reserve is drawn upon to a certain extent to support growth, the process ceases and there is no further increase in height or in length of bone. From this point on, the animal's chief business seems to be to sustain hfe. This law applies to animals on a stationary live weight as well as those being fed so that the live weight is steadily declining, and indeed to those whose ration, while above maintenance, and causing a gain in hve weight, is less than the normal growth rate of the individual. Such an animal will, while gaining in weight, get thinner, because it is drawing upon its reserve to supplement the ration in its effort to grow at a normal rate." " On all the animals under observation the retardation in height growth did not manifest itself at all until after the sparse nourishment had been continued for several months. On the other hand, the influence upon the width development was observable much earlier, and width development ceased altogether, in the case of animals on a maintenance or submain- tenance ration, long before the height development had ceased." " Our experiments have shown that within certain limits which are not yet at all well defined, retarded growth means retarded development of the organism. Thus an animal at twelve months of age and weighing on account of sparse nour- ishment only 400 pounds when it should under natural nourish- ment have weighed 800 pounds, has not its tissues as fully developed and matured as they would have been had the nourishment been normal. For example, we find that the flesh of steers 14-16 months old that had been sparsely fed through- out their lives presented the general characteristics such as color, flavor, etc. of veal or the flesh of calf. At this age the flesh of a highly nourished animal possessed the characteristic color, texture, and flavor of beef. Prof. Eckles has shown that 538 CHEMISTRY OF FOOD AND NUTRITION dairy heifer calves heavily fed reach sexual maturity at from eight to ten months of age, whereas similarly bred individuals that were sparsely fed did not reach the stage of puberty under from 16 to 19 months of age." " An animal which has been retarded and which in its earlier hfe has shown an asymmetric development, may tend later to correct this asymmetry, and it is not inconceivable that this may be fully corrected before the animal has reached a state of complete maturity, or a point where growth ceases altogether." Somewhat similar experiments have been performed upon dogs by Aron. Here also when the food was suitable in char- acter but too Hmited in amount to support normal growth the young animals grew in length and height but became thinner. Because of the " growth impulse " such an underfed young animal burns his reserve of body material to cover the deficit in the energy intake " in his endeavor to grow at a normal rate." Such a condition continued indefinitely results after a time in cessation of all growth and finally in death from star- vation. A dog which by underfeeding had been kept for a year at the weight which he had when 5 weeks old and had be- come long, tall, and very thin, and was then fed liberally im- mediately gained in weight and circumference but appeared to have lost the capacity for further growth in length and height. If, however, the period of underfeeding be not too prolonged, the animal on subsequently receiving ample food may regain normal proportions and grow to full normal size. Since stationary weight in the young animal which is at- tempting to grow with an insufficient energy supply does not mean cessation of all growth but growth of bone and brain at expense of adipose tissue and to some extent also of muscle, it follows that the body of such an animal gradually changes in composition, the percentages of fat and perhaps protein becom- ing less while the percentages of water and ash increase. If, however, the diet is rich in fat, as in experiments upon mice FOOD IN RELATION TO GROWTH 339 recently reported by Mendel and Judson, a simple diminution of the amount of food to a point where gain in weight ceases may not result in any such general replacement of fat by water, perhaps because in such a case the stunting may be due to in- sufficiency of some of the other factors rather than to an energy deficit. The experiments of Mendel and Judson also yield interesting data regarding the changes which normally occur in the water, fat (ether extract), and ash content of the body during its most active growth. From 88 analyses of the entire bodies of mice the following changes in composition were found : (a) increase in solids from 16 percent at birth to a maximum of 35 per cent at fifty days with a subsequent decrease to ^t, per cent ; (b) de- crease in the proportion of water in the fat-free substance from 85.5 per cent at birth to 73 per cent in the adult mouse ; (c) rapid increase in fat from 1.85 percent at birth to about 10 percent followed by slow increase to 1 2 per cent ; (d) increase in ash content from 1.86 per cent at birth to 3 per cent in the adult mouse. (2) Protein As explained in earlier chapters (text and figures, pages 55-68 and 224-226), it was shown by Osborne and Mendel that with a diet adequate in all other respects any one of a number of purified proteins such as casein, lactalbumin, or edestin might serve as the sole protein both for maintenance and for growth, while gliadin as sole protein food sufficed for maintenance but not for growth, and zein as sole protein did not suffice even for maintenance. Gliadin contains adequate tryptophane but only about I per cent of lysine ; addition of lysine to the gliadin ration made it adequate for growth. Zein contains neither tryptophane nor lysine ; addition of tryptophane to the zein diet makes it adequate for maintenance; addition of both tryptophane and lysine makes it adequate for growth. 340 CHEMISTRY OF FOOD AND NUTRITION When " adequate " proteins were fed in progressively re- stricted amounts, i.e. in diminishing percentage of the food mixture, Osborne and Mendel found that with diflerent pro- teins different amino acids prove to be the limiting factors — e.g. lysine in the case of edestin, cystine in the case of casein. With 15 to 18 per cent of casein in the food mixture the rate of growth was normal; with 9 to 12 per cent of casein the rats grew more slowly but normal rate of growth was resumed upon adding 3 per cent of cystine to the food mixture. With only 4.5-6 per cent of casein the addition of the 3 per cent cystine did not make the growth normal, indicating that with casein reduced to this point the supply of some other amino acid had become insufficient.* Another case in which cystine appears to have been a de- termining factor in tissue growth has been recorded by Evvard, Dox, and Guernsey in connection with their feeding experiments upon pregnant swine. Here a difference in the hair coats of the new-born pigs appeared to be due to the different intake of cystine in the food protein consumed by the mother, hair being rich in sulphur, and cystine the sulphur-bearing amino acid of the food. A so-called incomplete protein, i.e. one which when fed alone is quite inadequate to meet the requirements of protein metab- olism, may nevertheless contribute toward these requirements to an important degree and may even play a prominent part in promoting growth, as was strikingly demonstrated by Osborne and Mendel in experiments in which they added zein to a ration containing a small percentage of lactalbumin. (See Fig. 4, page 66.) Here the addition of zein to the ration more than doubled the rate of growth. Still more recently McCollum, Simmonds, and Pitz, feeding rats on rations composed of a single grain with supplementary additions, find that gelatin supplements wheat proteins excellently though it apparently does not ap- * Journal of Biological Chemistry, Vol. 20, page 351. FOOD IN RELATION TO GROWTH 341 preciably improve the proteins of maize or oats. Since gelatin, althougli lacking tyrosine and tryptophane is relatively rich in lysine, these results are interpreted as indicating that lysine is probably the limiting factor in wheat proteins but not in the proteins of the maize or of the oat kernel. In view of such evidence it is important to guard against the erroneous impression that incomplete proteins are useless for growth. The illustrations just given show that the growing organism may use such proteins to extremely good advantage ; but the " incomplete " proteins must not be permitted to dis- place the " complete " proteins to too great an extent if the young organism is to grow and develop at a fully normal rate. When growth is retarded by inadequate intake of protein or of a particular amino acid, the emaciated appearance char- acteristic of animals attempting to grow on an insufficient en- ergy intake is not to be expected. Osborne and Mendel have recorded numerous cases of suspension of growth of young rats, especially when kept on rations containing gliadin as a sole protein food. Here the inadequacy of the lysine intake results in retardation or even complete suspension of growth, but the animal may remain quite healthy and symmetrical. Moreover rats may be subjected to this type of stunting for a remarkably long time (even as long as would normally cover the entire growth period) and still retain their capacity to grow when given an adequate diet. In some cases*" after periods of suppression of growth, even without loss of body weight, growth may proceed at an exag- gerated rate for a considerable period. This is regarded as something apart from the rapid gains of weight in the repair or recuperation of tissue actually lost. Despite failure to grow for some time the average normal size may thus be regained be- fore the usual period of growth is ended." Statistical studies * Osborne, Mendel, Ferry, and Wakeman. A merican Journal oj Physiology, Vol. 40, pages 16-20 (1916). 342 CHEMISTRY OF FOOD AND NUTRITION on children also indicate that retardation in early growth can usually be made up by extra rapid growth later.* Mendel and Judson have studied the influence of different types of protein stunting upon the composition of the body in the case of the mouse. They find that when abundance of fat is furnished in the diet, but not enough protein to maintain normal growth, the percentage of fat in the animal becomes greater than when the food contains an adequate amount of protein with the same proportion of fat. They suggest that: " There seems to be a tendency to protect the limited amount of protein by increasing the available supply of fat in the body." " This does not occur when growth is arrested by lack of lysine, as in the use of gliadin as the only protein in the diet, since in this case the Umiting factor lies not in the amount but in the nature of the protein." (3) Ash Constituents Ash constituents have long been recognized as playing an important part in the growth of young animals and of these, as we have already seen, the elements most likely to be deficient are calcium, phosphorus, and iron. Infants (and young mam- mals generally) are born with a reserve store of iron usually sufficient to supply the growth requirement up to about the end of the normal suckhng period. At any time after this initial reserve supply has been used, the iron in the body will be found very largely localized in the blood. The blood con- stitutes less than 7 per cent of the weight of the body but con- tains more than 70 per cent of its iron content. Hence a deficit of iron becomes more noticeable in the blood than in the other tissues — growth may not cease but the child (or young animal) may grow anemic ; experiments illustrating this have been cited in the chapter on iron, and it has been shown that inorganic forms of iron are not of equal nutritive value with the organic * Mendel. Biochemical Bullclin, Vol. 3, page 167. FOOD IN RELATION TO GROWTH 343 forms which occur naturally in food materials. To an even greater extent than the iron is localized in the blood, the cal- cium of the body is localized in the bones ; it is estimated that the bones contain over 99 per cent of the body calcium. An inadequate supply of calcium in the food during growth retards the development and calcification of the bones. The calcium needed by the growing organism can be assimilated from inor- ganic forms. Both of these facts are illustrated by the experiment of raising pup- pies on meat with and with- out bones to gnaw as de- scribed in Chapter XI. It has also been found that the addition of calcium chloride and calcium carbonate to a basal ration of corn and common salt in the case of pregnant swine resulted in greater size, more vigor, bigger bone, and better general condition of the new-born pigs (Eward, Dox, and Guernsey). Bone development may also be interfered with by inadequacy of the phosphorus supply. Several investigators, in studying the effect of diet upon growth of bone, have found that the bones formed in a young animal kept on phosphorus poor diet are apt to be soft, spongy, and weak (of low breaking strength), and that this may be prevented by the simple addi- tion of calcium phosphate to the food. Since phosphorus is a prominent constituent not only of bones but of all the soft tissues as well, the effects of a phos- Time "> Months Fig. 13. — Efifect upon growth of adding to a diet otherwise adequate a salt mixture of such composition as to make the composition of the total ash similar to that of milk ash ; immediate resumption after entire suspension of growth.' Courtesy of Dr. E. V. McCoUum. 344 CHEMISTRY OF rOOD AND NUTRITION phorus deficiency may be far-reaching. In the experiments of Hart, McCollum, and Fuller, young pigs on phosphorus-poor food continued to grow for some time but finally developed not only the bone defects just noted but also weakness of the legs, stupor, and a more or less comatose condition accompanied by twitching of the muscles, dragging of the hind quarters, and Time in Months Fig. 14. — Growth at much less than half the normal rate through the greater part of the normal growth period, followed by accelerated growth upon adding a suitable salt mixture to the diet. Courtesy of Dr. E. V. McCollum. ultimately loss of weight and collapse. These effects were all prevented by simple addition of calcium phosphate to the food. Hart and McCollum record cases in which swine restricted to a ration of corn meal and corn gluten showed little or no growth, but began to make good growth upon addition to the food of such salts as to make the ash content of the ration similar to that of milk. FOOD IN RELATION TO GROWTH 345 McCollum, Simmonds, and Pitz have likewise shown that a defective inorganic content of the diet may also result in re- tardation or suspension of the general growth of the young animal, which may be followed by prompt resumption of growth (even at an accelerated rate so that the normal weight for the age may be regained) when a salt mixture is added such as to make the total ash of the ration similar in composition to milk ash (Figs. 13 and 14). (4) VlTAMINES OR FoOD HORMONES Osborne and Mendel (1913) found that the use of highly puri- fied salts in rations of isolated food substances resulted in less growth than when salts of only ordinary purity were fed. This suggested to them that other inorganic salts might be needed, and a ration containing very small addi- tions of salts of iodine, fluorine, manganese, and aluminum was fed with somewhat more favorable results than had attended the use of the usual (simpler) salt mix- ture ; but none of their diets composed entirely of pure substances gave as good results as the corresponding food mixtures in which " protein-free milk " was used, and they concluded that the latter was unquestionably superior to any purely arti- ficial food mixture. This superiority now seems to be attributable primarily to the soluble A " to a diet ade- presence in the " protein-free milk " of the quate in all other respects. "water soluble B," probably identical with °|"''^^*y ° the antineuritic " vitamine." If the latter is the case, the substance is not confined to milk but is fairly widely distributed among natural food materials. Less widely distributed is the other "essential accessory" furnished by milk, Time '" Months Fig. 15. — Effect upon growth of adding "fat 346 CHEMISTRY OF FOOD AND NUTRITION the so-called " fat soluble A," to the presence of which in butter * is attributed its marked growth-promoting property as shown independently by McCollum and Davis and by Osborne and Mendel. The latter find that in a diet containing " protein- free milk " and an adequate protein, 5 per cent of butter fat usually suffices to insure normal growth and in a few cases from I to 3 per cent has seemed sufficient. When butter fat is fractionally crystal- lized from alcohol the growth-promoting factor remains in the oil fraction , the fractions of higher melting point being in- effective. Lard and olive oil were also found in- efifective, while cod liver oil resembled butter fat in its growth-promoting property, and beef fat shows the same property to a less degree. Mc- Collum finds the same property in the fat of egg yolk and of animal organs such as the kidney, but in no com- mercial fat of vegetable origin thus far examined, although feed- ing experiments with whole grains and grain embryos indicate that their fats must carry appreciable amounts of this growth- promoting substance. He finds also, as noted earlier in the chapter, that the same " fat soluble A " (as demonstrated by A / ^r \ ^Y / "y "y" marks bir of young Th V / ^ ithout "^y iter Soluhl , B" ■ E Time '" Months Fig. 16. — Effect upon growth of adding " water soluble B " to an otherwise adequate diet. Courtesy of Dr. E. V. McCollum. ♦According to McCollum, "fat soluble A" is about 30 times more soluble in fat than in w^ater. In milk about half of it is dissolved in the small volume of fat and about half in the large volume of water present. Skimmed milk is, therefore, not whullv devoid of this substance. FOOD IN RELATION TO GROWTH 347 specially arranged feeding experiments) occurs in relative abun- dance in alfalfa and cabbage leaves and probably in green vege- tables and forage plants generally. The accompanying charts (Figs. 15 and 16) show the effects of presence or absence of A or B upon the growth curves of young rats. Recognition of the in- dependent need for each of these substances or groups of sub- stances is too recent for definite correlation of each with a dis- tinct type of stunting. Both " fat soluble A " and " water soluble B " are held to be essential for the maintenance of health as well as for growth. The fat soluble A appears to be dis- pensable, when maintenance alone is involved, for a somewhat longer period than is the water soluble B, which accounts for the polyneuritic symptoms in birds kept on polished rice diet and the cure of these symptoms by the feeding of extracts of foods rich in the water soluble B. Thus McCoUum and Ken- nedy find " that pigeons can be brought into the polyneuritic state by feeding a diet free from both the essential factors A and B, and can be completely cured and maintained in a normal condition for at least 35 days on the same diet which brought on the disease, plus the water extract of a foodstuff (rolled oats) on which rats cannot grow without the addition of butter fat, but on which they do grow when the latter is added." Dietary Deficiencies of Individual Articles of Food McCoUum and his associates are now applying the above conceptions to the study of the dietary deficiencies of individual articles of food. In a recent paper * they present their plan of investigation as follows : " If a single natural food product fails to nourish an animal adequately, it may be due to : (a) lack of suflScient protein, or to proteins of poor quality ; (b) an unsatisfactory mineral con- tent due either to inadequacy of certain elements in amount, or * McCoUum, Simmonds, and Pitz. Journal of Biological Chemistry, Vol. 25, pages 105, 132 (May, 1916). 348 CHEMISTRY OF FOOD AND NUTRITION to unsatisfactory proportions among them ; (c) an inadequate supply of the fat soluble A ; (d) of the water soluble B ; (e) or some toxic substance contained therein. One, two, three, four, or all of these factors may operate in inducing nutritive dis- turbances. " It should be obvious that a systematic procedure in which we feed the substance under investigation supplemented with (a) pure protein only, (b) salt mixture additions only, (c) but- ter fat only, (d) extracts known to carry the water soluble B and as little else as is possible, will reveal whether the failure of nutrition involves one factor only, or more than one. If more than one factor is involved, a similar procedure, but with the addition of all possible combinations of pairs of the isolated food ingredients listed above, followed if need be by another series of feeding experiments in which animals are fed the natural foodstuff supplemented with three such uncomplicated additions, in all possible combinations, and if necessary another experiment in which all four additions are made, \vill give us results which make it possible to consider the components of our rations in an entirely new light. Provided the foodstuffs contain a toxic substance, special procedures will have to be devised for studying its effects. " Similar studies must also be made by this method of pro- cedure, with pairs of the important foodstuffs (food materials) in varying proportions, the variation of the mixture including sufficient range to reveal the degree to which the deficiencies of the protein mixture of one grain are corrected by the peculiar quantitative relationships among the amino acids yielded by the proteins of the other grain. The same may be said for the factors other than protein. In this way we shall become able to interpret the biological value of the. mixtures of natural foodstuffs which make up the rations which are in common use, in which the attempt is now made to make for safety through variety. We have carried our inquiry into the nature of the FOOD IN RELATION TO GROWTH 349 dietary deficiencies of several natural products far enough to convince us of the practicability of this method of study." Following this general plan McCoUum and his associates have studied the dietary deficiencies of wheat, wheat embryo, rice, maize, oats, and beans. While some of the results thus obtained have already been cited, it may be well to summarize here the chief findings with reference to each of these food materials in succession. In all cases the experiments were chiefly upon rats. The wheat kernel when fed alone did not induce normal growth in the experimental animals. Addition of either (i) purified casein, (2) butter fat, or (3) a suitable salt mixture such as to make the total ash of the ration resemble milk ash in composition, was found to improve conditions to some degree in each case, but in no case did such a single addition result in normal growth. Neither could fully satisfactory results be secured by the addition to the wheat ration of any two of these three factors mentioned ; but when all three were added, the animals showed complete growth and normal reproduction. Hence McCollum concludes that the wheat kernel is deficient as a food (i) in the poor quality of its protein, (2) in that it furnishes an inadequate supply of " fat soluble A," (3) in that it has an unsatisfactory inorganic content. He also believes that when the diet is chiefly made up of the entire wheat kernel, including embryo, the possibility of a mild toxicity, due to a toxic constituent in the embryo, must also be reckoned with. Wheat embryo when fed alone did not induce growth although it is rich in proteins of high nutritive efficiency and in water soluble B, and not deficient in fat soluble A. It is deficient in its inorganic content; even so simple a modification as the addition of 2 per cent of calcium lactate to the wheat embryo diet may induce noteworthy growth where otherwise no growth takes place. To an important extent, according to these authors, the failure of rats and swine to grow on diets consisting 350 CHEMISTRY OF FOOD AXD NUTRITION largely of wheat embryo is attributable to a toxic substance contained therein, which appears to be associated with the fat. Extraction of the fat by ether removes in great measure the toxicity of the embryo without necessarily making the food deficient in the fat soluble A. According to the authors the toxicity may be overcome by the simple addition of casein to the diet. That diet may greatly influence susceptibiHty to toxicity was reported by Hunt in 1910. Hunt found great differences in susceptibility to acetonitrile poisoning, which differences appeared to be due to diet alone.* Polished rice as a diet for growth was found to be deficient in four respects : (i) its protein content seemed too low for maxi- mum growth; (2) it contained inorganic elements in insuf- ficient amounts and also not in proper proportions ; (3) it was found deficient in fat soluble A ; (4) it lacked water soluble B. Maize when fed alone induced no appreciable growth, nor could a suitable diet be made by mixing the parts of the maize kernel in different proportions. The proteins of the maize kernel contain all the amino acids essential for growth, but it is held that the proportion of certain of them is such that * "In extreme cases mice after having been fed upon certain diets maj' recover from forty times the dose of acetonitrile fatal to mice kept upon other diets. It is, moreover, possible to alter the resistance of these animals at will and to overcome the effects of one diet by combining it with another. . . . The experiments with oats and oatmeal and eggs are of especial interest. In the earUer parts of this paper many experiments were quoted showing that a diet of oatmeal or of oats usually leads to a mark^ed resistance of mice to acetonitrile ; the experiments quoted in this section which show that the administration of certain iodine compounds with or subsequently to such a diet further increases this resistance, and the experiments previously reported showing that as far as the resistance toward acetonitrile is con- cerned iodine exerts its action through the thyroid gland, all point to the conclusion that the resistance caused by an oat diet is in part an effect e.xertcd upon the thyroid. This effect is obtained much more markedly and constantly with young, growing mice. From these experiments and considerations it seems very probable that it is possible to influence, in a specific manner, by diet, one of the most important hormones in the body ; this is a comparatively new principle in dietetics and one which may prove of much importance " (Hunt, The Effect of a Restricted Diet and of Various Diets upon the Resistance of Animals to Certain Poisons, pages 56, 73). FOOD IN RELATION TO GROWTH 351 when this is the sole source of protein the growth is never more than about two thirds normal. The maize diet always requires the addition of a suitable salt mixture (or food of suitable ash content). Also the amount of fat soluble A is insufficient in maize to induce growth at the normal rate. Normal growth and reproduction, however, occurred when maize was supplemented by butter fat, purified casein, and a suitable salt mixture. The oat kernel, according to McCoUum's investigations, con- tains protein of poorer quality than either the maize or wheat kernel. When all other dietary factors are properly adjusted, nine per cent of oat protein in the diet serves to induce slow growth for a time, but never for more than about a month (ex- periments with rats). Casein, which serves as such an efficient adjunct to the wheat and maize proteins, does not seem to sup- plement oat protein in a very satisfactory manner ; a diet with 9 per cent of protein from the oat kernel and 10 per cent purified casein did not induce growth at a maximum rate as did similar combinations of casein with wheat and maize proteins. In this connection McCollum reports the unexpected finding that gelatin supplements the protein of the oat kernel more effectively than does casein. The ash constituents of the oat kernel must always be sup- plemented in order to induce growth. Fat soluble A is present in the oat kernel in very small amounts. The amount of water soluble B is adequate. Growth at more than half the normal rate may be obtained when the oat diet is supplemented by the addition of a suitable salt mixture and either butter fat or a suitable protein. When all three of these supplements are added, growth is normal but somewhat slow. McCollum be- lieves that excessive feeding of the oat kernel causes some injury to the animal. The -white bean, when fed as the chief component of the diet, gave results indicating that its proteins are of lower nutritive efficiency than those of the cereal grains. The bean protein 352 CHEMISTRY OF FOOD AND NUTRITION can be supplemented by ihe addition of g per cent of casein to the diet. The inorganic content of the white bean is not such as to induce growth, but must be supplemented by a suitable salt mixture (or by food of a different ash content from that of the bean alone). The white bean seems to contain less of fat soluble A than do the cereal grains. It contains water soluble B in abundance. The bean diet appeared to exert an unfa- vorable effect in that animals fed on a diet containing a smaller proportion of beans (25 percent of the total food) seemed better nourished than those whose diet contained a larger proportion. It is suggested that beans may contain some unknown sub- stance which is harmful when taken in too large an amount ; or that the pressure of the intestinal gases resulting from fer- mentation of the hemicelluloses for which the higher animals have no digestive enzyme may result in a somewhat asphyxial condition of the intestinal wall, thus interfering with the normal processes of absorption and unfavorably affecting the general condition of nutrition. Seeds in general are held by McCollum to require supplement- ing in order to make a diet which will support normal growth and reproduction. As supplement to a diet consisting largely of the products of cereal grains or other seeds, milk is found to be especially effective. It is also found that while seeds are not effectively supplemented by other seeds, they may be sup- plemented by the leaves and probably also by the roots and tubers of plants so that it is feasible, if desired, to draw a bal- anced diet, adequate for all the requirements of growth and reproduction in an omnivorous animal, entirely from the prod- ucts of plants. Thus McCollum kept rats through four generations upon a carefully adjusted ration of maize, alfalfa, and cooked peas. Growth and reproduction were normal. The mothers successfully suckled young up to the normal age of weaning, after which they took the same food mixture as the adults. In this connection it is interesting to note that rats FOOD IN RELATION TO GROWTH 353 which were free to make their own selection from a much greater variety of vegetable foods never grew beyond half the normal adult size. In practice milk is found to be most highly efficient as a sup- plement to diets consisting largely of seeds or their products: " The dietary should be built around bread and milk." The chemical constitution of its proteins and its high calcium and \itamine contents are all factors in the unique nutritive efficiency of milk, and make it possible for a moderate addition of milk to render adequate a diet otherwise composed entirely of seeds. Cotton-seed meal or flour * constitutes an abundant and con- centrated source of protein and energy which as yet has been but Httle utiHzed in human nutrition. This is doubtless largely because bad results have sometimes followed its use in stock feeding, leading to the general belief that it is somewhat toxic, at least when used in considerable quantities. Withers and Carruth succeeded in extracting from the kernels of the cotton- seed a substance, gossypol, which shows deleterious action when fed and to which the toxicity of raw cotton seed and of some cotton-Seed meals was attributed. This substance, however, is thermo-labile, and apparently is more or less completely de- stroyed by the heating to which cotton-seed meal or flour is ordinarily subjected in connection with the processes of crush- ing and pressing. Feeding experiments to determine whether the well-prepared cotton-seed meal or flour now available for human food has any appreciable toxicity, and to what extent it meets the nutritive requirements of normal growth and re- production, have recently been reported by Richardson and Green and by Osborne and Mendel. Richardson and Green, feeding a high-grade commercial cotton-seed flour, found that no evidence of toxicity appeared although this flour constituted * Cotton-seed flour is prepared by finely grinding, sifting, and perhaps also as- pirating the meal so that particles of lint, hulls, etc., are removed more completely than from the ordinary cotton-seed meal used in stock feeding. 2 A 354 CHEMISTRY OF FOOD AND NUTRITION 45 to 50 per cent of the ration of albino rats through four successive generations or during 565 days of the life of an individual (about two thirds the entire normal Hfe span) ; that the cotton-seed flour met all protein requirements of main- tenance and growth, and when supplemented with protein-free milk and butter fat was able to support normal growth and re- production. They found that no better growth was induced, but more frequent reproduction with lower mortahty and more general well-being of animals were obtained when 5 per cent of casein was added to a diet containing 50 per cent cotton-seed flour with butter fat, protein-free milk, lard, and starch. Nor- mal growth and reproduction did not result from diets con- taining 50 per cent cotton-seed flour in which there was a lack of butter fat, protein-free milk, or both. On a diet containing fifty per cent cotton-seed flour with the addition of casein and butter fat, but with no mineral matter other than that from the cotton seed, rats grew normally and reproduced, but the second generation did not make quite normal growth. Osborne and Mendel also found the proteins of cotton-seed flour to be efficient in nutrition, not only when fed alone in relatively abundant amounts but also when used as supple- ments to maize protein. They obtained toxic effects from the feeding of cotton-seed kernels but not from the cotton-seed flour. Like Withers and Carruth they demonstrated that the harmful substance could be removed from the kernels by extraction with ether ; but the kernels can also be rendered harmless by steam- ing, which is a step in the usual commercial process of extracting the oil. The results of heating were, however, not altogether uniform and Osborne and Mendel suggest that undue heating may render the meal unpalatable or otherwise unsuitable for nutrition, in addition to destroying the original deleterious substance, and that these facts may help to explain the con- flicting evidence regarding the alleged suitabihty of different samples of commercial meals. FOOD IN RELATION TO GROWTH 355 These recent investigations upon cotton-seed flour are worthy of careful study both because of the great economic importance of this material and because they illustrate well the application of modern methods of nutrition research to the solution of a long-standing problem regarding the utility of an abundant but relatively neglected food material. REFERENCES AcKROYD AND HoPKiNS. Feeding Experiments with Deficiencies in the Amino Acid Supply. Biochemical Journal, Vol. 10, page 551 (1916). Aron. Nutrition and Growth. Philippine Journal of Science, Vol. 6 B, pages 1-52 (191 i). Chittenden and Unt)erhill. The Production in Dogs of a Pathological Condition Closely Resembling Human Pellagra. American Journal of Physiology, Vol. 44, page 13 (191 7). Daniels and Nichols. The Nutritive Value of the Soy Bean. Journal of Biological Chemistry, Vol. 31, page 91 (19 17). Eward, Dox, and Guernsey. Effect of Calcium and Protein Fed Pregnant Swine upon the Size, Vigor, Bone, Coat, and Condition of the Offspring. American Journal of Physiology, Vol. 34, pages 312-325 (1914). Forbes.' Specific Effects of Rations upon the Development of Swine. Ohio Agricultural Experiment Station, Bull. 213 and 283 (1909 and 1915)- GoETSCH. Influence of Pituitary Feeding upon Growth and Sexual Develop- ment. Johns Hopkins Ilospilal Bulletin, Vol. 27, page 29 (1916). Hart, Halpin, ant) McCollum. The Behavior of Chickens Fed Rations Restricted to the Cereal Grains. Journal of Biological Chemistry, Vol. 29, page 57 (February, 191 7). Hart and McCollum. Influence on Growth of Rations Restricted to the Corn or Wheat Grain. Journal of Biological Chemistry, Vol. 19, page 373 (1914)- Hart, McCollum, and Fuller. The R6le of Inorganic Phosphorus in the Nutrition of Animals. Wisconsin Research Bulletin i ; Amer- ican Journal of Physiology, Vol. 23, page 246 (1908-1909). HLiRT, ]McCollum, Steenbock, and Humphrey. Physiological Effects upon Growth and Reproduction of Rations Balanced from Restricted Sources. Wisconsin Agricultural Experiment Station, Research Bull. 17 (1912); Wisconsin Bull. 228, page 33; Journal of Agricultural 356 CHEMISTRY OF FOOD AND NUTRITION Research, Vol. lo, page 175; and Proceedings of llic National Academy of Sciences, Vol. 3, page 374 (191 7). Hart, Miller, and McCollum. Further Studies on the Nutritive De- ficiencies of Wheat and Grain Mixtures and the Pathological Condi- tions Produced in Swine by their Use. Journal of Biological Chemislry, Vol. 25, page 239 (June, 1916). HoGAN. Corn as a Source of Protein and Ash for Growing Animals. Jour- nal of Biological Chemistry, Vol. 29, page 485 (191 7). Hopkins. Feeding Experiments Illustrating the Importance of Accessory Factors in Normal Dietaries. Journal of Physiology, Vol. 44, page 425 (1912). LusK. Science of Nutrition, Third Edition, Chapters 13 and 14. McCoLLUM. The Value of the Proteins of the Cereal Grains and of Milk for Growth in the Pig, and the Influence of the Plane of Protein In- take on Growth. Journal of Biological Chemistry, Vol. 19, page 323 (November, 191 4). McCoLLUM. The Supplementary Dietary Relationships among Our Natural Foodstuffs. Journal of the American Medical Association, Vol. 68, page 1379 (May 12, 191 7). McCoLLUM AND Davis. The Substance in Butter Fat Which Exerts a Stimulating Influence on Growth. Journal of Biological Chemistry, Vol. 19, page 245 (October, 1914). McCoLLUM AND Davis. Influence of the Plane of Protein Intake on Growth. Journal of Biological Chemistry, Vol. 20, page 415 (1915). McCollum and Davis. Nutrition with Purified Food Substances. Jour- nal of Biological Chemistry, Vol. 20, page 641 (.A.pril, 1915). McCollum and D.wis. Influence of Certain Vegetable Fats on Growth. Journal of Biological Chemislry, Vol. 21, page 179 (May, 1915). McCollum and Davis. Influence of Mineral Content of the Ration on Growth and Reproduction. Journal of Biological Chemislry, Vol. 21, page 615 (July, 1915)- McCollum and Davis. The Nature of the Dietary Deficiencies of Rice. Journal of Biological Chemislry, Vol. 23, page 181 (November, 1915). McCollum and Davis. The Essential Factors in the Diet during Growth. Journal of Biological Chemislry, Vol. 23, page 231 (November, 1915). McCollum and Simmonds. A Biological Analysis of Pellagra-Producing Diets. Journal of Biological Chemistry, Vol. 31, pages 29 and 181; Vol. 32, page 347 (1917). , McCollum, Simmonds, and Pitz. The Nature of the Dietary Deficiencies of the Wheat Embryo. Journal of Biological Chemistry, Vol. 25, page 105 (May, 1916). FOOD IN RELATION TO GROWTH 357 McCoLLUM, SiMMONDS, AND PiTZ. The Relation of the Unidentified Diet- ary Factors, the Fat-soluble A, and Water-soluble B, of the Diet to the Growth-promoting Properties of Milk. Journal of Biological Chemistry, Vol. 27, page 33 (October, 1916). McCoLLXJM, SiMMONDS, AND PiTz. The Vegetarian Diet in the Light of Our Present Knowledge of Nutrition. A mcrican Journal of Physiology, Vol. 41, page 333 (September, 1916). See also Journal of Biological Chemistry, Vol. 30, page 13 (May, 1917)- McCoLLUM, SiMMONDS, AND PiTz. The Distribution in Plants of the Fat Soluble A, the Dietary Essential of Butter Fat. American Journal of Physiology, Vol. 41, page 361 (September, 1916). McCoLLUM, SiMMONDS, AND PiTz. Dietary Deficiencies of the Maize Kernel. Journal of Biological Chemistry, Vol. 28, page 153 (December, 1916). McCoLLUM, SiMMONDS, AND PiTZ. The Effects of Feeding the Proteins of the Wheat Kernel at Different Planes of Intake. Journal of Biological Chemistry, Vol. 28, page 211 (December, 1916). McCoLLUM, SiMMONDS, AND PiTZ. Is Lysine the Limiting Amino Acid in the Proteins of the Wheat, Maize, or Oat Kernel? Journal of Bio- logical Chemistry, Vol. 28, page 483 (January, 1917)- McCoLLUM, SiMMONDS, AND PiTz. The Nature of the Dietary Deficiencies of the Oat Kernel. Journal of Biological Chemistry, Vol. 29, page 341 (March, 191 7). McCoLLUM, SiMMONDS, AND PiTZ. The Dietary Deficiencies of the White Bean {Phaseolus vulgaris). Ibid., Vol. 29, page 521 (April, 1917)- McCrudden. Nutrition and Growth of Bone. Transactions of the 15th International Congress of Hygiene and Demography, Washington, 191 2. Mendel. Viewpoints in the Study of Growth. Biochemical Bulletin, Vol. 3, page 156 (January, 1914)- Mendel. Nutrition and Growth. The Harvey Lectures, Series 10, 1914-1915. Mendel. Abnormalities of Growth. American Journal of the Medical Sciences, Vol. 153, page i (January, 1917). Mendel and Judson. Some Interrelations between Diet, Growth, and the Chemical Composition of the Body. Proceedings of the National Academy of Sciences, Vol. 2, page 692 (December, 1916). Mendel and Osborne. Growth. Journal of Laboratory and Clinical Medicine, Vol. i, page 211 (January, 1916). Osborne and Mendel. Feeding Experiments with Isolated Food Sub- stances. Carnegie Institution of Washington, Publication No. 156, Parts I and II (191 1). 358 CHEMISTRY OF FOOD AND NUTRITION Osborne and Mentjel. R6le of Gliadin in Nutrition. Journal of Biologi- cal Chemislry, Vol. 12, pages 473-510 (1912). Osborne and Mendel. Influence of Butter Fat on Growth. Journal of Biological Chemislry, Vol. 16, pages 423-437 (1913). Osborne and Mendel. The Influence of Cod Liver Oil and Some Other Fats on Growth. Journal of Biological Chemislry, Vol. 17, page 401 (April, 1914). Osborne and Mentjel. Relation of Growth to the Chemical Constituents of the Diet. Journal of Biological Chemislry, \'o\. 15, pages 311-326 (1913)- Osborne and Mendel. Amino Acids in Nutrition and Growth. Journal of Biological Chemislry, Vol. 17, pages 325-349 (1914). Osborne and Mendel. Nutritive Properties of Proteins of the Maize Kernel. Journal of Biological Chemislry, Vol. 18, pages 1-16 (1914). Osborne and Men-del. The Comparative Nutritive Value of Certain Proteins in Growth, and the Problem of the Protein Minimum. Jour- nal of Biological Chemistry, Vol. 20, page 351 (1915). Osborne and Mentjel. Resumption of Growth after Long-Continued Failure to Grow. Journal of Biological Chemislry, Vol. 23, page 439 (December, 1915). Osborne and Mentjel. The Stability of the Growth-Promoting Sub- stance of Butter Fat. Journal of Biological Chemislry, Vol. 24, page 37 (January, 1916). Osborne ant) Mentjel. Acceleration of Growth after Retardation. Atner- ican Journal of Physiology, Vol. 40. page 16 (March, 1916). Osborne and Mentjel. The Amino Acid Minimum for Maintenance and Growth, as exemplified by further experiments with lysine and trjpto- phan. Journal of Biological Chemislry, Vol. 25, page i (May, 1916). Osborne and Mendel. The Growth of Rats upon Diets of Isolated Food Substances. Biochemical Journal, Vol. 10, page 534 (1916). Osborne and Mentjel. The Relative Value of Certain Proteins and Pro- tein Concentrates as Supplements to Corn Gluten. Journal of Bio- logical Chemislry, Vol. 29, page 69 (February, 191 7). Osborne and Mendel. Nutritive Factors in Animal Tissues. Journal of Biological Chemislry, Vol. 32, page 309 (191 7). Osborne and Mendel. The Use of Soy Bean as Food. Journal of Bio- logical Chemistry, Vol. 32, page 369 (191 7). Osborne, Mentjel, and Ferry. The Eflect of Retardation of Growth upon the Breeding Period and Duration of Life of Rats. Science, Vol. 45, page 294 (1917)- Pearl. Effect of Feeding Pituitary Substance and Corpus Luteum on FOOD IN RELATION TO GROWTH 359 Egg Production and Growth. Journal of Biological Chemistry, Vol. 24, page 123 (February, igi6). Rettger. Influence of Milk Feeding on Mortality and Growth. Journal of Experimental Medicine, Vol. 21, page 365 (19 15). Richardson and Green. Nutrition Investigations upon Cotton-seed Meal. Journal of Biological Chemistry, Vol. 25, page 307 (1916); Vol. 30, page 243; Vol. 31, page 379 (1917)- Robertson. (Experimental Studies of Growth and the Growth-control- ling Substance of the Pituitary Body.) Journal of Biological Chemistry, Vol. 24, pages 347, 2,(^3^ 385. 397, 409; Vol. 25, pages 625, 647, 663; Vol. 27, page 393 (1916). Waters. The Capacity of Animals to Grow under Adverse Conditions. Proceedings of the Society for the Promotion of Agricultural Science, Vol. 29, page 3 (1908). Waters. Influence of Nutrition on Animal Form. Proceedings of the So- ciety for the Promotion of Agricultural Science, Vol. 30, page 70 (1910). CHAPTER XIV DIETARY STANDARDS AND THE ECONOMIC USE OF FOOD The General Problem of a Dietary Standard It is sometimes asked whetlier a normal appetite does not indicate, as well as can any dietary standard, the amount of food which is desirable for an individual in any given circum- stances. In considering such a question we shall hardly expect the phrase " amount of food " to indicate equally the energy value, the protein content, the content of each of the necessary chemical elements, and each of the unidentified dietary essentials A and B (or fat soluble and water soluble " vitamines ")• Since different articles of food vary greatly in the relative amounts of the various nutrients which they contain, some one aspect of food value must be chosen as a basis in order to give definite meaning to the phrase " amount of food." Inasmuch as the most prominent of the nutritive requirements is the need for energy, and the yielding of energy is the one function in which practically all articles of food take part, it is logical to expect that " amount of food " will more nearly express number of calories than any other one factor of food value or nutritive requirement. Observation confirms this impression and shows that men or other animals when eating varied food under the unrestricted guidance of hunger and appetite tend to take such quantities as are proportioned to the energy requirement 360 DIETARY STANDARDS AND ECONOMIC USE OF FOOD 36 1 whether or not this amount meets also the requirements as to each of the sixteen chemical elements known to be necessary in nutrition. If then hunger and appetite be regarded as guides, primarily, to the eating of the right amount of food to meet the energy requirement, we may determine their adequacy in any given case by the fatness of the person concerned, since excess of fuel food of whatever kind can contribute to the storage of body fat. If from year to year the body keeps in good condition for its work and maintains a fairly constant weight which bears such a proportion to the height as to show that a proper amount of fat is being carried, it is reasonably certain that the amount (fuel value) of food eaten in the course of the year is substantially that which is suited to the degree of activity maintained. If, however, by following the appetite, one becomes unduly stout or unduly thin, or does not get sufficient fuel for the energy required for the day's work, or is annoyed by digestive disturb- ances indicative of improper feeding, it is certain that the appetite is in this case not a perfect standard. Still more often will the individual appetite prove an inadequate guide to such a quantitative combination of the different types of food as shall lead to a well-balanced intake of each of the many essential food constituents. Here the customs and traditions which govern the food economics of the household and which un- doubtedly to some extent reflect the accumulated experience of the race serve an extremely important purpose in checking the caprices of the palate and guiding the individual into food habits which are more likely to conform to actual needs than are the dictates of the individual appetite. But the fullest appreciation of the value of household and social traditions in the formation of good dietary habits does not justify the conclusion that such traditions will always lead to the best results, either physiologically or economically. Even if these traditions represented the experience of past generations to 362 CHEMISTRY OF FOOD AND NUTRITION the fullest imaginable extent, they could not be expected to guide us in the use of foods which were not available to our predecessors but have now within a generation become a common part of the dietary. Nor is it reasonable to suppose that dietary habits adapted to people engaged chiefly in outdoor occupations under frontier conditions will be equally suited to the sedentary city worker of to-day. Under modern conditions scientific dietary standards, based on a knowledge of food chemistry and nutritive requirements such as the preceding chapters have attempted to give, constitute the most rational guide to the formation of hygienic and economic habits in the use of food. The earliest attempts to set dietary standards in terms of nutrients were those of the German physiologists, among whom the most influential was Voit. He suggested as a proper daily allowance of foodstuffs for a man at moderate muscular work : Protein, 118 grams. Fat, 56 grams. Carbohydrates, 500 grams. This dietary would have a fuel value of approximately 3000 Calories. The allowance of 118 grams of protein, which has since provoked considerable discussion, is said to have been based upon the average protein metabolism of many laboring men who were living apparently upon unreetricted diet, so that it was practically the result of dietary study. In the division of the remaining calories between fat and carbohydrate, Voit made the allowance of fat low and of carbohydrates high in order to cheapen the dietary. In England, Playfair recommended as a standard for a man at moderate work : Protein, 119 grams. Fat, 51 grams. Carbohydrates, 531 grams. DIETARY STANDARDS AND ECONOINIIC USE OF FOOD 363 This would yield 3060 Calories and is evidently based quite directly upon Voit's recommendations. In France, Gautier has proposed as a standard for men with Uttle muscular work : Protein, 107 grams. Fat, 65 grams. Carbohydrates, 407 grams. This allowance of nutrients — which is based in part upon carbon and nitrogen balance experiments, in part upon studies of French famiUes selected as typical, and in part upon the statistics of food consumed in Paris for a period of ten years — would supply 2630 Calories. In America, dietary standards have been discussed chiefly by Atwater, Chittenden, and Langworthy. Atwater, in his later writings,* ceasing to make distinction between fats and carbohy- drates as sources of energy in ordinary dietaries, but making allowances for different degrees of muscular activity, rec- ommended the following standards: Standards for Protein, Grams TuEL Value, Calories Man at hard muscular work Man at moderately active muscular work Man at sedentary or woman with moder- ately active work - Man without muscular exercise or woman at light to moderate work 150 125 100 90 4150 3400 2700 2450 That these standards were not intended simply as expressions of the actual needs of the body is plainly shown by the allowance of 150 grams of protein for a man at hard work, as against 100 grams for a sedentary man. By his own experiments with men at rest and at work in the respiration calorimeter Atwater had * Farmers' Bulletin No. 142, U. S. Department of Agriculture. Also Fifteenth Annual Report Ag,rkullural Experiment Sldtion, Starrs, Conn., 1903. 3^4 CHEMISTRY OF FOOD AND NUTRITION demonstrated that muscular work need not increase protein metabolism, if a sufficient amount of fuel be provided in the form of carbohydrates and fats. Hence, when, in providing for muscular work, he proposes to increase the protein in practically the same ratio as the calories, the idea evidently is not that such an increase is necessary, but simply that it was considered advisable on general grounds not to alter very greatly the nature of the diet in increasing its amount. Langvvorthy's Compilation of Results of Dietary Studies Occupation of Head of Family United States : Man at very hard work (average 19 studies) . Farmers, mechanics, etc. (average 162 studies) Business men, students, etc. (average 51 studies) Inmates of institutions, little or no muscular work (average of 49 studies) Very poor people, usually out of work (average of 15 studies) Canada: Factory hands (average 13 studies) England : Workingmen Scotland : Workingmen Ireland : Workingmen Germany : Workingmen Professional men France : Men at light work Japan : Laborers Professional and business men China : Laborers Egypt : Native laborers Congo : Native laborers Food per Man* PER Day Protein, Fuel value, Grams Calories 177 6000 100 3425 106 3285 86 2600 69 2100 108 3480 89 2685 108 3228 98 3107 134 3061 III 2511 no 2750 118 4415 87 2igo 91 3400 112 2825 108 2812 *In calculating these results it is assumed that women consume o.S as much food as men, and children of different ages from 0.3 to 0.8 as much as the man of the family. DIETARY STANDARDS AND ECONOMIC USE OF FOOD 365 In explanation of the liberality of his standards Atwater suggested that " the standard must vary not only with the conditions of activity and environment, but also with the nutri- tive plane at which the body is to be maintained. A man may live and work and maintain bodily equilibrium on either a higher or a lower nitrogen level, or energy level. One essential question is, What level is most advantageous? The answer to this must be sought, not simply in metabolism experiments and dietary studies, but also in broader observations regarding bodily and mental efficiency and general health, strength, and welfare." Langworthy, maintaining a similar point of view, has collected the data of large numbers of dietaries believed to be fairly representative of the food habits of people of different occupa- tions in the United States and other countries, and stated them in terms of protein and calories per man per day with the results shown on the preceding page, Langworthy concludes that the results obtained, the world over, for persons of moderate activity, " do not differ very markedly from a general average of 100 grams of protein and 3000 Calories of energy, and that it is fair to say that, although foods may differ very decidedly, the nutritive value of the diet in different regions and under different circumstances is very much the same for a like amount of muscular work." He also points out that in some cases this may not be apparent until allowance is made for differences in body weight. Thus he estimates the average weight of the Japanese professional and business men at 105 pounds, so that their food consumption of 87 grams protein and 2190 Calories corresponds to 105 grams protein and 3120 Calories for a man of 150 pounds, which agrees well with the American average for similar employment. As a standard for men with more muscular activity, such as mechanics at moderately active work, Langworthy sug- gests 3500 Calories including 105 grams of protein. 366 CHEMISTRY OF FOOD AND NUTRITION Chittenden differs from those whose standards have been quoted in giving almost no weight to the results of dietary studies, holding that these serve chiefly as a measure of self- indulgence, and that the true measure of what the body will most profitably use is to be found in the results of experi- ments upon the protein metabolism, such as have been de- scribed in Chapter VIII. On the basis of these experiments he proposes as a standard allowance for the man of 70 kilograms body weight, 60 grams of protein and 2800 Calories per day. For business and professional men such as Chittenden evidently has in mind, the allowance of 2S00 Calories is in substantial agreement with earher estimates. Sixty grams of protein for a man of 70 kilograms is, however, decidedly lower than any standard previously current. Energy Allowances for Adults It has been shown in a previous chapter that different normal individuals of similar age and physique are substantially alike in their energy requirement when performing equivalent amounts of muscular work, and that it is primarily the muscular activity, and not personal idiosyncrasy or the amount of food eaten, which determines the amount of energy transformed in the body. A dietary standard of high fuel value, and designed to maintain metaboHsm on a high energy level, provides, therefore, primarily for a large amount of muscular work. If this work is not performed and the food continues to be eaten and digested, we may expect to find a storage of fuel in the body chiefly in the form of fat, and this is true whether the surplus food eaten is carbohydrate, fat, or protein. Thus the store of body fat which a person carries is the most reliable indication as to whether the amount of food habitually eaten is or is not properly adjusted to the work performed. The storage of fat does, however, in itself modify the food requirement. While it is true, as has been shown, that, as between a lean and a fat man DIETARY STANDARDS AND ECONOMIC USE OF FOOD 367 having the same weight, the lean man will have the greater food requirement, yet it is also true that when any given man becomes fat, his increased size of body calls for increased metaboHsm of energy. The work involved in walking, for example, will increase in proportion to the weight moved {i.e. to the weight of the body as a whole) ; and the work of respiration will in- crease about in proportion to the weight of that part of the body which must be moved with the expansion and contraction of the lungs; while, if fat is deposited in such a way as to interfere directly with the free play of the muscles, there may be an actual lowering of muscular efficiency, so that a larger expendi- ture of energy may be required in order to produce a given amount of work. If the liberal diet is continued and the digestion remains normal, the storage of fat will continue until it raises the energy expenditure of the body to a point where the food is no longer in excess. If the store of fat carried when this point is reached is excessive, the fuel value has been too high ; if the store of fat is not excessive, the fuel value of the diet, although greater than would have been necessary to maintain the body at its former weight, has not been too high, and the body has acquired an asset whose utility may not always be recognized in health, but which may be of great value in case of accident, illness, or exposure. Opinions differ somewhat as to the desirable degree of fatness as indicated by the relation of height to body weight. Hill * estimates the average height at 25 years of age as 5 feet 3 inches for women and 5 feet 8 inches for men, and the corresponding average weights as 119 and 150 pounds respec- tively. He considers that variations of 10 to 15 per cent above or below the average should be considered normal. According to this estimate the woman of 5 feet 3 inches should weigh not less than 102-107, ^or more than 131-136 pounds, and the man of 5 feet 8 inches not less than 128-135, nor more than * Recent Advances in Physiology and Biochemistry, 368 CHEMISTRY 01" FOOD AND NUTRITION 165-173 pounds. These figures arc exclusive of clothing. Hill considers as " fal " those persons whose weight exceeds the average by 15 to 30 per cent, and as " over fat " those who exceed by more than 30 per cent, i.e. over 155 pounds for a woman 5 feet 3 inches or over 195 pounds for a man 5 feet 8 inches. Symonds has published * the average relation of height to weight in both men and women at different ages, as computed from the records of accepted apphcants for life insurance in the United States and Canada. The results are found in the following tables; that for men being based on 74,162 and that for women on 58,855 cases. In all these cases the height in- cludes shoes and the weight includes ordinary clothing. Symonds's Table of Height and Weight for Men at Different Ages BASED ON 74,162 accepted APPLICANTS FOR LIFE INSURANCE {Medical Record) Agds iS-24 25-29 30-34 3S-39 40-44 45-49 50-54 55-59 60-64 65-69 5 ft. in. 120 125 128 131 133 134 134 134 131 I in. 122 126 129 131 134 136 136 136 134 2 in. 124 128 131 133 136 138 138 138 137 3 in. 127 131 134 136 139 141 141 141 140 140 4 in. 131 135 13H 140 143 144 145 145 144 143 5 in. 134 138 141 143 146 147 149 149 148 147 6 in. 13H 142 145 147 150 151 153 153 153 151 7 in. 142 147 150 152 155 156 158 158 158 156 8 in. 146 151 154 157 160 161 163 163 163 162 g in. 150 155 159 162 165 166 167 168 i68 168 10 in. 1 54 159 164 167 170 171 172 173 174 174 1 1 in. 159 164 169 173 175 177 177 178 180 180 6 ft. in. i6s 170 175 179 180 183 182 183 185 18S I in. 170 177 181 185 186 189 188 189 189 189 2 in. 176 184 188 192 194 196 194 194 192 192 3 in. 181 190 195 200 203 204 201 198 * Medical Record, September 5, 1908; and McClurc's Magazine, January, igog. DIETARY STANDARDS AND ECONOMIC USE OF FOOD 369 Symonds's Table of Height and Weight for Women at Different Ages based on 58,855 accepted applicants for life insurance (McClure's Magazine) Ages 1S-19 20-24 25-29 30-34 35-39 40-44 45-49 so-54 55-59 60-64 4 ft. II in. III "3 115 117 119 122 125 128 128 126 5 ft. in. 113 114 117 119 122 125 128 130 131 129 I in. "5 116 118 121 124 128 131 133 134 132 2 in. 117 118 120 123 127 132 134 137 137 136 3 in. 120 122 124 127 131 135 138 141 141 140 4 in. 123 125 127 130 134 138 142 145 14s 144 5 in. 125 128 131 13s 139 143 147 149 149 148 6 in. 128 132 135 137 143 146 151 153 153 152 7 in. 132 135 139 143 147 150 154 157 156 IS'? 8 in. 136 140 143 147 151 155 158 161 161 160 9 in. 140 144 147 151 15s 159 163 166 166 165 10 in. 144 147 151 15s 159 163 167 170 170 169 From a study of the records of body weight in relation to the mortahty records Symonds concludes that among young people the greatest vitality coincides with a weight somewhat above the accepted average, while with middle-aged and elderly people a condition of slightly less than average fatness is most favorable to vitality and longevity. Another way of stating the same facts is: That the average of healthy men and women keep themselves slightly too thin while young, and allow themselves to grow slightly too stout as they grow older. Evidently, however, the optimum is very near the average of the accepted applicants as shown in the tables, and Symonds uses these figures as standards in his computations and dis- cussions of the influence of overweight and underweight on longevity and on mortality from specific diseases. Symonds's data therefore support the opinion that the average degree of 370 CHEMISTRY OF FOOD AND NUTRITION fatness of healthy American people is just about the most advantageous fatness for them to maintain. Whatever we accept as the ideal relation of weight to height, it is obvious that the proper standard for fuel value of the diet is that which will preserve the desired degree of fatness while sustaining the desired amount of activity. If good authorities differ in standards for fuel value, it is because, consciously or uncon- sciously, they contemplate different amounts of muscular activity or the maintenance of a different physique. That the amount of food required per day to maintain a healthy adult at the desired body weight will vary considerably with age and size and enormously with extremes of muscular activity has already been explained at some length in Chapter VII and need not be discussed further here. Unless it is desired to increase or decrease the body weight, the optimum energy intake of the healthy adult will be that which coincides with the total energy expenditure; in other words the "standard" and the " requirement " will in this case be the same. Energy Allowances for Children Food allowances or dietary standards for children differ from those for adults in that they must provide not only for all expenditures but also for growth. Recently a considerable number of accurate measurements of energy expenditure of children have been made — especially of infants in the first year of life and of boys twelve and thirteen years old. These data whether obtained by the method of direct or indirect calorimetry give precise information as to the energy output at the time of the experiment, but naturally the observations cannot cover the entire 24 hours of the day, nor can experiments of a few hours' duration give any direct information as to how much the intake must exceed the output in order to provide amply for a normal rate of growth. Observations of the un- restricted food consumption (ordinary dietary studies) of DIETARY STANDARDS AND ECONOMIC USE OF FOOD 371 healthy children who are making normal growth, and nitrogen balance experiments which show both gain in weight and storage of nitrogen (growth of protein tissue) may be expected to furnish evidence of some value though of a somewhat inferential nature. As a result of compilation and study of all available data whether of dietary studies, nitrogen balance experiments, observations of the respiratory exchange, or direct measurements of energy output, the following standards are suggested: Food Allowances for Healthy Children (Gillett) Age Calories per Day Years Boys Girls Under 2 900-1200 900-1 200 2-3 1000-1300 980-1280 3-4 I 100-1400 1060-1360 4-S 1200-1500 I 140-1440 5-6 1300-1600 1 2 20-1 5 20 6-7 1400-1700 1300-1600 7-8 1500-1800 I 380-1 680 - 8-9 1600-1900 1460-1760 9-10 I 700-2000 I 5 50-1 850 lO-II 1900-2200 1650-1950 11-12 2100-2400 1750-2050 12-13 2300-2700 1850-2150 13-14 2500-2900 1950-2250 14-15 2600-3100 2050-2350 15-16 2700-3300 2150-2450 16-17 2 700-3400 2250-2500 In earlier allowances no distinction was made between boys and girls below ten years of age. The averages of recorded data show, however, a slightly higher energy exchange (or metabolism) in boys than in girls of the same age, though the difference is often less than the range allowed to cover differences of size and activity at a given age. Beyond 10 372 CHEMISTRY OF FOOD AND NUTRITION years of age, the energy exchange in boys evidently increases more rapidly than in girls, probably because of their greater restlessness and muscular activity through this period of de- velopment and their greater average rate of growth during and after the fifteenth year. In this connection the accompanying table adapted from that of Manny based on data from Holt, Burt, and Boas is of interest. Average Weights and Rates of Growth of Boys and Girls at Different Ages (Manny) Boys Gtrls Age Weight Increase Weight Increase Kgms. Lbs. Per Year Lbs. Per Week Grams Kgms. Lbs. Per Year Lbs. Per Week Grams At birth . . 3-43 7-55 3-25 7.16 6 months . . 7.27 16.00 16.90 147 7-05 15-50 16.68 145 I year . . 9-32 20.50 9.00 78 9.00 19.80 8.60 75 2 years . . 12.05 26.50 6.00 52 "-59 25-50 S-70 50 3 years . . 14.18 31-20 4.70 41 13-63 30.00 4-50 39 4 years ' . . 15-91 35-00 3-80 33 15-45 34-00 4.00 35 5 yr. 6 mo. 18.73 41.20 4-13 36 18.09 39-80 3.87 34 6 yr. 6 mo. 20.55 45.20 4.00 35 19-73 43-40 3-60 31 7 yr. 6 mo. 22.50 49-50 4-30 38 21.68 47.70 4-30 38 8 yr. 6 mo. 24.77 54-50 5.00 44 23-86 52.50 4.80 42 9 yr. 6 mo. 27.09 59.60 5.10 45 26.09 57-40 4.90 43 lo yr. 6 mo. 29-73 65.40 5-80 51 28.59 62.90 5-50 48 II yr. 6 mo. 32.14 70.70 5-30 46 31-59 69.50 6.60 S8 12 yr. 6 mo. 34-95 76.90 6.20 54 35-77 78.70 9.20 80 13 yr. 6 mo. . 38.55 84.80 7-90 69 40.32 88.70 10.00 87 14 yr. 6 mo. 43-27 95-20 10.40 91 44.68 98.30 9.60 84 15 yr. 6 mo. 48.82 107.40 12.20 107 48-50 106.70 8.40 73 16 yr. 6 mo. S5-00 121.00 13.60 119 51.02 112.30 5.60 49 Children, like adults, will vary in muscular activity and this will influence their energy requirements irrespective of other conditions. Among other conditions to be considered are differences in size and physical development among children DIETARY STANDARDS AND ECONOI^IIC USE OF FOOD 373 of the same age and sex. Children of more than average size, if normally active and not over-fat, will require somewhat more food than an average child of the same age. An estimate of energy requirement per unit of weight at different ages has been given in Chapter VII (page 196). A child who has become somewhat emaciated, either through rapid growth * or other causes, should have a larger food allowance than would ordinarily be required either for his age or for his weight. In calculating the food requirements of a family it is best not to estimate the needs of other members in terms of that of the man of the family (because men on account of the great differences in activity of their occupations are Hkely to be more variable in their energy requirements than are children of any given age) but rather to estimate the Calories for each mem- ber of the family separately according to his or her own needs and then sum up the total. Not infrequently other members of the family may require more food than the man^ especially if he be of less than average size and engaged in sedentary or other light work. The Problem of a Standard for Protein In attempting to set a standard for the amount of protein in the dietary we find no such definite and satisfactory basis for judgment as in the case of total food (or fuel) value. There is no indication that any kind of work necessarily increases the * Large as are the appetites of growing children it is not uncommon for the "growth impulse" to outrun the food intake so that the child although always having had access to ample food may as the result of very rapid growth be brought into a condition somewhat resembling that of the young animals described in the preceding chapter (page 338) which become emaciated through "attempting to grow" on rations suiBcient only for maintenance, i.e. through the growth of some tissues at the expense of others. As Aron points out a child in this condition has an abnor- mally low percentage of fat and high percentage of water in his body content. Hence he needs extra food not only to increase his weight up to that which corre- sponds to his height, but also to restore the normal percentage of fat in the body weight which he already has. 374 CHEMISTRY OF FOOD AND NUTRFriON expenditure of protein as muscular work increases the expendi- ture of fuel, and the body cannot store up protein to anything like the extent that it stores fuel in the form of fat ; the feeding of protein above what is required for maintenance increases only slightly the store of protein which the body carries. When one writer proposes an amount of protein but little above the minimum required for equilibrium, while another advocates a much larger amount, there is implied a diflerence of view regarding protein such as no longer exists with respect to the energy metabolism. The difference, it is true, is hardly so great as might appear from a casual examination of the pro- posed standards. It may perhaps be most fairly expressed in terms of the relation between protein and energy in the different standards. Protein would contribute, according to the standards of Voit, Playfair, and Gautier, about i6 per cent of the fuel value of the food; of Atwater, about 15 per cent; of Langworthy, 12 per cent; of Chittenden, 8^ per cent. It will be of interest to examine some of the arguments which have been advanced in favor of a high protein or of a low pro- tein diet. The following extracts, given in chronological order, are from writings of those who had given special study to the subject and chiefly from the literature of the first decade of this century, when Chittenden's investigation of the protein require- ment was a subject of active discussion. The time of pubhca- tion of these opinions must not be overlooked, since some of the phenomena then attributed to differences in protein intake might perhaps now be attributed, in part at least, to the ash constituents and vitamines of the food. Opinions regarding the Value of Liberal Protein Diet Liebig believed that fats and carbohydrates were burned in the body primarily to supply it with warmth, and that protein alone served as the source of muscular work and other forms of tissue activity. He therefore classed the non-nitrogenous as DIETARY STANDARDS AND ECONOMIC USE OF FOOD 375 " respiratory " and the nitrogenous as " plastic " foodstuflfs, and treated the proteins as playing a " nobler " part in nutrition than can be taken by fat or carbohydrate. Although it was soon demonstrated that carbohydrates and fats as well as protein serve the body in the production of muscular energy, yet the influence of Liebig's teaching, and of the great attention given to protein in Voit's classical researches on nutrition, together with the fact that protein is the most prominent constituent of protoplasm, has resulted in a strong tendency to associate high protein feeding with increased stamina and muscular power. The reasoning of those who appreciated the results of more recent experimental work, and yet believed the general attitude of Liebig and Voit to have been largely sustained by experience, is well expressed by Von Noorden, who wrote in 1893 : * " When one considers that the dietary habits of peoples are the results of biological laws, it would seem that the action of these laws, extending through the thousands of years of existence of the species, would have resulted in the estabUshment of suit- able habits regarding the amounts of protein consumed. The data gathered by Voit may be taken as showing that this normal habit involves the consumption of about 105 grams of digestible protein f per day, a smaller protein consumption being usually associated with weak individuals or inactive peoples. While men can maintain equilibrium on less, still it can rightly be said that a liberal protein consumption makes for a full development of the man. A single individual may for years, or even decades, offend against this biological law unpunished. When, however, the small consumption of protein continues for generations, there results a weak race." Von Noorden, however, is careful to add : * Freely translated from the first edition of Von Noorden's Pathologic der Stoff- wechsel. t Corresponding to Voit's allowance of 118 grams of total protein when the food for the sake of economy, as contemplated by Voit, is taken somewhat largely from vegetable sources. 376 CHEMISTRY OF FOOD ANT) NUTRITION " On ihc other hand, the importance of protein must not be overestimated. A diet is not necessarily good because the amount of protein is right ; it must have the proper proportions of the non-nitrogenous nutrients as well, since the protein is not to be depended upon for the necessary fuel value. Better somewhat less protein with a liberal amount of total food than more protein with insufficient fuel value; the latter brings a rapid loss of strength, the former can be endured very well, at least for a long time, and very Hkely throughout the life of the individual." Chittenden, in 1905, had reached exactly the opposite conclu- sion, — that the products of protein metabolism are a constant menace to the well-being of the body, and that any excess of protein over what the body actually needs is likely to be directly injurious, and at best puts an unnecessary and useless strain upon the liver and kidneys. Chittenden had satisfied himself by his numerous and long-continued experiments that both physical and mental stamina are promoted by decreasing the amount of protein in the food : " Greater freedom from fatigue, greater aptitude for work, greater freedom from minor ailments, have gradually become associated in the writer's mind with this lowered protein metabolism and general condition of physiologi- cal economy " . . . (Physiological Economy in Nutrition, pages 51, 127). Hutchison, in 1906, concluded that the normal amount of protein in a diet furnishing 3000 Calories should be placed at about 75 grams. This allows some margin above the results of Chittenden's experiments and agrees with the rela- tion of protein to calories in mother's milk, which Hutchison regards as nature's hint as to the proper balance of nitroge- nous and non-nitrogenous food for the human species {Chemi- cal News, Vol. 94, page 104). Folin held that the argument for a high protein diet based on the fact that large amounts of protein are commonly eaten DIETARY STANDARDS AND ECONOMIC USE OF FOOD 377 by those who can afford it can be equally well applied to the dietetic use of alcoholic beverages and is no more convincing in one case than in the other ; while on the other hand, study of protein metabolism has given rather strong evidence that the body has no need of such amounts as are commonly eaten. The loss of body nitrogen which occurs in the early periods of restricted protein feeding, and which was not determined nor specifically discussed by Chittenden, is treated by Folin as follows: " All the Hving protoplasm in the animal organism is suspended in a fluid very rich in protein, and on account of the habitual use of more nitrogenous food than the tissues can use as protein, the organism is ordinarily in possession of approxi- mately the maximum amount of reserve protein in solution that it can advantageously retain. When the supply of food protein is stopped, the excess of reserved protein inside the organism is still sufficient to cause a rather large destruction of protein during the first day or two of protein starvation, and after that the protein catabolism is very small, provided sufficient non- nitrogenous food is available. But even then, and for many days thereafter, the protoplasm of the tissues has still an abundant supply of dissolved protein, and the normal activity of such tissues as the muscles is not at all impaired or diminished. When 30 grams or 40 grams of nitrogen have been lost by an average-sized man during a week or more of abstinence from nitrogenous food (but with an abundance of carbohydrate and fat) the Hving muscle tissues are still well supplied with all the protein that they can use. . . . The continuous excessive use of protein may lead, however, to an accumulation of a larger amount of reserve protein than the organism can with advantage retain in its fluid media. It is entirely possible that the con- tinuous maintenance of such an unnecessarily large supply of unorganized reserve material may sooner or later weaken one, or another, or all, of the living tissues. At any rate, it seems scarcely conceivable that the human organism, having all the 378 CHEMISTRY OF FOOD AND NUTRFFION time access to food, can gain in efficiency on account of such an excess of stored protein. The carrying of excessive quantities of fat is considered as an impediment, the carrying of exces- sive quantities of unorganized protein may be none the less so because more common and less strikingly apparent" {American Journal of Physiology, Vol. 13, pages 131-132, 136-137). Benedict argued that general experience in animal feeding favors the use of liberal quantities of protein, and that " while men may for some months reduce the proportion of protein in their diet very markedly and apparently suffer no deleterious consequences, yet, nevertheless, a permanent reduction of the protein beyond that found to be the normal amount for man is not without possible danger. The fact that a subject can so adjust an artificial diet as to obtain nitrogenous equilibrium with an excretion of nitrogen amounting to about 2 or 3 grams per day is no logical argument for the permanent reduction of the nitrogen in food for the period of a lifetime. . . . Dietary studies all over the world show that in those communities where productive power, enterprise, and civilization are at their high- est, man has instinctively and independently selected Uberal rather than small quantities of protein" {American Journal of Physiology, Vol. 16, page 409). A similar position was taken by Meltzer, who compared the ap- petite for a liberal surplus of protein with the liberal way in which the body is provided with organs and tissues for nearly all of its functions, and concludes that " valuable as the facts which Chit- tenden and his colaborer found may be, they do not make obvious their theory that the minimum supply is the optimum — the ideal. The bodily health and vigor which people with one kidney still enjoy does not make the possession of only one kidney an ideal condition. The finding that the accepted standard of protein diet can be reduced to one half can be com- pared with the finding that the inspired oxygen can be reduced to one half without affecting the health and comfort of the DIETARY STANDARDS AND ECONOMIC USE OF FOOD 379 individual, but no one deduces from the latter fact that the breathing of air so rarefied would be the ideal. . . . The storing away of protein, like the storing away of glycogen and fat, for use in expected and unexpected exceptional conditions is exactly like the superabundance of tissues in an organ of an animal, or like an extra beam in the support of a building or a bridge — a factor of safety " {Science, Vol. 25, page 481). In view of the arguments of Benedict and of Meltzer, it is of especial interest that in his later book Chittenden says : " It is certainly just as plausible to assume that increase in the con- sumption of protein food follows in the footsteps of commercial and other forms of prosperity, as to argue that prosperity or mental and physical development are the result of an increased intake of protein food. Protein foods are usually costly and the ability of a community to indulge freely in this form of dietetic luxury depends in large measure upon its commercial pros- perity." Moreover, Chittenden contends that his allowance of 60 grams of protein per day for a man of average size is a perfectly trustworthy figure, with a reasonable margin of safety; that "dietetic requirements, and standard dietaries, are not to be founded upon the so-called cravings of appetite, but upon reason and intelligence reenforced by definite knowl- edge of the real necessities of the bodily machinery" ; that " we must be ever mindful of the fact, so many times expressed, that protein does not undergo complete oxidation in the body to simple gaseous products like the non-nitrogenous foods, but that there is left behind a residue not so easily disposed of " ; and that " there are many suggestions of improvement in bodily health, of greater efficiency in working power, and of greater freedom from disease, in a system of dietetics which aims to meet the physiological needs of the body without undue waste of energy and unnecessary drain upon the functions of digestion, absorption, excretion, and metabolism in general ..." {The Nutrition of Man, pages 160, 164, 227, 269). 380 CHEMISTRY OF FOOD AND NUTRITION Plainly the dietary habit of well-to-do people and the diet- ary standards which have been generally accepted in the past tend to be decidedly hberal with respect to protein, and to prescribe it in quantities which may be believed to be benefi- cial but certainly are not known to be necessary. It does not seem advisable, however, to adopt as a standard the lowest amount of protein to which the body can adjust itself, but rather to regard as the normal requirement an amount which will enable the body to maintain not only its equilibrium, but also some such reserve store of protein as we are accustomed to carry. An allowance of about 75 grams of protein per man per day, which is 50 per cent above the average estimate of actual requirement (page 220), seems fully adequate in view of our present knowledge. A reasonable surplus of protein, from suitable food materials, can hardly be injurious and may be advantageous. Whether such a surplus should be especially recommended or not is largely an economic question. Where little can be spent for food and there is danger that too little food may be eaten, it would be a mistake to use a surplus of protein which could economically be replaced by other food of greater fuel value. In such cases one must not be misled by the popular state- ment that " protein builds tissue " into supposing that a lib- eral amount of protein can keep the body strong in spite of a deficiency in the total food. This impression is still somewhat prevalent, but is certainly incorrect. The body is weakened through getting too little food, be- cause body material must then be burned for fuel. So long as the total food be deficient, the loss of body substance will continue, because not only the food protein, but body tissues as well, must be burned to meet the energy requirement. To strengthen the body through the diet we must increase, not the protein alone, but primarily the total calories. Strengthening or weakening of the body by feeding ordi- DIETARY STANDARDS AND ECONOMIC USE OF FOOD 381 narily depends much more upon the sufficiency or insufficiency of the energy value of the total food than upon the amount of protein which it contains. Protein Standards for Children and for Family Dietaries Little can be said with confidence regarding the best amount of protein for children after the nursing period. In practice well-planned dietaries for children usually contain between 10 and 15 per cent of the total energy in the form of protein. During the years of rapid growth a considerable fraction of the protein of the food is utilized in the synthesis of body pro- teins ; and since the amount of food protein required to form a gram of body protein is variable, depending upon the amino acid make-up of the former, it is evident that the kind of protein supplied becomes a matter of great importance. Here chemical and physiological laboratory evidence, clinical experience, and its e\'ident place in nature all indicate plainly the superiority of milk as source of supply of protein for growth, whether the case be that of the growing child after weaning or of the nursling fed through the mother. The recommendation that family dietaries should whenever possible include " a quart of milk a day for every child " was aimed primarily to insure an appro- priate protein supply. Needless to say, the milk also supphes important amounts of many other substances essential to growth. Since the energy requirement is greatly increased by muscular activity and the protein requirement is not, it is evident that in the metabolism of normal adults the energy and protein require- ments will not run parallel. The protein requirement of the healthy adult depends chiefly upon his size, while his energy requirement depends chiefly upon his activity. In childhood both the energy requirement and the protein requirement are high — often two to three times as high per unit of weight as for adults without muscular work. More- over the high protein and energy requirements of the child as 382 CHEMISTRY OF FOOD AND NUTRITION compared with the man are found to run ai)proximately parallel and as shown in a previous chapter the same proportion of pro- tein in terms of the total energy which seems rational for the adult dietary suffices also for the food requirements of the child provided in the latter case the food is of appropriate kind. In most family groups the differences in age and size will constitute a more prominent factor than the differences in activity, and since the former affect energy and protein require- ments in about the same proportion, it becomes feasible and convenient to set the protein allowance for ordinary family groups in terms of a proportion of the total food value. To allow for varying conditions and for individual preferences as well as to provide a Uberal margin for safety it is customary to consider that from 10 to 15 per cent of the total calories may be in the form of protein. In cases where the nutritive requirements of growth, preg- nancy, or lactation are to be met, the kind of protein is perhaps as important as the amount. Standards for the Calcium, Phosphorus, and Iron Content of the Dietary Formerly dietary standards took no account of the ash constituents because it was assumed that dietaries furnishing sufficient energy and protein would always be adequate as regards the " inorganic " elements. As explained in previous chapters this assumption is not safe in the case of calcium, phosphorus, or iron. In the light of present knowledge ade- quate dietary standards must provide for these elements. The experimental evidence regarding the minimum requirements of the body for each of these elements has been reviewed in earlier chapters and there has been but brief discussion of the relation between minimum and optimum amounts. The evidence thus far available indicates an average minimum requirement for equilibrium, per man per day, of 0.45 gram DIETARY STANDARDS AND ECOxNOMIC USE OF FOOD 383 calcium (0.63 gram CaO), 0.96 gram phosphorus (2.20 grams P2O5), and about o.oio gram (10 milligrams) of iron. To allow only these quantities in the daily food would corre- spond to an allowance of only 50 grams per man per day of protein. If the standard allowance be set 50 per cent above the indi- cated average minimum corresponding to an allowance of 75 grams of protein we obtain Calcium, 0.68 gram (equivalent to 0.95 gram of calcium oxide, CaO)". Phosphorus, 1.44 grams (equivalent to 3.30 grams of P2O5). Iron, 0.015 gram (15 milligrams). If these be taken as proper allowances per man of 70 kilograms whose energy requirement averages 3000 Calories per day, then the corresponding allowances for other adults or for families containing children could also be stated as follows : For Adults PER Kilogram of Body Weight For Children (or Families Containing Chlldrex) per 100 Calories Protein Phosphorus Calcium Iron 1.07 grams 0.0206 gram 0.0097 gram 0.00022 gram 2.5 * grams 0.048 gram 0.023 gram 0.0005 gram If it be desired to provide as Hberal a margin of safety here as in the case of a protein allowance of 100 grams per man per day, then the above figures must obviously be increased by one third. The Unidentified Essentials Of the unidentified fat-soluble and water-soluble substances essential to normal metaboHsm we have as yet no direct quanti- tative measures, either of the proportions in which they occur in food or are needed in nutrition. In view of their importance * In the case of the child this should be mainly milk protein. 384 CHEMISTRY OF FOOD AND NUTRITION it is plain that they should not be ignored in the planning of dietaries, either of children or adults. McCoUum and Sim- monds have recently shown that a low intake of either " fat soluble A " or " water soluble B " not only retards or suspends the growth of young animals but is also distinctly detrimental to adults. A diet furnishing barely enough of these essentials to support slow growth of young regularly resulted in sub- normal vitaUty when fed to adults; but the symptoms were not always the same, e.g. some of the adults lost weight while others maintained weight but lost vitality. They state : " Our results indicate that there is no low plane of intake of either of these substances which can be said to maintain an animal without loss of vitality. When the minimal amount necessary for the prevention of loss of weight is approached, the life of the animal is jeopardized if the diet is persisted in." They also find that " the animal can tolerate being limited to a very low intake of either the dietary A or B much better with an otherwise excellent diet than when it is less well constituted," and also that " it is better to have a liberal supply of one and a minimal supply of the other of the A and B than the minimal allowance of both." The presence of sufficient quan- tities of these substances is insured by making prominent in the diet the types of foods rich in them. These are chiefly : milk and its products, eggs, vegetables, fruits, and the outer portions of the cereal grains — all foods which it is wise to make prominent in the diet for other reasons as well. It will be remembered that " fat soluble A " and " water soluble B " may or may not occur abundantly in the same articles of food. Milk, eggs, and green vegetables appear to be rich in both; butter in " fat soluble A " and whole grains in " water soluble B." Thus either milk or eggs alone, or both butter and whole grain products, would provide the two kinds of un- identified essentials. When both economy and efficiency are considered, it appears that milk and vegetables are especially DIETARY STANDARDS AND ECONOMIC USE OF FOOD 385 worthy of a more prominent place in the diet than is commonly given them in present American practice. Limitations 0} Dietary Standards. — At the risk of repetition let it be clear that too much weight must not be attached to any of the so-called dietary standards, i.e. to any attempt to state the requisites of an adequate diet in terms of quantities of cer- tain nutrients. As Atwater sought strongly to emphasize, a dietary standard at best is " only an indication, not a rule." Some of those who have been most active in recent investiga- tion are most emphatic in warning against the expectation that dietary standards can be made to embrace all the quahties which a diet must have in order to be permanently adequate. Thus Hart, McCollum, Steenbock, and Humphrey in a very recent article * say : " With this recognition of all the normal factors for adequate nutrition there must not simultaneously arise a desire for a mathematical expression of these factors in feeding standards. It is doubtful if this can ever be done, at least for certain of them. For example, the r61e of the mineral nutrients is so varied, in- cluding such widely separate functions as construction and con- trol through antagonism, as to make it seem futile to attempt an expression of absolute requirements when natural foods, with their diversity of mineral content, are involved. Even the recognition of differences in the quality of proteins and their relation to nutrition will make it more diflficult to continue ex- pressing protein requirements in exact quantities than before the development of such knowledge; and what can be said of the quantitative requirements of fat soluble A and water sol- uble B and their supply in feeding materials? We need more effort placed on the accumulation of information on the phys- iological behavior of feeding stuffs than on the attempts to bring out new mathematical expressions of feeding standards." * Proceedings of the National Academy of Sciences, Vol. 3, page 374 (May, igi7). 20 386 CHEMISTRY OF FOOD AXD NUTRITION The Economic Use of Food True economy in the use of food must be physiological as well as pecuniary economy. The diet must supply amply all the requirements of nutrition (not merely the appetite nor the need for energy and protein) and this must be accomplished without the expenditure of too large a proportion of the income. The majority of famiUes in the United States have had in recent normal times incomes of less than $800 per year, of which not over 45 per cent can be spent for food if other living conditions are to be at all satisfactory. This implies an allowance of ap- proximately one dollar per day for food for the " normal " family of five,* or 20 cents per capita per day. If this be taken as approximating the average expenditure in normal years, f it would follow that the sum annually spent for food in the United States is in- the neighborhood of $7,000,000,000. From such statistical estimates of the value of the different food industries as the writer has been able to find it would appear that this is distributed somewhat as follows : Meats, poultry, fish, and shellfish about $2,800,000,000 — or about 40 per cent. Eggs about $400,000,000 — or about 6 per cent. Milk about $500,000,000 — or about 7 per cent. Cheese about $50,000,000 — or less than i per cent. Butter and other fats . about $500,000,000 — or about 7 per cent. Grain products . . . about $1,000,000,000 — or about 14 per cent. Sugar, molasses, etc. . about $500,000,000 — or about 7 per cent. Vegetables about $500,000,000 — or about 7 per cent. Fruits about $300,000,000 — or about 4 per cent. Nuts t about $50,000,000 — or less than I per cent. Miscellaneous, § by difference about 6 to 7 per cent. * If the family of five be reckoned as equivalent in food requirements to 3.7 men, the amount here suggested as available for food would correspond to 27 cents "per man per day" or "per unit." t No attempt is made in this chapter to quote the fluctuations of prices under war conditions. The economic relationships here discussed will be found to be but little disturbed by a general raising or lowering of the level of prices. I This estimate doubtless includes considerable quantities of nuts not used as such for human food but pressed for oil and the residue fed to farm animals. § Including beverages, condiments, and minor uncla-ssitied food materials. DIETARY STANDARDS AND ECONO^IIC USE OF FOOD 387 Any such estimates as these can be no more than rough ap- proximations since they depend upon data which are by no means complete and accurate for the year in which gathered and are subject to fluctuation from year to year. It also appears im- possible to avoid arbitrary assumptions regarding the relations of wholesale and retail values. They are intended, therefore, only to indicate in the most general way the relative prominence of expenditure for the different types of food materials as judged from the statistics of the food industries. Another statistical estimate may be obtained from the data published by the U. S. Bureau of Labor Statistics, who report that of the total value of food consumed in 2567 workingmen's families the distribution of expenditure was as follows : Per Cent of Total Cost of Food Meat, poultry, and fish . . . Eggs MUk Cheese Butterand lard Grain products Sugar and molasses . . . . Vegetables Fruit Other food and food adjuncts 33-80 S-H 6.52 0.80 11.66 9-57* S-34 9.72 505 7-50 These averages are based upon data which were apparently obtained, for the most part at least, by simply asking questions of the housewife regarding the kinds, amounts, and costs of her food purchases and relying upon her memory for the facts. The probable errors in data for individual families would thus be large, but the great number of families included in the inquiry would tend to minimize the errors in the final average. * Low partly because of purchase of flour rather than bread, partly because oat- meal, etc., were often not reported under this head but under "other foods." 388 CHEMISTRY OF FOOD AND NUTRITION A different kind of data bearing on this same problem is found in the dietary studies made under the auspices of the United States Department of Agriculture or of the New York Associa- tion for Improving the Condition of the Poor. These dietary studies are accurate records of the kinds and amounts of foods consumed by given groups of people during a period of a week or more. From such studies, chiefly of family groups, 208 have been taken as presumably representative of American food habits generally, and the cost of these dietaries has been studied with reference to the distribution of expenditure under headings corresponding to those used in the case of the above statistical estimates with the following results: Per Cent of Total Cost OF Food Meats and fish (including poultr>' and shellfish if used) Eggs MUk (including cream if used) Cheese Butter and other fats Grain products Sugar, molasses, etc Vegetables Fruit (and nuts if used) Miscellaneous * 34-3 5-7 9.6 i.o 8.6 17-4 4-5 lO.I 3-8 Of the dietaries included in the above average, 92 constituted a series observed during 1914-1915 in connection with the food investigations of the New York Association for Improving the Condition of the Poor. These studies were not entirely confined to New York City nor to families of low incomes. The cost of food per man per day ranged from 12 to 76, averaging 34 cents. The median cost was 31.5 cents per man per day. In * Tea, coffee, and other food adjuncts were usually but not always reported under this heading. The reported average is therefore somewhat below the truth. DIETARY STANDARDS AND ECONOMIC USE OF FOOD 389 one fourth of the famiUes the cost was below 25 cents; in one fourth it was above 40 cents ; in one half it was between 25 and 40 cents per man per day. The average distribution of expenditure in these 92 famihes was as follows : Per Cent of Total Cost OF Food Meat and fish (including poultry and shellfish when used) Eggs MUk (and cream if used) Cheese Butter and other fats Grain products Sugar, molasses, etc Vegetables Fruit Nuts Miscellaneous (chiefly beverages, condiments, and other food adjuncts) 33-19 5-55 9.08 I-I3 8.14 17.85 3-8o 9.12 6.03 0-35 5.76 When these 92 studies were grouped according to the amount spent per man per day for food, it was apparent that as the scale of expenditure became more liberal a larger proportion of the money was spent for butter and fruit and a smaller proportion for breadstuffs. The distribution of expenditure among other types of food was, however, very similar in the dietaries of low, medium, and high cost. Each of the three kinds of evidence used in arriving at the above estimates of distribution of expenditure for food may readily be criticized as inaccurate or inconclusive or both. Yet the trend of the data derived from the different kinds of evi- dence is so consistent that it can hardly be devoid of signifi- cance. It can scarcely be doubted that of the money devoted to the purchase of food the average American family spends 390 CHEMISTRY OF FOOD AND NUTRITION from 30 to 40 per cent for meats and fish (including poultry and shellfish when used), about 5 or 6 per cent for eggs, about 7 to 10 per cent for milk, from 7 to 12 per cent for butter and other fats, from 10 to 20 per cent for bread and other cereal and bakery products, 3 to 7 per cent for sugar and other sweets, 7 to 10 per cent for vegetables, 2 to 8 per cent for fruit, and less than 2 per cent for cheese and nuts. At the same time it is plain that such a food budget, however prevalent, need not be regarded as fixed. Many people occasionally, and some habit- ually, put the last and smallest of the items just mentioned in the place of the first and largest by using cheese or nuts as so- called " meat substitute," more properly as an alternative to meat, — a custom which on the whole appears to be growing. The place of each type of food in the diet has been discussed in a general way elsewhere * and space does not permit us to go over the same ground here. That the writer does not regard the usual distribution of expenditure for food in American famiHes as being either inevi- table or ideal may be indicated by the fact that in his own house- hold, consisting of three adults and four growing children, the distribution of money expended for food is about as follows : Per Cent of Total Cost OF Food Meats, poultry, and fish Eggs Milk Cheese Butter and other fats Bread, cereals, and other grain products Sugar, molasses, and syrups .... Vegetables and fruits 10-15 5-7 25-30 2-3 10-12 12-15 about 3 1S-18 * Sherman, Food Products, pages 74-81, io8-iii, 139-141, 212-216, 288-295, 346-351. 357, 388-393, 440-444- DIETARY STANDARDS AND ECONOMIC USE OF FOOD 39 1 Just what prominence should be given to each type of food in the provisioning of a given family or community is a problem calling for consideration of many factors. One important fea- ture of the problem is to ascertain how the normal distribution of expenditure among the various types of food materials affects the relative proportions of nutrients in the resulting mixed diet. The accompanying table permits a comparison between the expenditures for the different types of food and the returns from each in terms of energy, protein, calcium, phosphorus, and iron in the case of the series of 92 family dietaries described on page 389. In individual dietaries the returns will naturally vary according as an economical or an expensive food of its kind is chosen, but in the average of 92 different dietaries, each of a week's duration, the danger of error due to such individual variations is minimized. Each Type of Food in Percentage of Total (Average of 92 Dietaries) Meats and fish . . Eggs ..... Milk* . . . . Cheese .... Butter and other fats Grain products Sugar and molasses Vegetables . . . Fruits Nuts Miscellaneous . . Cost Calories Protein Calcium Phosphorus Iron 33-19 5-55 9.08 I-I3 8.14 17.85 3.80 9.12 6.03 0-3S 5-76 16.54 1-75 8.11 0.94 10.29 37-79 10.78 9-03 3-87 0.27 0.65 36.29 4-49 10.13 2.08 0.28 35-86 0.07 8.91 1.08 0.22 0-59 3-68 3-25 50.19 7.28 0.67 15-31 0.69 13-25 4.66 0.14 26.70 4.00 18.52 2.96 0.33 28.85 0.06 14.65 2.41 0.26 1.26 31-43 6.18 4.72 0-5S 0.39 24-95 0.20 26.22 4.09 0.18 1.09 If we compare the cost of each type of food with the energy and individual nutrients which it furnishes, we find that because of the differing prominence of the several factors of food value in the various types of food it is often difficult to decide which expenditures were more economical. Thus in the averages just given meat and fish cost one third of the total expenditure * Cream, in those cases in which it was purchased, is here included with milk. The amount of cream was small, if any. 392 CHEMISTRY OF FOOD AND NUTRITION for food and furnished about one third of the protein, phosphorus, and iron but only one sixth of the energy and only about one thirtieth of the calcium. Eggs furnished protein, phosphorus, and iron about in proportion to their cost, but less calcium and much less than a proportionate amount of energy. Milk furnished calories and protein about in proportion to cost, twice as much phosphorus, and fu'e times as much calcium in proportion, but only half as much iron. By adopting the principle of a score card and assigning weights to the different factors of food value, it becomes feasible to compute a "com- posite valuation" or "score" for each food or group of foods which may then be compared with its cost. Since the most frequent deficiency in American dietaries is inadequacy of total food or energy value and most dietaries actually observ^ed are of such composition as would furnish enough of each essential element if the total amount of food eaten were sufficient to provide a liberal energy supply, it seems reasonable to assign to the energy value of a diet a weight of about half of its composite valuation. It also seems reasonable to assign the remaining "points" equally to protein, calcium, phosphorus, and iron.* If then we giv^e to energy a weight of 60 on a scale of 100 and to protein, calcium, phosphorus, and iron each a weight of 10, or to energy 40 and to protein, calcium, phosphorus, and iron each 15, we obtain from the data of the table above the "score values" or "composite valuations" under the designations "I" and "II" respectively in the table which follows : Meats and fish Eggs Milk (and cream) Cheese . . . . - Butter and other fats Grain products Sugar and molasses Vegetables ... Fruit Nuts Miscellaneous . . * In reality this amounts to giving a higher valuation to the protein since this is counted both as protein and as a part of the energy supply as well. DIETARY STANDARDS AND ECONOMIC USE OF FOOD 393 By comparing the composite valuation with the cost it will be seen that if either of these methods of estimating comparative values is at all valid, the mone)^ spent in these q2 families for milk and cheese, grain products, and vegetables brought a better relative return in food value and was there- fore in this sense better invested than the money spent for meats and fish, eggs, and fruit. In making any such comparison it must be kept prominently in mind : (i) that the weights assigned to the different factors of food value must necessarily be more or less arbitrarily chosen so that the resulting "com- posite valuations" or "food values" rest partly on facts and partly on assumptions; (2) that not all the important factors of food value are taken into account in these valuations, "vitamine values" for instance being wholly omitted from the calculation because as yet we have not the data necessary to permit us to give them numerical expression. It is quite possible that when it becomes feasible to state the vitamine values in numer- ical terms and give them due weight in the composite valuation, the expendi- tures for eggs and butter may appear more economical than is indicated by the above table. Any comparisons based on the use of such arbitrary weights or valuations as can at present be assigned must therefore be used with much discretion if misconceptions are to be avoided ; but if so used they may be found serviceable in guiding the economical choice of food and to some extent in teaching relative food values. Individual articles of food may be given " score values " or " composite valuations " in a similar manner. Thus if 100 Calories be given a value of 40 on the scale of 100, and such quantities of protein, phosphorus, calcium and iron as should accompany 100 Calories in an adequate economical diet be given a value of 15 each, the score for almonds might be ascertained as follows : To every 100 Calories of almonds there are 3.23 grams of protein, 0.071 gram of phosphorus, 0.039 gram of calcium, and 0.0006 gram of iron. If we accept the allowance* of 75 grams of protein, 1.44 grams of phosphorus, 0.68 gram of calcium, and 15 milligrams of iron per man per day, then to every 100 Calories of the 3000 ordinarily taken as the requirement of a man at ordinary labor, there should be 2.5 grams of protein, 0.048 gram of phos- phorus, 0.023 gram of calcium, and 0.0005 gram of iron. Then to every 100 Calories of almonds there is 1.3 (3.23 divided by 2.5) times the amount of protein required to " balance " the energy value ; 1.48 times the amount of phosphorus, 1.61 times the amount of calcium, and 1.2 times the amount of iron. Scoring these as indicated above, we have the score value for almonds as follows : * Sec page 383. 394 CHEMISTRY OF FOOD AND NUTRITION Assumed Values Calories (looj 40 Protein i-3 X 15 Phosphorus 1.48 X 15 Calcium i. 61 X 15 Iron 1.20 X 15 Score Points 40 19-5 22.2 24.2 18.0 123.9 Since a pound of almonds contains 16.14 loo-Calorie portions, then a pound of almonds has a score value of 2000 (123.9 multiplied by 16.14). The following table gives the score value of common typical foods : Approximate Score Valxje (Composite Valuation) per Pouxd of Some Common Typical Foods as Purchased Meat — Beef, sirloin Bacon Eggs Cheese — Cottage . . . . Hard American . . Milk — Condensed sweetened . . . unsweetened . . Skimmed . . . . Whole Butter Cream — 18.5% fat . 40% fat . . . . Lard Olive oil Sugar Grain Products — . . Bread, entire wheat Bread, white 1290 1770 1092 1287 4460 2000 1556 500 600 2320 860 1350 2450 2450 1090 1250 1098 II* 1460 1460 1341 1688 5690 2200 1955 670 700 1750 860 1150 1650 1650 725 1320 1060 Grain Products (Con Bread, rye . Corn meal . Crackers . Corn flakes Farina . Flour, graham Flour, rye . Flour, white Hominy Macaroni . Oatmeal Rice, white Vegetables — Asparagus, fresh Beans, dry, white Beans, dry, Limas Beans, fresh Limas Beans, string . Beets .... 1125 1444 1579 1270 1418 2000 1502 1372 1301 1502 2245 1289 279 2750 2380 3(>3 374 246 II* nil 1360 1433 1090 1308 2150 1459 1257 1147 1444 2465 1 139 368 3350 2780 420 472 286 *The two sets of arbitrary score values correspond to the two systems of "weights" or "points" explained above. The score value will vary slightly with the data of the particular analysis and should perhaps be expressed only in round numbers, DIETARY STANDARDS AND ECONOMIC USE OF FOOD 395 Approximate Score Value (Composite Valuation) per Pound of Some Common Typical Foods as Purchased — Continued I* II* 285 367 278 338 487 640 256 350 497 523 125 153 2834 3464 280 380 280 330 2510 2960 400 475 349 405 399 374 377 414 161 195 630 890 130 144 162 192 246 307 175 156 1075 955 II* Vegetables {Con) Cabbage . Carrots Cauliflower Celery . . Corn, canned Cucumbers Lentils . Lettuce . Onions . Peas, dry Peas, fresh Parsnips Potatoes, sweet Potatoes, white Radishes Spinach . Squash . Tomatoes Turnips Fruit — Apples, fresh Apples, dry Fruit {Con.) Bananas . Dates Grapefruit . Grapes . Lemons . Olives Oranges . Peaches, fresh Pears . . Pineapple Plums . . Prunes Raisins . Nuts — Almonds* Cocoa Filberts* Peanuts* Pecans* . Walnuts* 254 1298 167 286 199 1000 209 169 236 234 345 1 144 1500 1900 2900 1676 2010 1556 730 236 1240 169 266 228 1000 228 177 228 253 337 "35 1550 2000 3231 1752 2078 1440 670 By dividing the " Score Value " of a pound of any food by the price in cents per pound one finds the number of score units or points of food value obtained for each cent, and a comparison of different foods on this basis gives some indication of their relative economy, if the limitations of such comparisons are held strictly in mind. Among these limitations may be mentioned (i) the fact already noted that such valuations necessarily in- volve the arbitrary assignment of weights to the different factors or phases of food value so that facts and assumptions are inseparably combined in the final results notwithstanding the numerical form in which these are expressed; (2) the further tacit assumption that a given amount of protein, of phosphorus, of calcium, or of iron is of th3 same value in whatever food * With sh^l. 396 CHEMISTRY OF FOOD AND NUTRITION found, wliich is certainly not true in detail and may be very far from true in many cases ; (3) that any such attempt to reduce the values of different types of food to a single basis for comparison necessarily tends to obscure those differences of composition and character between the different types of food, which must be kept in mind in order that one may give each type of food its proper place and thus secure a well-balanced dietary. Let us return then to the consideration of the average data of the 92 dietaries as given in the table on page 391. The average food value of these 92 dietaries calculated per man per day was as follows : Energy 2928 Calories Protein loi Grams Calcium 0.72 Gram (i. 01 Grams CaO) Phosphorus 1.52 Grams (3.48 Grams P2O5) Iron 0.0166 Gram Comparing these averages with the amounts actually required for normal nutrition (page 383) it will be seen that the freely chosen dietaries contained a liberal surplus of protein and a fair supply of phosphorus and iron but scarcely more than is ac- tually necessary of calories or of calcium. Correspondingly we find that the number of individual family dietaries actually deficient in calcium and in total food value (calories) is high enough to cause serious concern, while the cases of deficiency of phosphorus or iron were considerably less frequent and there were few if any cases showing an actual deficiency of protein. Tfhis suggests that there would be true economy in a some- what different distribution of expenditure by which less should be spent for expensive high protein food, unless it is also rich in calcium or furnishes a high energy value in proportion to its cost, while more prominence should be given to those foods which are rich in calcium or are advantageous sources of energy without being conspicuously poor in phosphorus and iron. In general this would mean somewhat less meat and somewhat more of milk and vegetables, of the cheaper sorts of fruit, and of bread or other grain products in the diet. DIETARY STANDARDS AND ECONOMIC USE OF FOOD 397 Breadstuffs and other staple grain products always give a high energy return as compared with their cost, and usually also a high return in protein and ash constituents, the latter, however, depending largely upon whether " whole grain " or highly milled products are used. In general the more economical the dietary must be the higher should be the proportion of ex- penditure for bread (or other grain products) and the more restricted the dietary the more desirable it becomes to use " whole grain " rather than highly milled products. Meats give usually, as compared with their cost, a fair return in protein, phosphorus, and iron, a low return in energy, and an extremely low return in calcium. Milk, on the other hand, is very rich in calcium and furnishes in proportion to its cost more energy and phosphorus than does meat of average fatness, and proteins and iron of at least equal value if not of equal amount. Milk also excels other foods in respect to the advantageous quantitative relationships of its ash constituents and is probably the best possible source of the growth-promoting substances needed by all young mammals. The well-known dietary rule of " a quart of milk a day for every child," already amply justified by practical results, has received additional support from several angles through the recent advances in our knowledge of the chemistry of nutrition. Armsby estimates that of the energy value of grain about 18 per cent is recovered for human consumption in milk and only about 3.5 per cent in beef. While milk is somewhat poor in iron, that which it contains is exceptionally efficient in nutrition. Moreover, the supply of this element may readily be safeguarded either by the use of whole grain products or by increasing the proportion of fruits and vegetables in the diet. It will be recalled from what has been said in earlier chapters that an abundance of fruits and vege- tables in the diet is also advantageous in other important ways. Vegetables and some fruits, economically selected, bring a good 398 CHEMISTRY OF FOOD AXD NUTRITION return in nutrients for the money expended and their liberal use adds greatly to both the attractiveness and the wholesome- ness of the diet. It therefore seems advisable to spend at least as much for fruit and vegetables as for meat and fish ; also to spend at least as much for milk as for meat (or for milk and cheese as for meat and fish). At ordinary prices eggs are about as cheap a food as meat, and cheese (like milk) is much cheaper than meat in proportion to its food value. Eggs and cheese can therefore be substi- tuted for meat to any extent desired in the individual dietary without detriment to its nutritive value and usually with good economy. General adoption of a dietary such as we now believe to be best would call for more milk and perhaps more vegetables and fruit than now come to our city markets; but more of these foods will be produced and marketed as the demand for them increases. Moreover an increased demand for these foods and a correspondingly decreased (per capita) demand for meat, so far from causing any serious " dislocation of industry," will help to facilitate natural evolution of American agriculture. With increasing population on stationary area farming neces- sarily becomes more intensive. Beef is produced less by the grazing of cattle on free ranges of unbroken prairie and more by the feeding of grain and other cultivated crops. For a given amount of food consumed a dairy herd yields a product of greater food value than does a herd of beef animals. An increasing ratio of milch cows to beef cattle is naturally to be expected with the development of a more intensive agriculture and will be to the advantage of producer and consumer alike. In re- gions adapted to dairy farming but too remote from large mar- kets to ship in the fresh state we may anticipate an increasing production of condensed and dried milk and of butter and cheese. An increased production of fruit and vegetables should also be DIETARY STANDARDS AND ECONOMIC USE OF FOOD 399 a natural result of a more stable and intensive agriculture. At the same time the concentration of population in large cities increases the expense of transportation and makes the cost of retail distribution a serious item, especially in the case of bulky products with a relatively low value per pound. Cabbage, potatoes, and root crops can be produced at a low cost per ton, but the percentage of the cost of production which must be added when they are distributed through modern retail agencies tends constantly to increase. The more highly perishable fruits and vegetables having a higher cost per pound or ton are now successfully transported in transcontinental carload shipments. Precooling and low- ered temperatures in refrigerator cars, secured by the use of salt with ice, promise to reduce still further the losses incident to their transportation. Cold storage tends to equalize prices throughout the season on such perishable foods as butter, cheese, and eggs, and secures a supply of other fresh foods such as apples, of good quality, throughout almost the entire year. With the perfection of faciUties for more rapid distribution in cities after removal from freezing temperatures the number and quantity of vegetables and fruits so preserved should increase greatly. The canning industry has already developed to enormous proportions and it seems likely that drying processes will be applied to a con- stantly increasing number of the more bulky vegetables. The physical and economic wastes in marketing are being reduced by various agencies in the United States Department of Agriculture, now largely consoUdated in the Bureau of Markets, and in general the supply may be trusted to keep pace with the demand in the gradual shifting of emphasis from meat toward dairy products, vegetables, and fruit, which seems to be clearly desirable both in view of our present knowledge of nutrition, and in the light of our agricultural situation. The broader and more accurate conception of food values 400 CHEMISTRY OF FOOD AND NUTRITION which is made possible by the recent advances in the chemistry of food and nutrition will guide the judgment both as to the proper emphasis to be placed upon each type of foods in the dietary and as to the wise selection among foods of the same type. It supplies the economic justification for the purchase of certain foods which would appear expensive if considered simply as sources of proteins, fats, and carbohydrates, and, on the other hand, it shows that some foods which are economical sources of protein and energy are also of high nutritive value in other respects. Making due allowance for all known factors which affect the nutritive value of foods, there remain large discrepancies be- tween nutritive value and market cost, and correspondingly ample opportunity for the exercise of true economy in the choice of food materials, REFERENCES Armsby. The Food Supply of the Future. Science, Vol. 30, page 817 (1909). See also Ibid., Vol. 46, pages 160-162 (191 7). Atwater. Methods and Results of Investigation on the Chemistry and Economy of Food. Bull. 21, Ofiice of E.xpcrimcnt Stations, U. S. Dept. Agriculture (1895). Atwater. The Demands of the Body for Nourishment and Dietary Stand- ards. Fifteenth Report of the Storrs (Conn.) Agricultural E.xperi- ment Station, pages 123-146 (1903). Atwater. Neue Versuche ueber Stoff- und Kraftwechsel im menschlichcn Korper. Ergebnisse der Physiologie, Vol. 3,!, pages 497-604 (1904). Benedict. The Nutritive Requirements of the Body. American Journal of Physiology, Vol. 16, page 409 (1906). Chittenden. Physiological Economy in Nutrition (1905). Chittenden. The Nutrition of Man (1907). FoLiN. A Theory of Protein Metabolism. A mcrican Journal of Physiology, Vol. 13. page 117 (1905). Gephart AND LusK. Analysis and Cost of Ready to Serve Foods (Amer- ican Medical Association, Chicago). GiLLETT. Food Requirements of Children (Association for Improving the Condition of the Poor, New York). DIETARY STANDARDS AND ECONOMIC USE OF FOOD 401 HiNDHEDE. Protein and Nutrition. Hutchison. Food and Dietetics. Kellogg and Taylor. The Food Problem. Langworthy. Food and Diet in the United States. Reprinted from the Yearbook of the U. S. Department of Agriculture for 1907. LusK. Science of Nutrition, Third Edition, Chapters 12 and 21. LusK. Food Economics. Journal of the Washiiiglon Academy of Sciences, Vol. 6, page 387 (June, 1916). LusK. Food Values. Science, Vol. 45, page 345 (April 13, 1917). McKay. The Protein Element in Nutrition. Meltzer. Factors of Safety in Animal Structure and Animal Economy. Harvey Society Lectures, 1906-1907, and Science, Vol. 25, page 481 (1907). Mendel. Changes in the Food Supply and their Relation to Nutrition (Yale University Press). Sherman and Gillett. A Study of the Adequacy and Economy of Some City Dietaries (New York Association for Improving the Condition of the Poor). Taylor. The Diet of Prisoners of War in Germany. Journal of tlie Amer- ican Medical Association, Vol. 69, page 1575 (1917). U. S. Bureau of Labor Statistics (Bulletins and Reports). U. S. Census Bureau Reports. U. S. Department of Agriculture. Bureau of Markets (Bulletins, Cir- culars and Reports). U. S. Department of Agriculture, Office of Experiment Stations, Bulls. Nos. 21, 29, 31, 38, 46, 52, 53, 55, 71, 75, 84, 91, 98, 107, 116, 129, 132, 149, 150, 221, 223 (data and discussion of dietary studies). U. S. Department of Agriculture, Office of the Secretary. Reports 109, no, III, 112, 113. Meat Situation in the United States (1916). "The World's Food" (Papers by several authors). Annals of the American Academy of Political and Social Sciotce, Vol. 74, pages 1-293 (November, 1917). APPENDICES APPENDIX A NOMENCLATURE AND CLASSIFICATION OF PROTEINS Joint Recommendations of the Committees on Protein Nomen- clature of the American Physiological Society and Ameri- can Society of Biological Chemists Since a chemical basis for the nomenclature of the proteins is at present not possible, it seemed important to recommend few changes in the names and definitions of generally accepted groups, even though, in many cases, these are not wholly satis- factory. The recommendations are as follows: First. — The word " proteid " should be abandoned. Second. — The word " protein " should designate that group of substances which consist, so far as at present is known, es- sentially of combinations of a-amino acids and their derivatives, e.g. oe-amino acetic acid or glycocoll ; a-amino propionic acid or alanine ; phenyl-ct-amino propionic acid or phenylalanine ; guanidin-a-amino valerianic acid or arginine, etc., and are therefore essentially polypeptids. Third. — That the following terms be used to designate the various groups of proteins : I. Simple Proteins. — Protein substances which yield only a-amino acids or their derivatives on hydrolysis. Although no means are at present available whereby the chemical individuality of any protein can be established, a number of simple proteins have been isolated from animal and vegetable tissues which have been so well characterized by 403 404 APPENDIX A constancy of ultimate composition and uniformity of physical properties that they may be treated as chemical individuals until further knowledge makes it possible to characterize them more definitely. The various groups of simple proteins may be designated as follows : (a) Alhumins. — Simple proteins soluble in pure water and coagulable by heat. {h) GlohuJins. — Simple proteins insolul)lc in pure water, but soluble in neutral solutions of salts of strong bases with strong acids.* (c) Glutelins. — Simple proteins insoluble in all neutral sol- vents but readily soluble in very dilute acids and alkalies. f (d) Alcohol-soluble Proteins. — Simple proteins soluble in relatively strong alcohol (70-80 per cent), but insoluble in water, absolute alcohol, and other neutral solvents. | (e) Albuminoids. — Simple proteins which possess essentially the same chemical structure as the other proteins, but are char- acterized by great insolubihty in all neutral solvents.§ (/) Histones. — Soluble in water and insoluble in very dilute ammonia, and, in the absence of ammonium salts, insoluble even in an excess of ammonia ; yield precipitates with solutions of other proteins and a coagulum on heating which is easily solu- ble in very dilute acids. On hydrolysis they yield a large number of amino acids, among which the basic ones predominate. (g) Protamins. — Simpler polypeptids than the proteins in- * The precipitation limits with ammonium sulphate should not be made a basis for distinguishing the albumins from the globulins. t Such substances occur in abundance in the seeds of cereals and doubtless rep- resent a well-defined group of simple proteins. % The subclasses defined (a, b, c, d) are exemplified by proteins obtained from both plants and animals. The use of appropriate prefixes will suffice to indicate the origin of the compounds, e.g. ovoglobulin, myoalbumin, etc. § These form the principal organic constituents of the skeletal structure of ani- mals and also their external covering and its appendages. This definition does not provide for gelatin, which is, however, an artificial derivative of collagen. APPENDIX A 405 eluded in the preceding groups. They are soluble in water, un- coagulable by heat, have the property of precipitating aqueous solutions of other proteins, possess strong basic properties, and form stable salts with strong mineral acids. They yield com- paratively few amino acids, among which the basic amino acids greatly predominate. II. Conjugated Proteins. — Substances which contain the protein molecule united to some other molecule or molecules otherwise than as a salt. (a) Nudcoprotcins. — Compounds of one or more protein molecules with nucleic acid. {h) Glycoproteins. — Compounds of the protein molecule with a substance or substances containing a carbohydrate group other than a nucleic acid. (c) P ho spho proteins. — Compounds of the protein molecule with some, as yet undefined, phosphorus-containing substance other than a nucleic acid or lecithin.* {d) Hemoglobins. — Compounds of the protein molecule with hematin or some similar substance. (e) Lecitho proteins, — Compounds of the protein molecule with lecithins (lecithans, phosphatids). III. Derived Proteins. I. Primary Protein Derivatives. — Derivatives of the protein molecule apparently formed through hydrolytic changes which involve only slight alterations of the protein molecule. (a) Proteans. — Insoluble products which apparently result from the incipient action of water, very dilute acids, or enzymes. {b) M eta proteins. — Products of the further action of acids and alkalies whereby the molecule is so far altered as to form products soluble in very weak acids and alkalies, but insoluble in neutral fluids. * The accumulated chemical evidence distinctly points to the propriety of classifying the phosphoproteins as conjugated compounds, i.e. they are possibly esters of some phosphoric acid or acids and protein. 4o6 APPENDIX A This group will thus include ihc familiar " acid proteins " and " alkali proteins," not the salts of proteins with acids. (c) Coagulated Proteins. — Insoluble products which result from (i) the action of heat on their solutions, or (2) the action of alcohols on the protein. 2. Secondary Protein Derivatives* — Products of the further hydrolytic cleavage of the protein molecule. (a) Proteoses. — Soluble in water, uncoagulated by heat, and precipitated by saturating their solutions with ammonium sul- phate or zinc sulphate. f (b) Peptones. — Soluble in water, uncoagulated by heat, but not precipitated by saturating their solutions with ammonium sulphate. J (c) Peptids. — Definitely characterized combinations of two or more amino acids, the carboxyl group of one being united with the amino group of the other, with the ehmination of a molecule of water.§ * The term " secondary hydrolytic derivatives " is used because the formation of the primary derivatives usually precedes the formation of these secondary- deriva- tives. t As thus defined, this term does not strictly cover all the protein derivatives commonly called proteoses, e.g. heterproteose and dysproteose. t In this group the kyrins may be included. For the present we believe that it will be helpful to retain this term as defined, reserving the expression "peptid" for the simpler compounds of definite structure, such as dipeptids, etc. § The peptones are undoubtedly peptids or mixtures of peptids, the latter term being at present used to designate those of definite structure. APPENDIX B COMPOSITION OF FOODS Explanation of Headings Food as purchased may or may not consist entirely of edible material. When an article of food contains inedible matter or refuse, this may be stated separately and the composition of the edible portion then given, or the percentages of refuse and of edible nutrients in the original matter may be given so as to show directly the percentage of each edible nutrient obtained in the material as purchased. For example ; loo pounds of beef contains i6 pounds of bone and 84 pounds of moist flesh, of which 15.4 pounds are protein, 15 pounds fat, 53 pounds water, and 0.6 pound ash. The composition may be stated in either of the following forms : Composition of Beef Refuse Per Cent Water Per Cent Protein Per Cent Fat Per Cent Ash Per Cent 16.0 53-0 15-4 15.0 0.6 Composition of Beef Refuse. Edible Portion Per Cent Water Per Cent Protein Per Cent Fat Per Cent Ash Per Cent 16.0 63.1 18.3 17.9 0.7 407 4o8 APPENDIX B For most purposes it is convenient to include in one table the nutrients calculated both on the basis of edible material and of material as purchased. In such a case the percentage of refuse in the material as purchased may be given or may be omitted as in the following form : Composition of Beef Water Protein | Fat 1 Ash Edible portion (E. P.) As purchased (A. P.) 63.1 53-0 18.3 154 17.9 15.0 0.7 0.6 In order to avoid confusion and possible errors in taking data from tables of composition it is important to note in which form the percentages are stated. Data given in either form are of course readily convertible into the other. In Table I which follows, the percentages of nutrients and the correspond- ing energy values are stated in the form last illustrated above. Table II shows percentages of ash constituents in the edible portion only. Table III shows grams of protein and of cal- cium, phosphorus, and iron in loo-Calorie portions, which esti- mates may obviously be used equally well whether the food be originally recorded in terms of edible material or of material as purchased. A word of explanation regarding the sources and reliability of the data may also be offered. The percentages of proteins, fats, and carbohydrates given in Table I are in the great ma- jority of cases taken from the tables of composition of American food materials compiled by Atwater and Bryant and published in Bulletin 28 of the Office of Experiment Stations, U. S. De- partment of Agriculture. By reference to this bulletin the reader may find the number of analyses on which the average is based and the maximum and minimum of the recorded percentages of each constituent, as well as the percentages of moisture, APPENDIX B 409 ash, and in some cases crude fiber. The energy values given in Table I are computed from the average percentage of protein, fat, and carbohydrate by the use of the latest and most accurate factors (see page 143). The data for ash constituents given in Tables II and III are based on a critical compilation of all avail- able ash analyses, both American and European. In some cases only a single ash analysis could be found ; in other cases the data given are averages of many fairly concordant analyses. Between these extremes are data of all degrees of probable re- liability. It does not seem feasible to indicate the relative accuracy of the estimates for different articles of food. In general it may be said that only in the cases of the more impor- tant foods are the ash analyses as yet sufiiciently numerous and concordant to justify one in laying great emphasis upon comparisons of one article of food with another. More empha- sis can properly be laid upon estimates of the ash constituents of rations or dietaries made up of several food materials, since in such cases accidental errors will tend to offset each other. It is chiefly to facilitate such calculations that the tables have been made as complete as seemed practicable even though this necessitated including estimates of differing reliability on ap- parently equal terms. Data which are based in part at least upon assumptions are inclosed in parenthesis. They are not necessarily less accurate as estimates of average composition than are some of the di- rectly determined data of individual analyses. Since many unpublished ash analyses have been included in the present averages, Tables II and III will be found to present many differences in detail from those published elsewhere, or in the first edition of this book. The general trend of the aver- ages has, however, not been materially altered by the results of recent work. Attention may also be called to the fact that in Table II the data are uniformly given as percentages of the elements and 4IO APPENDIX B not of their oxides. For the convenience of those who may prefer to continue to calculate calcium and phosphorus in terms of the oxides as has been customary in the past, Table III shows the weights of CaO and PjO;, as well as of protein, calcium, phosphorus, and iron in loo-Calorie portions of foods. TABLE I Edible Organic Nutrients and Fuel Values of Foods* Protein (NX6.2S) PER CENT Almonds E. P.f A. P.t Apples E. P. A. P. Apricots E. P. A. P. Artichoke, French . . . . E. P. A. P. Asparagus, fresh . . . . A. P. cooked A. P. Avocado E. P. A. P. Bacon, smoked E. P. A. P. Bananas E. P. A. P. Barley, pearled Beans, dried Lima, dried Lima, fresh E. P. A. P. 2I.O "•5 •4 •3 I.I i.o 3-4 1-7 1.8 2.1 2.1 1.4 IO-5 9-5 1-3 .8 8.5 22.5 i8.i 7-1 3-2 Fat PER CENT Carbo- j HY- DRATE PER CENT 54-9 30.2 •5 •3 • 5 •3 .2 3-3 20.1 13-2 64.8 59-4 .6 •4 I.I 1.8 1-5 •7 •3 17-3 9-5 14.2 10.8 13-4 12.6 12.0 6.0 3-3 2.2 7-4 4.8 22.0 14-3 77.8 59-6 65-9 22.0 9.9 Fuel Value PER Pound Calo- ries 100 Calorie Portion GRAMS 2940 1615 285 214 263 247 •300 150 100 213 » 993 652 2840 2372 447 290 1615 i5<>5 1586 557 15 28 159 212 174 184 151 302 450 213 46 70 16 19 lOI 156 28 29 29 82 182 * The percentages of nutrients are taken from Bull. 28, Office of Experiment Stations, U. S. Department of .\griculture. The fuel values are calculated from these percentages by the use of the factors explained in Chapter V, viz. — protein, 4 calories ; fat, g calories ; carbohydrate, 4 calories per gram. t E. P. signifies edible portion ; A. P. signifies as purchased. APPENDIX B 411 Table I — Continued Food Beans — Continued string, fresh E. P. A. P. baked, canned . . . . A. P. red kidney, canned . . . Beef, brisket, medium fat . E. P. A. P. chuck, average . . . . E. P. A. P. corned, average . . . . E. P. A. P. cross ribs, average . . . E. P. A. P. dried, salted, and smoked, E. P. A. P. flank, lean E. P. A. P. fore quarter, lean . . . E. P. A. P. fore shank, lean . . . . E. P. A. P. heart E. P. A. P. hind quarter, lean . . . E. P. A. P. hind shank, lean . . . E. P. A. P. hind shank, fat . . . . E. P. A. P. liver E. P. A. P. loin E. P. A. P. neck, lean E. P. A. P. neck, medium fat ... E. P. A. P. Protein (NX6.2S) per cent Fat per CENT Carbo- hy- drate PER CENT Fuel Value PER Pound Calo- ries 100 Calorie Portion GRAMS 2-3 •3 7-4 184 241 2.1 •3 6.9 176 259 6.9 2-5 19.6 583 78 7.0 .2 18.5 471 96 15-8 2S.5 — 1449 31 12.0 22.3 — 1 130 40 19.2 15-4 — 978 46 15-8 12.5 — 797 S8 15-6 26.2 — 1353 34 14-3 23.8 — 1230 37 15-9 28.2 — 1440 32 13-8 24.8 — 1262 36 30-0 6.5 •4 817 56 26.4 6.9 — 760 60 20.8 "•3 — 838 54 20.5 II.O — 821 SS 18.9 12.2 — 842 54 14.7 9-5 — 655 69 22.0 6.1 — 647 70 14.0 3-9 — 414 no 16.0 20.4 I.O 1 140 40 14.8 24.7 •9 1292 35 20.0 13-4 — 907 50 16.7 II. 2 — 757 60 21.9 5-4 — 617 75 9.1 2.2 — 255 179 20.4 18.8 — 1171 40 9.9 9.1 — 552 83 20.4 4-5 1-7 584 78 20.2 3-1 2-5 537 85 19.7 12.7 — 877 52 17. 1 II. I — 764 60 21.4 8.4 — 732 62 iS-i 5-9 — 493 93 20.1 16.5 — 1040 44 14.5 II.O — 740 6t 412 APPENDIX B Table I — Continued Food PROTErN (NX6.25) PER CENT Beef — Continued plate, lean E. P. A. P. Porterhouse steak . . . E. P. A. P. rib rolls, lean A. P. ribs, lean E. P. A. P. ribs, fat E. P. A. P. round, lean E. P. A. P. round, free from visible fat rump, lean E. P. A. P. rump, fat E. P. A. P. sides, lean E. P. A. P. sirloin steak E. P. A. P. sweetbreads A. P. tenderloin A. P. tongue E. P. A. P. Beets, cooked E. P. fresh E. P. A. P. Blackberries A. P. Blackfish E. P. A. P. Bluefish E. P. A. P. Boston crackers Brazil nuts E. P. A. P. 15.6 13-0 21.9 19.1 20.2 19.6 IS-2 15.0 12.7 21.3 19-5 23.2 20.9 19.1 16.8 12.9 19-3 iS-S 18.9 16.S 16.8 16.2 18.9 14.1 2.3 1.6 1-3 1-3 18.7 7-4 19.4 10. o II.O 17.0 8.6 Fat PER CENT Carbo- hy- drate PER cent 18.8 15-5 20.4 17.9 IO-5 12.0 9-3 35-6 30.6 7-9 7-3 2-5 13-7 II.O 35-7 27.6 13.2 10.6 18.5 16.1 12. 1 24.4 9.2 6.7 i.o 1-3 ■7 1.2 .6 8.5 66.8 33-7 7-4 9-7 7-7 10.9 3-5 Fuel \'alue 100 PER Calorie Pound | Portion Calo- I grams RIES IO5I 867 1230 1077 795 84s 654 1721 1480 709 649 512 940 796 1763 1361 890 715 1099 960 799 1290 717 529 180 209 167 262 393 163 402 206 1835 3162 1 591 APPENDIX B 413 Table I — Continued Food Protein Fat (NX6.2S) PER PER CENT CENT 6.0 6.3 8.9 1.8 9.0 3-0 "•5 1.6 9.1 1.6 9.6 1.4 9.4 1.2 9.2 1-3 0.7 •9 6.4 1.2 I.O 85.0 3-0 •5 27.9 61.2 3.8 8.3 1.6 •3 1.4 .2 4-3 — I.I •4 •9 .2 1.8 •5 I.I .1 •9 .1 2.1 2.8 9.6 I.I 3-2 .6 28.8 35-9 29.6 38.3 27.7 36.8 20.9 1.0 259 33-7 15-9 21.0 18.7 27.4 29.9 38.9 22.6 29-5 27.6 34-9 1.0 .8 •9 .8 Carbo- hy- drate PER CENT Fuel Value per Pound Calo- ries 100 Calorie Portion grams Bread, Boston brown . . . graham rolls, water . . . . . toasted white, homemade . . . milk Vienna average white .... whole wheat Buckwheat flour .... Butter Buttermilk Butternuts E. P. A. P. Cabbage E. P. A. P. Calf's-foot jelly Carrots, fresh E. P. A. P. Cauliflower A. P. Celery E. P. A. P. Celery soup, canned . . . Cerealine Chard E. P Cheese, American pale . . American red .... Cheddar cottage full cream Fromage de Brie . . . NeufchS,tel pineapple roquefort Swiss Cherries, fresh E. P A. P S4-0 52.1 54-2 61.2 53-3 5I-I 54-1 53-1 49-7 77-9 4.8 3-5 •5 5-6 4.8 174 9-3 7-4 4-7 3-3 2.6 5-0 78.3 S-O •3 4.1 4-3 2.4 1.4 1-5 2.6 1.8 1-3 16.7 15-9 1345 1 189 1268 1385 1 199 1158 1 199 1182 1113 1580 3491 162 3065 417 143 121 394 204 158 139 84 68 243 1640 173 1990 2102 2080 499 1890 1 1 70 1484 2180 1645 1945 354 337 34 38 36 33 38 39 38 38 41 29 13 280 IS 109 317 376 115 221 286 328 542 672 187 28 262 23 91 24 39 31 21 28 23 128 134 414 APPENDIX B Table I — Continued Food Protefn (NX6.2S) PER CENT Fat PER CENT Carbo- hy- drate PER CENT Fuel Value ioo PER Calorie Pound Portion Calo- crams RIES Cherries — Continued canned A. P. Chestnuts, fresh . . . . E. P. A. P. Chicken, broilers . . . . E. P. A. P. Chocolate Cocoa Cod, dressed A. P. salt E. P. A. P. Consomme, canned . . . A. P. Corn, green, canned . . . sweet, fresh E. P. A. P. Corn meal Cowpeas, dried green E. P. Crackers, butter . . . . A. P. cream A. P. graham A. P. soda . A. P. water A. P. Cranberries A. P. Cream Cucumbers E. P. A. P. Currants, fresh dried Zante Dandelion greens .... -Dates, dried E. P. A. P. Doughnuts Eggplant E. P. Eggs, uncooked E. P. A. P. I.I 6.2 5-2 21-5 12.8 12.9 21.6 II. I 254 19.0 2.5 2.8 3-1 1.2 9.2 21.4 9.4 9.6 9-7 lO.O 9.8 10.7 •4 2.5 .8 • 7 1-5 2.4 2.4 2.1 1.9 6.7 1.2 13-4 11.9 5-4 4-5 2.5 1-4 48.7 28.9 .2 •3 •4 1.2 I.I •4 1.9 1.4 .6 lO.I 12. 1 9.4 9.1 8.8 .6 18.5 .2 .2 1-7 i.o 2.8 2-5 21.0 •3 IO-5 9-3 21. 1 42.1 35-4 30.3 37-7 •4 19.0 19.7 7-7 75-4 60.8 22.7 71.6 69.7 73.8 73-1 71.9 9.9 4-5 3-1 2.6 12.8 74.2 10.6 78.4 70.6 53-1 5-1 407 1098 920 493 289 2768 2258 209 473 361 53 455 459 178 1620 1550 603 1887 1938 1905 1875 1855 79 68 259 1455 277 1575 1416 1941 126 672 594 APPENDIX B 415 Table I — Continued Farina • Figs, dried Flounder A. P. E. P. Flour, r^'e wheat, California fine . . wheat, entire wheat, graham .... wheat, patent baker's grade wheat, straight grade . . wheat, average high and medium wheat, average low grade Fowls • E. P. A. P. Gelatin Grape butter Grapes E. P. A. P. Grapefruit E. P. A. P. Haddock E. P. A. P. Halibut steaks E. P. A. P. Ham, fresh, lean . . . . E. P. A. P. fresh, medium . . . . E. P. A. P. smoked, lean E. P. A. P. Herring, whole E. P. A. P. smoked E. P. A. P. Hominy Protein (NX6.2S) PER CENT Fat PER CENT Carbo- hy- drate PER CENT Fuel Value per Pound Calo- ries II. 1.4 76.3 1640 4-3 •3 74.2 1437 5-4 ■3 — no 14.2 .6 — 282 6.8 •9 78.7 1590 7-9 1.4 76.4 1585 13.8 1.9 71.9 1630 13-3 2.2 71.4 1628 13-3 1-5 72.7 1623 10.8 I.I 74.8 1608 11.4 1.0 75-1 1610 14.0 1.9 71.2 1625 19-3 16.3 — 1017 13-7 12.3 — 752 91.4 .1 — 1660 1.2 .1 58.5 1088 1-3 1.6 19.2 437 I.O 1.2 14.4 328 .6 .1 12.2 235 •4 .1 8.9 172 17.2 •3 — 324 8.4 .2 — 160 18.6 5.2 — 550 iS-3 4.4 — 457 25.0 14.4 — 1042 24.8 14.2 — 1030 15-3 28.9 — 1458 13-5 25-9 — 1303 19.8 20.8 — • 1209 17-5 18.S — 1073 19.5 7-1 — 644 II. 2 3-9 — 362 36.9 15-8 — 131S 20.5 8.8 — 731 8.3 .6 790 1609 TOO Calorie Portion grams 28 32 412 161 29 29 28 28 28 28 28 28 45 60 27 42 104 138 193 264 140 283 83 100 44 44 31 35 38 42 70 125 35 62 28 4i6 APPENDIX B Table I — Continued Food Protein (NX6.25) PER CENT Honey Huckleberries Kohl-rabi E. P. Koumiss ...... Lamb, breast E. P. A. P. chops, broiled . . . . E. P. fore quarter E. P. A. P. hind quarter E. P. A. P. leg, roast side E. P. A. P. Lard, refined Lemon juice Lemons E. P. A. P. Lettuce E. P. A. P. Liver, beef E. P. A. P. veal E. P. Lobster, whole E. P. A. P. canned A. P. Macaroni Macaroons Mackerel E. P. A. P. salt E. P. A. P. Marmalade, orange . . . Milk, condensed, sweetened skimmed whole •4 .6 2.0 2.8 19. 1 iS-4 21.7 18.3 14.9 19.6 16.S 19.7 17.6 14.1 i.o •7 1.2 1.0 20.4 20.2 19.0 16.4 5-9 18.1 134 6.5 18.7 10.2 21. 1 16.3 .6 8.8 3-4 3-3 Fat PER CENT .6 .1 2.1 23.6 19.1 29.9 25.8 21.0 19.1 16.1 12.7 23.1 •18.7 lOO.O •3 .2 4-5 3-1 5-3 1.8 • 7 I.I •9 15-2 7-1 4.2 22.6 17-4 .1 8.3 •3 J-.O Carbo- hy- drate PER CENT Fuel Value 100 PER Calorie Pound Portion Calo- crams KIES 8x.2 16.6 5-5 5-4 74- 65- 84-5 54-1 5-1 =;.o 1481 140 234 1311 1058 1614 1385 1127 1 149 953 876 1263 1015 4080 178 201 140 87 72 583 538 562 379 139 382 1625 1922 629 356 1305 1005 1548 1480 167 3T4 APPENDIX B 417 Table I — Continued Food M'-" I , commercial homemade Molasses, cane Mushrooms V. P. Muskmelons E. P. A. P. Mutton, fore quarter . . . E. P. A. P. hind quarter E. P. V. P. leg E. P. A. P. side A. P. E. P. Nectarines I'L P. A. P. Oatmeal Okra E. P. A. P. Olives, green E. P. A. P. ripe E. P. A. P. Onions, fresh E. P. A. P. Oranges E. P. A. P. Oxtail soup, canned . . . A. P. Oysters E. P. in shell A. P. canned A. P. Parsnips E. P. A. P. Pea soup, canned .... A. P. Peaches, canned . . . . A. P. fresh E. P. A. P. Protein (NX6.25) PER CENT Fat per CENT Carbo- hy- drate per CENT Fuel Value PER Pound Calo- ries 100 Calorie Portion grams 6.7 1.4 60.2 1280 36 4.8 6.7 32.1 942 48 2.4 69-3 1302 35 3-5 •4 6.8 204 223 .6 — 9-3 180 252 •3 — 4.6 89 5^0 15.6 30-9 — 1543 29 12.3 24-5 — 1223 37 16.7 28.1 — 1450 31 13-8 23.2 — 1197 38 19.8 12.4 — 863 52 16.5 IO-3 — 718 63 I3-0 24.0 — 1215 37 16.2 29.8 — 1512 30 .6 — 15-9 299 152 .6 — 14.8 280 162 16.1 7.2 67.5 1811 25 1.6 .2 7-4 172 264 1.4 .2 6.5 152 300 I.I 27.6 11.6 1357 33 .8 20.2 8.5 995 46 1-7 25.0 4-3 1 130 40 1.4 21.0 3-5 947 48 1.6 •3 9.9 220 206 1.4 •3 8.9 199 228 .8 .2 11.6 233 19s .6 .1 8.5 169 268 3-8 •5 4.2 166 274 6.2 1.2 3-7 228 199 1.2 .2 • 7 43 1065 8.8 2.4 3-9 328 138 1.6 •5 13-5 294 154 1-3 •4 10.8 236 192 3.6 • 7 7.6 232 196 • 7 .1 10.8 213 213 • 7 .1 9.4 188 242 •5 .1 7-7 153 297 2E 4i8 APPENDIX B Table I — Continued Food Peanuts E. P. A. P. Pears, fresh E. P. A. P. Peas, canned A. P. dried green E. P. A. P. Peppers, green E. P. Persimmons E. P. Pies, apple custard lemon mince squash Pineapples, fresh . . . . E. P. canned A. P. Pine nuts (pignolias) . . . Pistachios, shelled .... Plums E. P. A. P. Pomegranates E. P. Pork, chops, medium . . . E. P. A. P. chuck ribs and shoulder . E. P. A. P. fat, salt A. P. sausage A. P. side E. P. A. P. tenderloin A. P. Potato chips A. P. Potatoes, white, raw . . . E. P. A. P. sweet, raw E. P. A. P. Protein (NX6.2S) PERCENT Fat PER CENT Carbo- hy- drate PER CENT Fuel Value PER Pound Calo- ries 25.8 38.6 24.4 2490 19.5 29.1 18.5 1877 .6 •5 I4.I 288 •5 ■4 12.7 256 3-6 .2 9.8 252 24.6 1.0 62.0 161 1 7.0 •5 16.9 454 3.6 .2 9.8 252 I.I .1 4.6 109 .8 •7 31.5 615 3-1 9.8 42.8 1233 4.2 6.3 26.1 806 3-6 lO.I 37-4 1156 5-8 12.3 38.1 1300 4.4 8.4 21.7 817 ■4 •3 9-7 196 •4 • 7 36.4 695 33-9 49-4 6.9 2757 22.3 54-0 16.3 2900 I.O — 20.1 383 •9 — 19.1 363 1-5 1.6 19.5 447 16.6 30.1 — 1530 134 24.2 — 1230 17-3 311 — 1585 14.1 25-5 — 1298 1.9 86.2 — 3555 I3-0 44.2 I.I 2030 9.1 55-3 — 2423 8.0 49.0 — 2145 18.9 130 — 875 6.8 39-8 46.7 2598 2.2 .1 18.4 378 1.8 .1 14-7 302 1.8 • 7 27.4 558 1-4 .6 21.9 447 APPENDIX B 419 Table I — Continued Food Prunes, dried E. P. A. P, Pumpkins E. P. A. P. Radishes E. P. A. P. Raisins E. P. A. P. Raspberries, red .... black Rhubarb E. P. A. P. Rice Salmon, dressed A. P. whole E. P. A. P. Sausage, Bologna . . . . E. P. A. P. farmer E. P. A. P. Shad, whole E. P. A. P. roe Shredded wheat .... Spinach, fresh A. P. Squash E. P. A. P. Strawberries Succotash, canned .... Sugar Tomatoes, fresh A. P. canned A. P. Tuna (tunny fish) . . . . E. P. Turkey E. P. A. P. sandwich, canned . . . Protein (NX6.25) per cent Fat per CENT Carbo- hy- drate PER CENT Fuel Value per Pound Calo- ries 2.1 73-3 1368 1.8 — 62.2 1160 I.O .1 5-2 117 •5 .1 2.6 60 1-3 .1 5-8 133 •9 .1 4.0 91 2.6 3,-i 76.1 1562 2-3 3-0 68.5 1407 1.0 — 12.6 247 1-7 1.0 12.6 300 .6 ■ 7 3.6 105 •4 •4 2.2 63 8.0 •3 79.0 1591 13.8 8.1 — 582 22.0 12.8 — 923 15.3 8.9 — 642 18.7 17.6 •3 1061 18.2 19.7 — "35 29.0 42.0 — 2240 27.9 40.4 — 2156 18.8 9-5 — 727 94 4.8 ^ 367 20.9 3.8 2.6 582 10.5 1-4 77-9 1660 2.1 •3 3-2 109 1.4 •5 9.0 209 •7 .2 4-5 103 1.0 .6 7-4 169 3-6 1.0 18.6 444 — — 1 00.0 1815 •9 •4 3-9 104 1.2 .2 4.0 103 26.6 11.4 — 946 21. 1 22.9 — 1320 16.1 18.4 — 1042 20.7 29.2 — 1568 100 Calorie Portion grams 2>Z 39 389 753 341 488 29 32 184 151 433 714 29 78 49 71 43 40 20 21 61 124 78 27 417 217 443 269 102 25 438 443 48 34 43 29 420 APPENDIX B Table I — Continued Food 'luiiiips E. P. A. P. Veal, breast E. P. A. P. cutlet E. P. A. P. fore quarter E. P. A. P. hind quarter E. P. A. P. side E. P. A. P. Vegetable soup, canned . . Walnuts, California or Eng- lish E. P. A. P. black E. P. A. P. Watermelons E. P. A. P. Wheat, cracked Whitefish E. P. A. P. Zwieback Protein (NX6.2S) PER CENT Fat PER cent Carbo- HV- DRAIE PER CENT Fuel Value per Pound Calo- ries 1-3 2 8.1 178 ■9 .1 5-7 124 20.3 II.O — 817 15-3 8.6 — 629 20.3 7.7 — 683 20.1 7-5 — 670 20.0 8.0 — 690 15-1 6.0 — 517 20.7 8.3 — 715 16.2 6.6 — 534 20.2 8.1 — 697 15.6 6.3 — 539 2.9 — •5 62 18.4 64.4 13-0 3199 4.9 17-3 3 S 859 27.6 56.3 II 7 301 1 7.2 14.6 3 780 •4 .2 6 7 136 .2 .1 2 7 57 II. I 1-7 75 5 1635 22.9 6.5 — 680 10.6 3-0 — 315 9.8 9.9 73 S 1915 TOO Calorie Portion grams 256 367 56 72 66 68 66 88 64 85 65 84 735 14 53 IS 59 332 800 28 67 144 24 APPENDIX B 421 TABLE II Ash Constituents of Foods in Percentage of the Edible Portion (Compiled from Various Sources) Food Almonds . . . . Apples . . . . dried . . . . Apricots . . . . dried . . . . Asparagus . . . Bacon (See Meat) Bananas . . . . Barley, entire . . pearled . . . Beans, dried . . kidney, dry . . Lima, dry . . Lima, fresh . . - string, fresh . . Beef (See Meat) Beer Beets Blackberries . . . Blood (avg.) . . Blueberries . . . Bluefish (See Fish) Bread, Boston brown "entire wheat" graham . . rye .... white . . . Breadfruit . . Brussels sprouts Buckwheat flour Butter . . . Buttermilk . . •239 .007 .032 .014 (.066) .025 .009 ■043 .020 .160 .132 .071 .028 .046 .004 .029 .017 .008 .020 .129 (.05) (•05) .024 .027 .084 .027 ■039 •015 .105 Wm zS •251 .008 •037 .010 (■047) .011 .028 .iji (.070) .156 •139 .188 (.070) .025 .008 .021 .021 .004 .007 .078 (.OS) (.05) ■039 .023 .007 .040 .048 .001 .016 ciUi .741 .127 (.623) .248 (1-157) .196 .401 •477 (.241) 1.229 1. 144 1. 741 (■613) .247 .058 •353 .169 ■075 •051 (•232) (.208) (.291) •151 .108 •235 •375 .130 .014 •151 .019 .011 (-050) .038 (•177) .007 •034 .076 (•037) .097 .041 .249 (.088) .019 .013 •093 (.007) .261 .016 (■394) (•394) (■394) .701 (•394) .027 .004 .027 (.788) .064 r1 ^' •465 .012 .048 .025 (•117) •039 .031 .400 .181 .471 •475 •338 • 133 •052 .028 •039 •034 ■031 .008 • 185 (.175) (•218) .148 •093 .068 .120 .226 .017 .097 •037 .005 (•025) .002 (.009) •039 .125 .016 (.016) .032 .041 .026 (.009) .024 .006 .058 (.010) .280 .008 (.607) (.607) (.607) 1.025 (•607) .100 .040 .012 (1.212) .099 .160 .006 ? .010 ? .041 .010 •153 (.120) .215 .227 .161 (•057) .030 •015 .016 .020 • 137 .011 .201 (.120) .150 .104 .105 .049 .194 .071 (.010) .026 422 APPENDIX B Table II — Continued Food Cabbage .... Cabbage greens Cantaloupe . . . Capers . . . . Carp (See Fish) Carrots .... Cauliflower . . . Caviar .... Celery .... Chard Cheese .... Cherries .... Cherry juice . . Chestnuts . . . Chicken (See Meat) Chocolate . . . Cider Citron . . . . Clams, round . . soft, long . . . Cocoa Coconut, dried . fresh .... Coconut milk . . Cod (See Fish) Corn(maize),mature meal . . . sweet . sweet, dried . Cotton-seed meal Cowpeas . Crackers . . . Cranberries . . Cream . . . Cucumbers . . Currants, dried fresh . . . •045 .106 .017 .122 .056 .123 •137 .078 ■150 •931 .019 .017 •034 .092 .008 .121 .106 .124 .112 •059 .024 .020 .020 .018 .006 .021 .265 .100 .022 .018 .086 .016 .082 .026 < a SB •015 .030 .012 .022 .021 .014 .022 .014 .071 •037 .016 .011 •051 (-293) .011 .018 .098 .079 .420 •059 .020 .009 .121 .084 •033 .121 .462 .208 .011 .007 .010 .009 .044 .017 si Oh B .247 ■512 •235 .209 .287 .222 .422 .316 .318 .089 •213 .200 .560 (-563) •095 .210 •131 .212 .900 •597 .300 .144 •339 .213 •113 .414. 1.390 1.402 .100 .077 .126 .140 .873 .211 .027 .025 .061 •051 .101 .068 .874 .084 .086 .606 .023 .013 .065 .012 .020 .011 •705 .500 •059 •073 .036 .036 •039 .040 .146 •234 .161 (•594) .010 ■035 .010 .081 .007 ^£ .029 .099 •015 .062 .046 .061 .176 •037 .040 •683 •031 .018 •093 •455 .009 •033 .046 .122 .709 •155 .074 .010 .283 .190 .103 •376 i^i93 •456 .102 •013 .067 •033 •195 .038 .024 .068 .041 .036 .050 1.819 .156 •039 .880 .014 .003 .006 (■051) .006 •003 1.220 .910 ■051 • 239 .120 ■045 .146 .014 .050 •037 .040 (.910) .009 .080 •030 .060 .006 .066 ■'^73 .014 .022 .086 .022 .124. .263 .Oil .006 .068 .006 .020 .224 •213 •203 (■056) .028 .008 •151 .III .046 .167 •485 .240 •125 .007 .030 .020 .044 .014 .0011 .0018 .0003 .0006 .0006 .0005 (.0025) .0013 .0004 (.0003) .0007 (.0027) (.0002) .0027 .0029 .0009 .0008 .0029 .0015 .0006 .00022 .0002 (■0025) .0005 APPENDIX B 423 Table II — Continued Food Currant juice . . Dandelion . . . Dates Duck (See Meat) Eggplant .... Eggs Egg white . . . Egg yolk .... Endive . . . . Farina . . . . Figs, dried . . . fresh .... Fish* Flaxseed . . . . Flour, buckwheat . "entire wheat" . graham . . . white . . . . rye Fowl (See Meat) Gluten feed . . . Goose (See Meat) Gooseberries . . Grapefruit . . . Grapejuice . . . Grapes . . . . Guava . . . . Haddock (See Fish) Halibut (See Fish) Ham (See Meat) Hazelnuts . . . Herring (See Fish) Hominy .... .021 • 105 .065 .011 .067 •015 • 137 .104 .021 .162 •053 .204 .010 .031 •039 .020 .018 .247 •035 .021 .011 .OIQ .014 287 SB .010 .036 .069 •015 .011 .010 .016 .013 .025 .071 .022 .252 .048 (.090) (•133) .018 .081 .014 .009 .009 .010 .008 .140 .058 .185 .461 .611 (.140) .140 .160 • "5 .380 .120 .964 ■303 .901 .130 (•274) (.457) • 115 •463 .250 .197 .161 .106 .197 •384 .618 .174 (.006) .168 •055 (.010) ■143 .156 .075 .109 .065 .046 .050 .027 (•037) (•037) .060 .019 .420 .038 .004 •005 •015 .019 .018 .072 .056 •034 .180 .014 •524 .038 •125 .116 .036 .627 .176 .238 •364 .092 .289 •542 .031 .020 .011 ■031 .030 •354 .144 2^ .004 .099 .228 .024 .106 •155' .094 .167 .076 •043 .014 .022 .012 (.070) (.070) .074 •055 .090 .005 .002 .005 ■045 .067 .046 .005 .017 .070 .016 •195 .216 .166 •035 •155 .056 .010 .170 .071 (.180) .183 .177 .123 .558 .011 .010 .009 .024 (.136) * Average fish is estimated to contain per loo grams of protein as follows : o.iOQ gram Ca; 0.1.33 gram Mg; 1.671 grams K; 0.373 gram Na; 1.148 grams P; 0.528 gram CI; 1.119 grams S; 0.0055 gram Fe. 424 APPENDIX B Table II — Conlinued Food Honey .... Horseradish . . . Huckleberries . . Huckleberry wine Jam * JeUy Kohl-rabi . . . Lamb (See Meat) Leeks Lemons .... Lemon juice . . . Lemon, sweet . . Lentils, dry . . . Lettuce .... Limes Lime juice . . . Linseed meal . . Lupins, dry . . . Macaroni . . . Mackerel (See Fish; Mamey .... Mango .... Mangolds . . . Maple syrup . . Meat t Meat extract, solid Meat peptone . . Milk (cow's), whole (cow's), skimmed (cow's), con- densed . . . ^^ .004 .096 .020 .009 .014 .077 .058 .036 .024 .030 .107 •043 •055 •413 .191 .022 .009 .021 .026 .107 .085 .025 .120 (.122) (■300) SB .018 •039 .007 .004 (.010) .030 .014 .007 .010 .006 .101 .017 .014 •432 .191 •037 .012 .007 .030 •034 •363 .124 .012 (.012) (•032) < — 386 468 051 042 100) 370 199 175 127 442 877 339 350 083 840 130 345 23s 334 208 347 440 143 149) 374) .001 .062 .016 .006 (■013) .050 ■.081 .004 .009 .062 .027 .062 .251 •073 .008 .071 .010 2-394 .641 .051 (-052) (.134) .019 .076 .008 .004 .008 .071 .006 .022 .010 .042 •438 .042 .036 .741 .520 .144 .028 .017 .038 •013 2.800 1. 130 •093 (.096) 235 .029 .016 .008 .001 (.004) •053 .024 .002 .003 .013 .050 .074 •039 .08s •034 •073 .140 .019 .082 (.010) 3-II7 .561 .106 (.110) (.280) to — «t. .001 .0007 .190 .Oil I .0009 .006 (007) (.0003) .05 7 .0006 .072 .011 .006 .016 .277 .014 .010 .003 •396 .172 .013 .026 (■005) 034 (•035) (.090) .0006 .0086 .0007 (•003) .00024 .00025 .0006 * The perceatages of the ash constituents in jams are believed to average about two thirds those of the corresponding fruits. t Average meat is estimated to contain per 100 grams protein as follows: 0.058 gram Ca; 0.118 gram Mg; 1. 6g4 grams K; 0.421 gram Na; 1.078 grams P; 0.378 gram CI; 1.146 grams S; 0.0153 gram Fe. APPENDIX B 42s Table II — Continued Food Milk — Cont. buffalo 203 camel's . . . .143 goat's 128 human . . . .034 mare's 083 sheep's 207 Millet 014 Molasses 211 Mushrooms . . . .017 Muskmelon . . . .017 Mustard 492 Mutton (See Meat) Oatmeal 069 Okra 071 Olives 122 Onions 034 Oranges 045 Orange juice . . .029 Oysters 052 Paprika 229 Parsnips 059 Peaches 016 dried 034 Peanuts 071 Pears 015 Pear juice . . . .009 Peas, dried . . . .084 fresh 028 Pecan nuts . . . .089 Pepper, green, fresh .006 Pepper, black, dry .440 Pepper, white, dry .425 Perch (See Fish) Persimmons . . . .022 Pineapple . . . .018 Plums 030 .016 .021 •013 .005 .007 .008 .167 .068 .016 .012 .260 .110 .010 .002 .016 .012 .011 •037 .164 •034 .010 .056 .180 .011 .008 .149 .038 •152 .010 .156 • 113 .009 .011 .011 .099 .114 ■ 14s .047 .081 .187 .290 1-349 •384 •235 .761 •344 •035 1.526 .178 .177 .182 .091 2.075 .518 .214 (.830) •654 .132 .140 ■903 .285 (•332) (•139) 1. 140 .292 .321 .203 .038 .019 .079 .010 .010 .030 .085 .019 .027 .061 .056 .062 •043 .128 .016 .012 .008 •459 .178 .004 .022 .082 .050 .016 .104 .013 •131 .011 .016 .org .125 .098 .103 .CIS •054 .123 •327 .044 .108 •015 ■755 •392 .019 .014 ■045 .021 .016 •155 •341 .076 .024 .146 •399 .026 .011 .400 .127 •335 .026 .188 •233 .021 .028 .032 2^ oi — qu .062 .105 .014 •03s .029 .071 .019 •317 .021 .041 .016 .069 .004 .021 .006 .003 •590 •155 •030 .004 .056 .011 •035 .024 .050 .013 .312 .029 .002 •051 ■037 .129 •051 .014 1.230 .202 .027 .070 .Oil .009 .187 .036 .009 .212 .224 .010 .009 .219 .063 •113 .014 .005 .009 .OOQ .0073 .0003 .0038 .0029 .0006 .0002 .0002 .0045 .0006 .0003 (.0012) .0020 .0003 .0057 .0017 .0026 .0004 .0005 .COO 5 426 APPENDIX B Table II — Continued Food 6 ^5 en is 11 u u u OS in II Pomegranate . . .011 .005 .063 .085 • los .003 .0004 Pork (See Meat) Potatoes .... .014 .028 .429 .021 •058 .038 .030 .0013 sweet .... .019 .028 •397 •039 045 .094 .024 .0005 Prunes, dried . . •054 ■055 1.030 .069 •105 .017 •037 •0030 Pumpkin .... .023 .008 (■320) .065 •059 — .021 (.0008) Radishes . . . .021 .012 .218 .069 .029 •054 .041 .0006 Raisins .... .064 .083 .820 •133 .132 .082 •051 .0021 Raspberries . . . .049 .024 ■173 — .052 — .017 .0006 Raspberry juice .021 .016 ■134 .005 .012 — .009 — Rhubarb .... .044 .017 •325 .025 .031 .036 •013 .0010 Rice, brown . . . — — — .207 — — .0020 white .... .oog •033 .070 •025 .096 •054 .117 .0009 Romaine (salad) . •045 .032 .306 .016 •053 •073 .019 — Rutabagas . . . .074 .018 •399 .083 .056 •058 •083 — Rye, entire . . . •055 •130 •453 •03s •385 •025 .170 .0039 (See also Bread and Flour) Salmon (See Fish) Sapato .... .026 .008 .179 — .006 .087 — — Shredded wheat .041 .144 — •324 — — .0045 Shrimp .... .006 . . Soup, canned . . .036 — •033 — .030 — — — canned vegetable .025 .013 .101 — •038 — •02 s — Spinach .... .067 •037 •774 •125 .068 .074 •038 .0036 Squash, summer, seeds removed .018 .008 •150 .002 — — — (.0006) with seeds . . .024 .012 .180 .004 — — — (.0006) Squash, winter . . .019 .oil •320 .004 — — — (.0006) Strawberries . . .041 .019 .147 .050 .028 .006 .014 .0008 Tamarind . . . .007 .021 — .072 .007 .009 — Tapioca .... .023 — — .090 .018 .029 .0016 Tomatoes . . . .oil .010 •275 .010 .026 •034 .014 .0004 Tomato juice . . .006 .010 •310 •015 •015 •055 — — Truffles .... .024 .018 .404 •077 .062 •039 — — Turnips .... .064 .017 •338 .056 .046 .041 .065 .0005 Turnip tops . . ■347 .02S •30: .0S2 .040' .I6S .060 — APPENDIX B 427 Table II — Continued Food s SB li to a Bl? oS- CO is 5 Veal (See Meat) Vinegar (cider) .016 .008 .165 — •013 — .017 (.0003) Walnuts .... .080 •134 •034 (.332) .287 . .358 .005 .040 .172 .0021 Water cress . . . .187? .099 .061 .167 .0019 Watermelon . . .Oil .003 ■073 .008 .003 .008 .007 W^heat, entire . . •045 •133 •473 ■039 •423 .068 .181 .0050 (See also Bread and Flour) Wheat bran . . . .120 •511 1. 217 •154 1.215 .090 •247 .0078 Wheat germ . . .071 •342 .296 .722 1.050 .070 ■325 — Wheat gluten . . .078 •04s .007 .028 .200 .050 .920 — Whey .04.4 .008 •157 .038 ■035 .119 .009 ? Whortleberries, en- tire .... .031 .021 .261 .021 .042 — — flesh only . . . .020 .oil .087 .018 — — — Wine (avg.) . .000 .010 .104 .008 •015 .011 .015 (.0003) TABLE III Protein, Calcium, Phosphorus, and Iron in Grams per 100 Calories OF Food Material (Estimated from data compiled from various sources) Food Protein Cal- cium (Ca) Phos- phorus (P) Iron (Fe) CaO P2O5 Grams Grams Grams Grams Grams Grams Almonds 3-22 •037 .072 .00060 .052 .165 Apples 0.64 .012 .020 .00048 .016 •045 Apricots 1.90 .023 .044 .00052 •033 (.100) Asparagus 8.10 .122 .177 .00451 .171 •405 Bacon (See Meat) 428 APPENDIX B Table III - - Continued Food Protein Cal- cium (Ca) Phos- phorus (P) Iron (Fe) CaO P2O6 Grams Grams Grams Grams Grams Grains Bananas . 1.32 .009 •031 .00061 .012 .072 Beans, dried . . . 6.52 .047 •137 .00203 .065 •314 kidney .... S-83 (.040) (•143) (.00216) (.056) (.326) Lima .... 5.80 .020 .096 .00200 .028 .221 string .... 5-55 .110 .126 .00265 •154 .289 Beef (See Meat) Beer - — .008 .061 .00217 .Oil .140 Beets 347 .064. .084 .00130 .089 •193 Blackberries . . . 2.25 .029 .058 .00104 .042 •133 Blueberries . . . (0.8) (.027) (.011) (.0012) (.038) (.025) Bluefish (See Fish) Bread, Boston bro\\ n . 2.64 .056 .082 (.0013) .079 .187 "entire" wheat . 3-95 (.020) .071 (.00065) (.028) (.163) graham . . . 3.42 (.020) .084 (.00096) (.028) (.192) rye 3-54 .009 .058 .00039 •013 ■^33 white .... 3-50 .oil •035 .00035 •oiS .081 Brussels sprouts (7-30) (.086) (•380) (.00349) (.121) (.870) Buckwheat flour i.8s .011 .065 .00034 •015 .148 Butter .... 0.13 .002 .002 .00003 .003 .005 Buttermilk . . 8.40 .294 .271 .00070 .411 .621 Cabbage . . . 5-07 •143 .092 .00349 .200 .210 Cantaloupe . . I-5I .044 .038 .00071 .061 .088 Carp (See Fish) Carrots . . . 2.42 .124 .101 .00133 •173 .232 Cauliflower . . 5-9° •403 .200 .00197 •564 •459 Celery .... 1.28 .421 .201 .00270 .589 .460 Chard .... 8.37 ■393 •105 (•00655) •550 .240 Cheese 6.05 .212 .156 .00030 .297 ■357 Cherries . . . 1.20? .025 •039 .00051 •035 .090 Chestnuts . . 2-55 .014 .044 .00029 .019 .088 Chicken (See Meat Chocolate . . . 2. II ■015 •07s (.00044) .021 .171 Citron .... 0-15 •037 .010 .00099 .052 .023 Clams, long . . 19.82 .285 .282 (.00970) •399 .645 round . . . 14.01 .229 .100 (.00970) ■321 .228 Cocoa .... 4-35 .023 ■143 .00054 .032 ■327 Coconut . . . 0.9s .006 .018 (.00030) .009 .041 Cod (See Fish) ' APPENDIX B 429 Table III — Continued Food Protein Cal- cium Phos- phorus Iron (Fe) CaO P2O5 (Ca) (P) Grams Grams Grams Grams Grams Grams Corn 3.06 .006 .102 .00079 (.008) (.233) Corn meal .... 2-59 .005 •O.S3 .0003 .007 .121 Cotton-seed meal . . 12.80 .066 .298 — .092 .682 Cowpeas 6.20 .029 .132 — .041 ■303 Crackers, "soda" . . 2.37 .006 .025 .00036 .008 •057 Cranberries .... 0.85 •039 .027 .00129 •054 .062 Cream, 18.5 per cent fat 1.27 .050 .044 .0001 .072 .100 40 per cent fat . . 0.58 .020 .020 .00005 .032 •045 Cucumbers .... 4.60 .090 .191 .00115 .126 •437 Currants, dried (Zantc) 0-7S .026 .061 .00087 .036 • 139 fresh 2.62 •045 .066 .000S7 .063 •150 Dandelion greens . 3-93 .172 .117 .0044 .241 .269 Dates 0.60 .019 .016 .00086 .026 .037 Duck (See Meat) Eggplant 4-3° .041 .122 .00184 •057 .280 Eggs • 9-05 •04s .122 .00205 .063 .279 Egg white .... 24.12 .020 .022 .00020 .028 .050 Egg yolk 4-32 .036 .118 .00230 .050 .270 Farina 3-05 .006 •035 .00022 .008 .079 Figs 1 1-35 •051 •037 .00095 .072 .084 Fish (See footnote on page 423) Flour, buckwheat .... 1.84 .Oil .065 .00034 •015 .148 "entire" wheat . . 3.85 .009 .066 .0007 .012 •152 graham 3-71 .oil .101 .00100 ■015 .232 white (wheat) . . 3.20 .006 .026 .00023 .008 .060 rye i 1-95 .005 .082 .00037 .007 .188 Fowl (See Meat) Goose (See Meat) Grapefruit 1.15 .040 .036 .00058 .056 .083 Grapes 1-35 .019 .032 .0003 1 .027 .074 Grapejuice 0-35 (.oil) .on .0003 •015 .025 Haddock (See Fish) Halibut (See Fish) Ham (See Meat) Hazelnuts .041 .o%o .00057 •057 .115 Herring (See Fish) •^ J^ Hominy . 2.3=; .002 .027 .00025 .002 .063 430 APPENDIX B Table III — Continued Cal- Phos- Iron (Fe) Food Protein cium (Ca) phorus (P) CaO PK). Grams Grains Grams Grams Grams Grams Honey 0.12 .002 .006 .0003 .002 •013 Huckleberries 0.82 .027 .Oil .0012 .038 .025 Kohl-rabi 6.48 .249 .186 .00194 •349 .426 Lamb (See Meat) Lemons 2.25 .081 .049 ■00135 •113 .112 Lemon juice — .060 — — .084 •059 Lentils 7-37 •031 .126 .00247 •043 .288 Lettuce 6.27 .224 .224 .00785 •314 •513 Linseed meal — — ■ — — — — Lupins — — — — — — Macaroni 3-70 .006 .040 •00033 .008 .092 Mackerel (See Fish) Maple syrup — ■037 (■003) (.001) •053 (•007) Meat (See footnote on page 424) Milk, whole 4-75 .174 •134 .00035 •243 .308 skimmed 9-25 (.331) .262 (.00068) (.463) (.600) condensed, sweetened . . 2.70 (.096) .072 (.0002) (•135) .165 condensed, unsweetened . 5-75 .189 .146 (.0004) (.264) •335 Molasses 0.83 .074 •015 •00255 .102 •035 Muskmelon I-5I •043 .038 .0008 .060 .088 Mutton (See Meat) Oatmeal 4.20 .017 .099 .00096 .024 .226 Olives 0.37 .041 .004 .00097 •057 .010 Onions 3-30 .069 ■093 .0010 .097 .212 Oranges 1-55 .088 .040 •00039 •123 .091 Orange juice 1.44 .067 •037 .00046 •093 .082 Oysters 12.30 .106 .306 .00893 .149 .702 Parsnips 2.47 .091 .117 .0009 .128 .268 Peaches 1.70 .038 •057 .00073 •053 .130 4.70 .013 •073 .00036 .018 .166 Pears 0-95 .024 .041 .00047 •033 •093 Peas 6.92 .026 .120 .00165 •036 .274 Pecans 1.30 .012 •045 •00035 .017 .104 Pepper, green 4-59 •034 •145 .00222 •047 ■333 Perch (See Fish) Persimmons — — — — — — Pineapple, fresh .... 0.92 .041 .064 .00116 •058 .146 APPENDIX B 431 Table III — Continued FOOE Protein Cal- cium (Ca) Phos- phorus (P) Iron (Fe) CaO F^Os Grams . Grams Grams Grams Grams Grams Plums . . . . . . . 1.20 .024 .038 .00059 •033 .087 Pork (See IMeat) Potatoes . . .... 2.65 .016 .069 .00156 .023 .158 sweet . .... 1.45 .016 •037 .00041 .023 .084 Prunes . . . . . . 0.70 .018 •035 .00100 ■025 .080 Pumpkin . .... 3-90 .089 .229 (.00130) .125 •525 Radishes . .... 4.42 .073 .098 .00205 .102 .225 Raisins .... 0.75 .019 .038 .00139 .026 .088 Raspberries .... 2.57 .074 .078 .00091 .104 .178 Rhubarb . . . . . 2.60 .189 •134 •00433 .264 •307 Rice, brown .... 2.52 (.003) .060 .00058 (.004) .138 white . . ." . 2.27 .001+ .027 .00026 .003 .063 Rutabagas .... 3-15 .185 .140 — .259 .322 Rye, entire . . . . — — — — — — Salmon (See Fisl 1) Shredded wheat • . . • 3.50 .Oil .089 .00123 .016 .203 Spinach . . .... 8.79 .281 .285 .01506 ■393 •653 Squash, summer .... 3-05 ■039 •03s (.0013) ■054 .080 winter . . .... 3.10 .040 .061 (.0013) .056 •139 Strawberries .... 2.56 .104 .072 .00205 .146 .164 Tapioca . . . . . O.II .004 .025 .00045 .006 .058 Tomatoes .... 3-95 .050 ■113 .00175 .070 •259 Turnips . .... 3.30 .161 .117 .00127 .226 .269 Turnip tops . . . . — — — — — — Veal (See Meat) Vinegar (cider) . . . . — .Ill .090 .00213 .156 .206 Walnuts, Califor nia or Eng- lish . . . . . . 2.60 •013 •015 .00030 .018 .116 Water cress . . . . . — — — — — — Watermelon . .... 1.32 .038 .010 (.00099) •053 .023 Wheat, entire .... 3-63? •013 .118 .00140 .018 .270 Wheat germ . . — — — — Wheat gluten .... — — — — — — Whey . . . . .... 3.74 .165 •131 ? .231 .300 Whortleberries . . . . . — — — — — — Wine (average, 10 per cent alcohol) . 1 .oil .021 .00167 .016 .047 INDEX Abderhalden, amino acids in blood, 120 inorganic iron in nutrition, 203-295 physiological chemistry, 136, 257, 308 sulphides in alimentary canal, 293 Abel, amino acids dialyzed from blood, 120 Abel, Rowntree, and Turner, removal of diffusible substances from blood of living animals, 136 Absorption in small intestine, 93 Acetic acid, 108, 109 aldehyde, 108, 109 Acetoacetic acid, 116 Acetone bodies, 117 Acetonitrile poisoning, effect of diet on resistance to, 350 Acid, acetic, 108, 109 acetoacetic, 116, 126 adenylic, 132 a-ketonic, 216 amino, 43-48, 55-68, 119-122, 216, 217 aminoglutaric, 44 aminosuccinic, 44 aspartic, 44, 47, 60, 61, 120, 126 /3-hydroxy, 116 ^-ketonic, 116 |8-oxj^butj'ric, ii6 butyric, 22, 113, 116 capric, 22 caproic, 22, 116 caprylic, 22 carbonic, 108, 109, 275, 276 diamino, 44, 67 diaminomonocarboxylic, 44 diaminotrioxydodecanic, 61 crucic, 23 fatty, see Fat formic, 108, 109 glutamic (glutaminic), 44, 47, 60, 61, 120, 126 guanylic, 132 hypogasic, 23 Acid — Continued lactic, 75, 105, 109, 113, 124, 126, 216 lauric, 22, 116 linoleic, 24 linolenic, 24 monoaminodicarbox>'lic, 44 monoaminomonocarboxylic, 43 myristic, 22, 116 nicotinic, 327 nucleic, 130-137 octoic, 113, 114 oleic, 23 palmitic, 22, 116 phosphoric, 131, 132 phycetoleic, 23 phytic, 244 pyruvic, 108, 109, 114, 125, 216 stearic, 22, 116, 216 \ Acid-forming diet, 279, 282 Acid-forming and base-forming ele- ments in food, 279-283 Acidity, 76, 77, 274 Acidosis, 117, 179, 280 Ackroyd and Hopkins, deficiencies in amino acid supply, 136, 355 Acrolein, 19 Activating substances, 76 Activity, muscular, 179-188, 226-229 Adenine, 131, 132, 327 isomer of, 327 Adenosine, 132 Adenylic acid, 132 Agar-agar, 17 ..Age, influence on food requirement, 193-198, 229-233, 370-373 on protein metabolism, 229-233 Alanine, 43, 45, 47, 48, 60, 61, 120, 124, 125, 126, 216, 403 deamination of, 126, 216, 403 Albumins, 52, 54, 56, 60, 142, 404 ^ acid-, 54 alkali-, 54 coagulated, 54, 73, 406 2 F 433 434 INDEX Albuminates, 54 Albuminoids, 404 Albumoscs, src Proteases Alcohol-soluble proteins, 52, 56, 404 Aldoses, 3, 4, 5 Aldrich, chemical nature of pepsin, 72 Alexis St. Martin, observations upon, 70 Alimentary glycosuria, 6 Alkali (Alkaline) reserve, 280, 313 Alkalinity, 76, 77, 274 Allantoin, 323 Allen, metabolism in diabetes, 136 Allose, 5 Almonds, 146, 256, 26g, 302, 395, 410, 421, 427 a-ketonic acid, 216 aldehyde, 120 Altrose, 5 Amandin, 52, 60 Amino acids, 43-48, 55-68, 1 19-122, 126, 216, 217 absorption of, 1 20 dialyzed from blood, 120 disappearance of, 121 formation of, 125 saturation capacity of tissues for, 121 separation of, 120 yields of from flesh, 61 from proteins, 60-61 Aminooxypurine, 132 Aminopurine, 132 Aminoglutaric acid, 44 Aminolipins, 36 Aminosuccinic acid, 44 Ammonia, relation to nitrogen metabo- lism, 130, 136, 216, 284 to regulation of neutrality, 126, 278, 279, 281, 283 Ammonium carbamate, 129 carbonate, 129 Amylases, 73, 74, 76, 103 Amylolytic enzymes, see Amylases Amylopectin, 13 Amylopsin, 76 in pancreatic juice, 73, 90 occurrence and action of, 79 Amylose, 13 a-amylose, 13 Anaerobes, 97 Anderson, organic phosphoric acid com- pound of wheat bran, 257 Anderson and Lusk, relation between diet and energy production dur- ing work, 200 Antineuritic action, relation of chemical structure to, 326 substances, attempts to isolate, 323, 324 Antipcristalsis, 94 Antiscorbutic property, of food, 310, 311-318 effect of cooking upon, 314, 315 Appetite, 80, 81 as dietary standard, 361 Apples, 146, 256, 269, 302, 395, 410, 421, 427 Apricots, 410, 421, 427 Arabans, 5 Arabinose, 4 Araboketose, 4 Arachin, 52 Arginine, 44, 47, 60, 61, 72, 120, 126, 129, 403 Armsby, animal nutrition, 168, 200 experiments in heat production, 162, 164 food as body fuel, 168 food supply of the future, 400 Armsby and Fries, influence of standing or lying on metabolism, 200 Aron, calcium requirement of children, 282 experiments with limited rations, 338 nutrition and growth, 355 phosphorus in beriberi, 322 Aron and Frese, utilization of different forms of food calcium, 282 Aron and Sebauer, calcium for growing organism, 282 Artichoke, 410 Ash constituents, xii, 234-309, 342-345, 382, 391-401, 409, 421-431 Asparagus, 146, 394, 410, 421, ^127 Aspartic acid, 44, 47, 60, 61, 120, 126 Asymmetry of underfed animals, 338 Atwater, bomb calorimeter, 139, 140 chemistry and economy of food, 168, 400 coeflScients of digestibility in mLxed diet, loi INDEX 435 Atwater — Continued dietary standards, i8o, 36,3, 365, 400 muscular work and protein metabo- lism, 228 protein sparing action of carbo- hydrate and fat, 214, 215 respiration calorimeter experiments, 168, 200, 400 Atwater and Benedict, fats and carbo- hydrates as protectors of protein, 232 , mechanical efiSciency of man, 183, i8s metabolism during sleep and sitting at rest, 176 metaboUsm while fasting, 188 respiration calorimeter, 168 rest experiments, 164-166 Atwater, Benedict, et al., respiration calorimeter experiments, 200 Atwater-Rosa-Benedict, respiration calo- rimeter, 158-163 Atwater and Snell, bomb calorimeter, i3g, 140, 168 Aub and DuBois, basal metabolism of old men, 200 Babcock, metabolic water, 257 Bacillus, aerogenes capsulatus, 97 bifidus, 96 coli, 96, 97 lactis aerogenes, 96 Bacon, 146, 394, 410, 421, 427 Bacteria, in digestive tract, 95, 97-98 Bailey and Murlin, energy requirement of new-born, 200 Bananas, 146, 256, 269, 302, 395, 410, 421, 428 Barley, 410, 421 Barlow's disease, 316 Baumann and Howard, mineral metabo- lism of scurvy, 327 Bayhss, general physiology, 102, 257 nature of enzyme action, 102 Bayliss and StarUng, secretin, 91 Beans, 146, 241, 256, 269, 302, 321, 394, 410, 421, 428 Beaumont, observations upon Alexis St. Martin, 70, 71 on stomach contraction, 83, 84 Bed calorimeter, 160, 162, 167 Beef, 6t, 145, 241, 256, 269, 302, 394, 411, 412, 421, 428 Beer, 421, 428 Beets, 146, 394, 412, 421, 428 Beet sugar, see Sucrose Benedict (F. G.), metabolism during fasting, 200, 232, 239, 257, 263, 272, 273, 282 in relation to acidosis, 179 muscular work, 200 pulse rate, 175, 176 variations in metabolism, 179 nutritive requirements of body, 378, 400 per unit of area, 172 respiration apparatus, 151, 152, 168 Benedict (F. G.) and Carpenter, metabo- lism experiments, 176, 177, 178, 200 respiration calorimeter, 168 Benedict (F. G.) and Cathcart, metabo- lism during muscular work, 187, 188, 200 Benedict (F. G.) and Emmes, basal metabolism of men and women, 201 influence of non-oxidizable material upon metabolism, 200 Benedict (F. G.) and Murschhauser, metaboUsm during muscular work, 182, 183 Benedict (F. G.) and Osterberg, human fat, 30 Benedict (F. G.) and Roth, energy metabolism of vegetarians, 190, 191, 201 Benedict (F. G.) and Smith, metabolism of athletes, 201 Benedict (F. G.) and Talbot, energy metabolism in infants, 195 respiratory exchange of infants, 201 Benedict (S. R.), uric acid in metabo- lism, 136 Beriberi, 310, 318-324, 327-330 Berthelot, bomb calorimeter, 139 mixed glycerides, 26 Betaine, 327 /3-amylose, 13 /3-hydroxy acids in fat metabolism, 116 /3-ketonic acid, 116 iS-oxidation theory, 116, 216, 217 /3-oxybutyric acid, 116 436 INDEX Bile, 9 1 Blackberries, 412, 421, 428 Blacklish, 412 Blatherwick, effect of base-forming ele- ments in food, 281, 282 Blauberg, mineral metabolism of in- fants, 282 Blood, ash of, 421 glucose content of, 6, 104, 118 reaction of, 273-284 see also Amino acids Bloor, metabolism of fat, 34, 40 Blueberries, 421, 428 Bluefish, 412, 428 Blyth and Robertson, mixed glyccride of butter fat, 26 Body fat, composition of, 30-35, 142 influence of food fat, 32-34 Body, human, elementary composition of, 234 Body temperature, regulation of, 191- 193, 202 Boldireff, on hunger, 82 Bomb calorimeter, 139, 140 Bomer, mixed glycerides, 26 Bones, calcification and development of, 343 source of calcium for carnivora, 262 Boutwell, phytic acid of wheat kernel, 2S7 Braddon, cause and prevention of beriberi, 319, 327 Braddon and Cooper, carbohydrate and vitamine metabolism, 325, 328 Brazil nuts, 412 Bread, 146, 299, 394, 413, 421, 428 Breadfruit, 421 BreadstufTs, see Grain products Brcithaupt and Cetti, calcium elimina- tion, 263 British gum, 14 Browne, butter fat, 32, 40 definition of sugar, 2 Brussels sprouts, 421, 428 Buckwheat flour, 413, 421, 428 Bunge, metabolism of iron, 287, 288, 292, 293, 300, 301, 305, 306 physiological and pathological chem- istry, 257, 308 sodium chloride ehmination, 23S ase of salt, 238 Bunge and Abderhalden, phosphorus content of milk, 247, 248 Bureau of markets, 399 Butter, 21, 22, 146, 386-392, 394, 413, 421, 428 Butter fat, 31-32, 142, 346, 356-358 growth promoting property of, 346, 356-358 Buttermilk, 413, 421, 428 Butternuts, 413 Butyric acid, 22, 113, 116 Cabbage, 146, 269, 302, 395, 413, 422, 428 Cxcum, 94 CafTeine, effect on metabolism, ref., 202 Calcium, 234, 260-272, 343, 344, 383, 391-396, 421-431 amounts in dietaries, 267, 268 amounts in foods, 268, 269, 421-431 elimination, 263 function in body, 260, 261 in milk, 268 relation to metabolism of iron, 270, 298-299, 382-383 requirement, 262-268 of children, 265, 266 of women, 264, 265 Calf's foot jelly, 413 Calorie, 139, 140 Calorimetrj', direct, 15S indirect, 154 Camerer, calcium requirement at dif- ferent ages, 266 storage of food for growth, 194 Camerer and Soldner, ash constituents of new-born infant and human milk, 282 Cane sugar, see Sucrose Cannon, action of pylorus, 86 competency of ileocecal valve, 94 explanation of hunger, 81, 103 intestinal digestion, 90 mechanical factors of digestion, 102 movements of stomach and intestines during digestion, 84 passage of food through small intes- tine, 92, 93 psychic contraction, 88 Cannon and Washburn, investigation of hunger, 80, Si, 82, 103 INDEX 437 Canteloupe, 422, 428 Capers, 422 Capric acid, 22 Caproic acid, 22, 116 Caprylic acid, 22 Carbohydrates, 1-18, 131, 142, 143 classification, 2, 4-5 conversion into fat, iii, 112 fermentation of, 97 formation from fat, 117, 118 formation from protein, 123, 124 metabolism of, 104-115, 123-125, 136-137 oxidation of, 105 references, 17, 18 respiratory quotient of, no, in storing of, in synthesis of, i, 2 yield from protein, 124-127 Carbon, 234 Carbon and nitrogen balance, 115, 156 Carbonic acid, 108, 109, 275, 276 Carlson, hunger in health and disease, 84, 103 hydrochloric acid in gastric juice, 87 Carpenter, metabolism increase during tj'pewriting, 201 respirator>' exchange, 169 Carpenter and Murlin, metabolism of ■ mother and child, 201 Carrots, 146, 256, 269, 302, 395, 413, 422, 428 Casein, 47, 48, 49, 53, 58, 61, 64, 73, 142, 225, 226, 240, 243, 246, 339, 340, 354 Caseinogen, 53 ; see also Casein Catalysts, organic, 75 Catalyzers, 78, 79 Cathcart, protein metabolism, 232 Cauhflower, 413, 422, 428 Caviar, 422 Celery, 146, 395, 413, 422, 428 Cellulose, 5, 15 Cerealine, 413 Cereals, see Grain products Cetti and Breithaupt, metabolism of iron while fasting, 298 Chamberlain, beriberi, 320, 321, 323, 328 Chamberlain and Vedder, etiology of beriberi, 328 rice polishings in beriberi, 322 Chard, 413, 422, 42S Cheese, 256, 269, 302, 386-392, 394, 413, 422, 428 Chemical composition of foods, 407- 431 Cherries, 413, 422, 428 Chestnuts, 146, 414, 422, 428 Chick and Hume, distribution among foodstuffs of substances required for prevention of beriberi and scurvy, 328 Chicken, 61, 414, 422, 428 Children, food requirements of, 193- 198, 200-202, 229-233, 265-268, 300, 331-347, 355-35Q. 370-373. 381-383, 400 table of weights and rates of growth, 372, 373 Chinese moss, 17 Chittenden, dietary standard, 366, 376, 379 economy in nutrition, 232, 400 low protein metabolism, 376 nutrition of man, 232, 400 protein requirement, 218-220, 376, 379 Chittenden and Underbill, production of condition resembling pellagra, 35S Chloride metabolism, 236, 237, 271, 272 V Chlorine, 234, 236, 237, 271, 272 Chlorosis, 289, 290 Chocolate, 414, 422, 428 Cholesterol, 37, 91 Choline, 323 Chyme, 89 Chymification, 71 Cider, 422 Circulation, work of, 168 Citron, 422, 428 Clams, 422, 428 Cocoa, 395, 414, 422, 428 Coconut, 422, 428 Cod, 146, 414, 422, 428 CoeiEcient of digestibility of food, 99, loi, 102 Cold storage, 399 Collagen, 52 Colloidal platinum as catalyzer, 78 Colloids, 12, 51 Colon, 94 438 INDEX Combustion, heat of, 139-142 Combustion in body, 109 Common salt, use of, 236-238 Comparison of cost and food value, 391- 400 Composite valuation, 391-396 Composition of body, 156, 174, 175, 234. 300, 301, 336-339 Conarachin, 52 Conjugated proteins, 53 Consomme, 414 Corn, 146, 302, 395, 414, 422, 429 Cornflakes, 394 Corn meal, 146, 394, 414, 422, 429 Cottonseed meal, 3^3, 354, 422, 429 Cowpcas, 414, 422, 429 Crackers, 394, 412, 414, 422, 429 Cranberries, 414, 422, 429 Cream, 394, 414, 422, 429 Creatine, 134, 135 Creatinine, 134, 135, 142 Cramer, production of fat from protein, 127, 128 Cresol, 98 Cucumbers, 395, 414, 422, 429 Currants, 146, 414, 422, 429 Cystine, 35, 37, 43, 60, 61, 64, 126, 340 Cytodine, 132 Cytodinc-nucleotide, 132 Cytosine, 131, 132, 133 Dakin, beta oxidation theory, 116 interrelations of protein and car- bohydrate, 124-127 oxidations and reductions in animal body, T17, 136 Dakin and Dudley, intermediary me- tabolism, 136 Dandelions, 414, 423, 429 Daniels and Nichols, nutritive value of soy bean, 355 Darling, pathological affinities of beri- beri and scurvy, 328 Dates, 305, 414, 423, 429 Derived proteins, 53 Descartes, fermentation in stomach, 60 Dextrans, 5 Dextrin, s, 14, 83, 86 Dextrose, see Glucose Dezani, chemical nature of pepsin, 72 (/.Fructose, see Fructose % also under Food Dietaries, 255-257, 267-268, 271, 303, 360-401 Dietarj- deficiencies, 310, 347-359, 384. 396 Dietar>- standards, 361-367, 385 Dietary studies, 149, 150, 364, 370, 371, 389. 390 DigestibiUty of food, 99-103 Digestion, gastric, 85-87 intestinal, 89-90, 93-94 saHvary, 80, 82, 85 Dihexoses, 5 Dioses, 4 Dioxyacetone, 4 Dioxypurine, 132 Dipeptids, 45-46 Disaccharides, 4, 5, 8-1 1, 17-18, 79 Disaccharoses, 4, 5, 8-11, 17-18 Distribution of expenditures for food, 386-390, 396-398 "Double bonds," 23 Doughnuts, 414 Drying oils, 24 DuBois, basal metabolism of man, 178, 198, 201 metabolism of boys, 196, 201 respiration calorimetry, 201 DuBois and Associates, metabolism in disease, 201 DuBois and DuBois, formula to esti- mate surface area, 173, 201 relation of body surface to metabolism, 172, 173- 174 table of surface areas, 173 Duck. 423, 429 Duclaux, terminolog>' for enzymes, 76 I Duodenum, 89, 90 INDEX 439 Eberle, artificial digestive juice, 71 Eckles, efifect of sparse diet upon time required to reach maturity, 338 Economic use of food, 386-401 Edestin, 49, 52, 56, 60, 142, 225, 226, 240, 247, 339, 340 Edie, et al., antineuritic bases, 328 Efficiency, mechanical, of man, 181-185, 200 Effront, enzymes, 103 Egg albumin, 49, 52, 55, 60, 240 Egg white, 299 Egg yolk, 256, 269, 302 Eggplant, 414, 423, 429 Eggs, 146, 241, 256, 269, 302, 386, 387, 388, 389, 390, 391, 392, 414, 423, 429 Ehrlich and Lazarus, medicinal iron in hemoglobin formation, 297 Ehrstrom, phosphorus metaboUsm in man, 247, 257 Eijkmann, beriberi in fowls, 321, 322 Elementary composition, 30, 32, 49, 142, 156, 234, 421-431 Embden and Schmitz, amino acid forma- tion, 125, 136 Emmett and Grindley, phosphorus content of flesh, 257 Emmett and McKim, yeast vitamine fraction as supplement to rice diet, 328 Endive, 423 Energy allowances for adults, 366, 367, 370 for children, 370-373 expenditure, during muscular labor, 185, 186 metabolism, 148-201 experimental methods, 148-169 governing conditions, 170-201 of growing infant, 195, 196 influence of age and growth, 193, 194 influence of food, 188 effect of internal secretions, 178 influence of mental work, 177, 178 requirement, 148-201, 366-373 influence of sex, 199 methods of study, 149-166 Enterokinase, 92 Enzymes, 6, 8, 10, 11, 69-80 activity of, 75, 76, 77 amylolytic, 75 chemical nature of, 71 classification of, 74 coagulating, 75 colloidal nature of, 72 deaminizing, 75 digestive, 69 extracellular, 75 hydrolytic, 75 intracellular, 75, 103 introduction of word, 74 isolation of, 74 lipolytic, 75 properties of, 74 proteolytic, 75, 97 reducing, 75 sugar-spHtting, 75 Epigastrium, 81 Eppler, investigations of phosphatids, 258 Erepsin, 80, 92 Ergometer, 185 Erucic acid, 23 Erythrose, 4 Erj^thrulose, 4 Essential oils, 36 Esterase, 103 Ethereal sulphate, 98, 241, 242 Ethylene linkage, 23 Euler, chemistry of enzjTnes, 103 ' Evvard, Dox, and Guernsey, cystine, in tissue growth, 345 effect of calcium and protein fed preg- nant swine on offspring, 282, 355 Excelsin, 52, 60, 225 Factors determining dietary standard, 360, 361, 385 for calculating energy requirement, 186 for calculating fuel values of food, 143 T'alck, influence of body fat upon pro- tein metabolism, 205, 206 Falk and Siguira, lipase preparations, 74. 103 Falk, lipolytically active substances, 74, 103 Farina, 394, 415, 423, 429 440 IXDKX Fasting, iS8, i8g, 200, 203-206, 253, 272, 273, 2g8 Fats, 19-36, 40-41 calories per gram, 142, 143 composition of, 30-32 fish, 24 food, influence of, on body fat, 32-34 formation from carbohydrates, 27- 29, 112-115 formation in nature, 27-29 general properties, 19-21 hardened, 23 heart, 30-31 hydrolysis of, ig kidney, 30-31 liver, 30-31 metabolism of, 115 of organs, 30-3 1 oxidation of, 115 production from protein. 127 respiratorj' quotient of, no, in storage in body, 117 structure of, 19, 21-27 Fatty acids, 21-24, 36 in metabolism, 116 unsaturated, 23-24, 31 Fatty oils, 19, 36 Fat soluble A, xiii, 333, 346, 347, 383, 384; see also Growth Feces, 99, 100, loi, 103, 253, 254, 263, 286, 289, 299 Fermentation, 69, 97 Ferments, 75 ; see also Enzymes Fibrin, 53 Figs, 415, 423, 429 Filberts, 395 Fingcrling, phosphorus metabolism, 249 250, 258 Fischer, synthetic polypeptids, 46, 40- 50 Fischer and Abderhalden, diamino- trioxy-dodecanic acid from casein, 61 Fish, 386, 387, 389. 390, 391, 392, 423, 429 Fitz, Alsberg, and Henderson, excretion of phosphoric acid in acidosis. 282 Fixed oils, 19 I'lesh, amino acids of, 61 Fletcher, beriberi and rice, 320 Flounder, 415 Flour, 241, 256, 269, 302, 394, 41S, 423, 429 Fluorine, 234 Folin, distribution of excreted nitrogen, 135 protein metabolism, 137, 376, 377, 400 Folin and Denis, protein metabolism, 137 relation of amino acids to metabolism, 119 Food, allowances for healthy children, 371 analy.ses, 408, 409, 410-431 antineuritic properties of, 310, 318- 330 antiscorbutic properties of, 310- 318, 327-329 composition of, 407, 408, 409, 410-431 digestibility of, 99-103 economic use of, 386-401 fuel value of, xii, 138-147, 407-420 functions of, xi, 335 influence of, on growth, sec Growth ; on metabolism, 188-101 ; see also under names of the different food- stuffs nutritive ratio of, 147, 148 passage through intestine, 92-94 passage through stomach, 83-89 requirements, 170-233, 252-267, 297- 301, 331-385. 400- 401 Foods, sec Food, sec also under name of each Foodstuffs, see under the name of each definition, xii Forbes, pho.sphorus, 242, 251, 258 mineral elements in nutrition, 258, 283 effect of rations upon development of swine, 258, 355 Forbes and Beegle, mineral metabolism of milch cow, 265, 283 Forbes and Keith, functions of phos- phorus, 242, 243 organic and inorganic phosphorus, 251 phosphorus compounds in animal metabolism, 25S Formaldehyde, i, 2 Formic acid, 108, 109 Fowls, 415, 423, 429 INDEX 441 Fraser and Stanton, study of beriberi due to use of polished rice, 320, 322 Frohlich, infantile scurvy, 328 Fructose, 3, 5, 7 Fruits, 386, 387, 388, 389, 390, 391, 392, 395 Fruit sugar, see Fructose Fucose, 4 Fuel requirements, sec Food require- ments, Dietary standards, Energy metabolism Fuel value of food, 138-143, 144, 145, 147, 407-421 Fundus, 84 Funk, acidosis, 315, 328 deficiency diseases, 328 isolation of antineuritic substance, 323 Funk and Schonborn, influence of vita- mine-free diet upon carbohydrate metabolism, 328 Furst, experimental scur\'y, 328 Galactans, 5, 8, 16 Galactose, 5, 8, 104 Galactosides, 8, 10 Garrod, scurvy, 312 Gastric digestion, 80, 84, 85-89 Gastric fistula, 70 Gastric juice, 70, 85-88 Gaule, absorption of inorganic iron, 290, 308 Gautier, dietary standard, 363 Gelling, nutritive value of diamino acids, 67 Gelatin, 48, 52, 55, 57, 61, 142, 225, 341. 41S as supplement to oat diet, 351 Gephart and DuBois, basal metabolism, 201 Gephart and Lusk, analysis and cost of ready-to-serve foods, 400 Gies, classifications of the lipins, 36, 40 Gillett, food requirements of children, 400 Givens and Mendel, calcium and mag- nesium metabolism, 283 Gliadin, 47, 48, 49, 50, 52, 56, 58, 61, 63, 68, 142, 224, 225, 240, 339, 342 Globulins, 52, 54-56, 60, 224, 404 Glucose, 3, 5, 6-7, 75, 104-110, 117, 118, 124-126, 142, 216 Glucosides, 4 Glutamic acid, 44, 47, 60, 6i, 120, 126 Glutaminic acid, sec Glutamic acid Glutelins, 52, 56, 6i, 404 Gluten, 56 Glutenin, 52, 61, 225 Gluten feed, 423 Glyceric aldehyde, 105-109, 115, 117, 125, 216 Glycerides, 19, 20, 24-27, 332; sec also Fats Glycerin, see Glycerol Glycerol, 19, 107, 115, 117, 216 Glycerophosphate, 322 Glycerose, 4 Glyceryl radicle, 19 Glycine, 42, 45, 47, 48, 60, 61, 62, 119, 120, 126, 403 Glycinin, 60, 225 Glycocoll, see Glycine Glycogen, 5, 14-15, 104, 109, in, 123, 137, 142, 204, 205 Glycolaldehyde, 3, 4 Glycolipins, 36 Glycolose, 3, 4 Glycoproteins, 53, 405 Glycosuria, 6, 124 Glycyl glycine, 45 Glyoxals, 124, 125 Goetsch, influence of pituitarj' feeding, 355 Goodall and Joslin, chloride excretion, 238 Goose, 423, 429 Gooseberries, 423 Gossypol, 353 Gottlieb, intestinal elimination of iron, 288, 308 Grain products, 386, 387, 388, 389, 390, 391. 392, 394. 397 Grapes, 395, 415, 423, 429 Grape butter, 415 Grapefruit, 395, 415, 423, 429 Grape sugar, see Glucose Growth, 56-68, 193-198, 224-226, 229- 231, 247-249, 266-267, 300-301, 310, 331-359 Griitzner, muscular activity of stomach, 83,84 442 INDEX Guanine, 130, 131, 132 Cluanosine, 132 Guanylic acid, 132 Gulosc, 5 Gumpcrt, metabolism of phosphorus, etc., 250, 258 Gums, 5 Guava, 423 Haddock, 415, 423, 429 HaHbut, 61, 415, 423, 429 Ham, 145, 415, 423, 429 Hammarsten's rennin, 78 Harden and Zilva, a-hydroxypyridine and adenine, 328 Hart, nutritive values of milk and grain proteins, 66, 67 Hart, Halpin, and McCoUum, behavior of chickens fed rations restricted to cereal grains, 355 Hart and Humphrey, protein require- ments of milch cows, 226 Hart and McCollum, effects of restricted rations, 328, 344, 355, 356 Hart, McCollum and Fuller, phosphorus in nutrition of animals, 248, 258, 344. 355 Hart, McCollum and Humphrey, ash constituents of wheat bran in metabolism of herbivora, 258 Hart and Steenbock, effect of magne- sium upon calcium metabolism. 270, 2S3 Hartley, fat of organs, 3C3-31, 40 Hasselbach, influence of food upon car- bon dioxide tension of expired air, 281 Hausermann, inorganic iron in place of food iron, 291, 292, 293 Hawk, water in nutrition, 258 Hazelnuts, 423, 429 Heat of combustion, 139-143 Heat production in body, sec Metab- oUsm Hematin, 59 Hematogen, 287 Hemicellulose, 16 Hemoglobin, 53, 59, 85, 285, 297, 40S Henderson, acid excretion, 273-279, 283 acidosis, 283 Henderson — Continued carbonic acid and neutrahty, 275, 276 equilibrium in solutions of phosphates, 283 fitness of the environment, 283 regulation of neutrality in animal body, 273-279, 283 Henriques and Andersen, nutrition through intravenous injection, 120, 137 Henriques and Hansen, influence of food fat and other conditions on body fat of swine, 29, 40 Heptoses, 5 Herbst, calcium and phosphorus in growth, 258, 266, 267 Herring, 415, 423, 429 Herter, bacteria of the digestive tract, 96-98 103 calcium metabolism, 263, 266 Hertz, absorption in large intestine, 93 Hess, infantile scurvy, 317, 318 Hess and Fish, infantile scurfy, 317, 329 Heterocyclic amino acids, 44 Hexobioses, 5 Hexosans, 5 Hexoses, 5, 132 Hill, estimation of relative heights and weights, 367 glj'cogen formation during sleep, 117, . 118 Hindhede, dissolving of uric acid as affected by food, 281 proteins and nutrition, 223, 232, 401 Histidine, 44, 45, 47, 60, 61, 72, 120 Histones, 52, 404 Hogan, corn as source of protein and ash, 356 Hoist and Frohlich, antiscorbutic prop- erty' of food, 313, 329 Hominy, 394, 415, 423, 429 Honey, 416, 424, 430 Hoobler, human milk production, 232 milk as food protein, 226 protein need of infants, 232 Hopkins, accessory factors in normal dietaries, 356 milk as growth-promoting food, 356 Hordcin, 52, 61 INDEX 443 Hormone, 88, 8g, 276, 34s Romberg, checking of secretion of gastric juice, 88 Horseradish, 424 Howell, arrangement of food in stomach, 8S physiology, 103 relation of amino acids to metabolism, t- 119 Huckleberries, 416, 424, 430 Hudek and Stigler, hunger, 82 Hull and Keeton, gastric lipase, 103 Human body, elementary composition, 234 Hundred-Calorie portions, 144-146, 410- 420 Hunger, 81-82 Hunt, acetonitrile poisoning, 350 Hutchison, food and dietetics, 401 normal amomit of protein in diet, 376 Hydrogen, 234 Hydrogenation of fats, 23 Hydrogen ion concentration, influence on enzyme activity, 76, 77 Hydrogen peroxide, decomposition of, 78 Hydrolysis, 6, 130 Hydrolytic cleavage, 130 Hypogteic acid, 23 Hypoxanthinc, 130, 132, 133 Ileocaecal valve, 92, 93, 94 Ileum, 93, 94 Indican, 98 Indol, 98 Infants, see Children Inorganic elements, 234-309, 342-345, 347-352, 355-359, 3S2-383, 391- 401, 421-431 distribution in body, 234, 260 in American dietaries, 271 relation to each other, 269, 270 requirements (quantitative), 252-255, 262-268, 297-300, 382-383 Inorganic foodstuffs, 234, 309: see also Inorganic elements Inositol, 244 Intestinal digestion, 89-94 Intestinal juice, 91, 92 Inulin, 5, 17 Inversion oi sugar, 9 Invertase, 77, 103 ; sec also Sucrase Invert sugar, 9 Iodine, 234, 345, 350 Iodine number of fats, 23 Irish moss, 17 Iron, 234, 271, 383 assim.ilation of, 287, 288 function in nutrition, 285, 286 in dietaries, 303, 308, 382-383, 409 in eggs, 304 in food, 285, 303, 308, 421-431 in food materials, tables, 302, 421- 431 in grain products, 305, 306 in meat, 303 in milk, 300, 301, 304, 305 in modified milk, 304, 305 metabolism, 285-301, 306-309 nutritive relations of, 297, 298 per cent in body, 285 requirement, 297-300, 382-383 reserve supply at birth, 300 utilization of different forms, 287- 297, 306, 308-309 value of inorganic, 286, 296, 297 vegetables and fruits as sources of, 306, 307 Isomaltose, 5 Isomerization, 326 Jackson et al., experimental scurN^', 31S, 316, 329 Jam, 424 Janney, metabolic relationship of pro- teins to glucose, 137 Jelly, 424 Jones, nucleic acids, 131, 134, 137 Jones and Read, yeast nucleic acid, 137 Jordan, Hart, and Patten, metabolism and physiological effects of phos- phorus of wheat bran, 258 Jordan and Jenter, formation of milk fat from carbohydrate, 28, 40 Kafirin, 52 Kastle, alkali in ash of human and cow's milk, 283 Katzenstein, oxygen consumption dur- ing muscular work, 181, 182 Kaufi^mann, metabohsm experiment with gelatin and amino acids, 55 444 INDEX Kayser, protein-sparing by fat or car- bohydrates, 211, 212 Keller, slorage of phosphorus, 247 Kellogg and Taylor, the food problem, 401 Kendall, bacteria of digestive tract, 95 Kephalins, 243 Ketoses, 3, 4, 5 Ketoxylose, 4 Knoop, formation of amino acids from ammonium salts. 125 Knoop and Embdcn, /3-oxidati()n theon,', 116 Knoop and Kertcs, a-amino acids and a-ketonic acids in the liver, 137 Kohlrabi, 416, 424, 430 Koumiss, 416 Krcis and Hafncr, mixed glycerides, 26 Krogh, respiratory exchange of animals and man, 201 Kiihnc, introduction of word "enzyme," 74 Kulz, carbohydrate formation from protein, 124 Kunkcl and Egers, regeneration of blood with medicinal iron, 291 KjTins, 406 Lactalbumin, 49, 52, 56, 60, 65, 66, 68, 225, 226, 339. 340 Lactase, occurrence and action, 79 Lactic acid, 75, 105-109, 113, 124, 125, 126, 216 Lactose, 5, 10 Lamb, 416, 424, 430 Landergren, nitrogen metabolism, 215 Langworthy, food and diet in United States, 401 results of dietarj' studies, 364, 365, 401 Langworthy and Milner, respiration calorimeter, 169 Lard, 394, 416 Laurie acid, 22, 116 Lawes and Gilbert, formation of fat from carbohydrate, 28 Leathes, synthesis of butyric acid from lactic acid, 113 Lecithans, 243 Lecithins, 38-39, 243, 322, 323 in human and cow's milk, 248 Lecithoproteins, 53, 243, 405 Leeks, 424 Legumelin, 52, 60 Legumin, 49, 52, 56, 60, 142, 240 I^eipziger, phosphoproteins, 246 Lemons, 395, 416, 424, 430 juice, 416, 424, 430 Lentils, 395, 424, 430 Lettuce, 146, 395, 416, 424, 430 Leucine, 43, 47, 60, 61, 72, 78, 120, 126 Leucosin, 49, 52, 60, 240 Levene and Meyer, carbohydrate metab- olism, 137 Levin, intestinal bacteria, 95, 96 Levulans, 5, 17 I^evulose, see Fructose Liebig, high protein diet, 374, 375 Limes, 424 Linoleic acid, 24 Linolenic acid, 24 Linseed meal, 424, 428 Lipases, 73, 74, 76, 79, 103 Lipins, classification of, 36 Lipoids, 21, 34-41 Lipolytic enzymes, sec Lipases Litten, scurvy, 329 Little, beriberi caused by fine white flour, 329 Liver, 416 Lloyd's reagent, 325 Ijobster, 416 Lowy and Zuntz, influence of war diet on metabolism, 201 Lupeose, 5 Lupins, 424, 428 Lusk, calcium rich diet during preg- nancy, 265 chemical regulation of temperature, 192 energy requirements, 180, 187 food economics, 401 food values, 401 formation of carbohydrate from pro- tein, 124 hydrolysis of nucleotides, 132 i-nfluence of food on metabolism, 190, 201 nutrition, 117, 137, 169, 201, 232, 283, 329, 356, 401 protein metabolism, 211 and muscular activity, 229 regulation of metabolism, 178 INDEX 445 Lusk — Continued specific dynamic action, igo, 201 yield of carbohydrate from protein, 127 Lusli, Rich, and Sodcrstrom, respiration calorimeter, 169 Lyman, metabolism of fats, 137 Lymphatic radicles, 90 Lysine, 44, 47, 48, 60, 61, 62, 63, 72, 120, 216, 224, 226, 339, 341, 342 Lyxose, 4 Macallum, absorption of iron, 290, 309 MacLean and Williams, fat of tissues and organs, 40 phospholipins in liver fat, 39 jMcClendon, formation of fat from pro- tein, 40 McCollum, causes of failure of food to nourish, 347, 348 deficiencies of individual foods, 334, 335, 347, 352, 356 dietary relationships among foods, 329, 356 effect of acid-forming food, 281 fat-soluble A in plant tissue, 333 growth and development, 334, 346 growth promoting property of butter fat, 346 rfuclein synthesis, 258 nutritive values of milk and grain pro- tein, 66, 67, 226, 232, 356 repair processes in protein metabolism, 232 value of inorganic phosphates, 248, 249 McCollum and Davis, essential factors in diet during growth, 356 growth promoting influence of butter fat, 39, 332, 356 influence of certain vegetable fats on growth, 356 influence of mineral content of ration on growth, 356 influence of plane of protein intake on growth, 232, 356 nature of dietary deficiencies of rice, 356 nutrition with purified food sub- stances, 356 McCollum, Halpin, and Drescher, syn- thesis of lecithin, 249, 258 McCollum and Hoagland, endogenous nitrogen metabolism, 283 McCollum and Kennedy, dietary fac- tors in production of polyneuritis, 329, 347 McCollum and Pitz, vitamine hypothesis and deficiency diseases, 329 McCollum and Simmonds, biological analysis of pellagra-producing diets, 356 McCollum, Simmonds, and Pitz, dietary deficiencies of the maize kernel, 357 of oat kernel, 357 of wheat embryo, 356 of white bean, 357 distribution in plants of fat soluble A, 357 effects of feeding proteins of wheat kernel at difi'erent planes of in- take, 232, 357 effect upon growth of adding salt mixture to ration, 34s is lysine the limiting amino acid in proteins of wheat, maize, or oat kernel?, 357 relation of unidentified dietary fac- tors to growth-promoting prop- erties of milk, 357 vegetarian diet in light of present knowledge of nutrition, 357 McCrudden, nutrition and growth of bone, 357 McCrudden and Fales, mineral metabo- lism in intestinal infantilism, 258 McKay, protein element in nutrition, 232, 401 Maize, 350, 351 Maize glutelin, 61, 225 Macaroni, 394, 416, 424 Mackerel, 416, 424, 428 Magnesium, 234, 271, 272 Magnus-Le\-y, respiratory quotient and metabolism, iii, 154, 155 Maltase, 79 Malt amylase, 77 Malt sugar, see Maltose Maltose, 5, 10, 11, 86 Manganese, 234 Mango, 424 Mangolds, 424 446 INDEX Mannans, s, i6 Mannohcptose, s Mannose, s Manny, average weights and rates of growth of children, table, 372 Manometer, 81 Maple syrup, 424, 430 Marcuse, value of phosphoproteins, 246 Marmalade, 416 Marshall, comparative value of organic and inorganic phosphorus, 2^2, 258 Masslow, metabolism of organic phos- phorus, 250 phosphorus for growing organism, 259 Mastication, iqi Mathews, fats and lipoids in the body, 35 influence of mental activity on metab- olism, 177 lipins, 36, 40 physiological chemistry, 103, 137, i6g, 202, 283 Means, basal metabolism and body sur- face, 174, 202 Means, Aub, and DuBois, effect of caffeine on heat production, 202 Meat, 386, 387, 388, 389, 390, 391, 392, 394. 397, 398, 424. 430 Mechanical efficiency of man, 181-185, 200 Meeh's formula for computing body surface, 172 Meischer, formation of organic phos- phorus compounds, 246, 259 Melezitose, 5 MeHbiose, 5 Meltzer, advantage of high protein diet, 378, 379 calcium, importance of, in the body, 270 factors of safety in animal structure and economy, 401 Mendel, abnormalities of growth, 357 changes in food supply and relation to nutrition, 401 gain in body weight of children, 230 nutrition and growth, 67, 233, 357 viewpoints in study of growth, 357 sec also Osborne and Mendel Mendel and Daniels, behavior of stained fats in body, 40 Mendel and Judson, changes in water, fat, and ash content of body dur- ing growth, 338, 339 influence of dilTerent types of stunting upon body composition, 342 relations between diet, growth, and composition of the body, 357 Mendel and Osborne, growth, 357 ; see also Osborne and Mendel Metabolism, at various ages, 193-198 basal, 151-168, 170-179 behavior of foodstuffs in, 104-137, 138 conditions affecting, 170-233 definition of, xi during fasting, 188, 189, 200 effect of muscular work, 179, 180 energy requirement in, 148-202 fate of foodstuffs in, 104-137 in disease, 178 influence of age and growth, 193-198 food, 188-191, 201, 202, 207-217 muscular work, 179-188, 226-229 previously stored fat and glycogen, 204-207 size, etc., 170, 171, 174, 175 temperature, 191-193 thyroid, 178 internal activities, and secretions, 175, 178 mineral, 234-309 calcium, 260-268, 272, 282-284 iron, 285-309 phosphorus, 242-259 sulphur, 239-242 protein, 1 18-137, 203-233, 374-382 of adults, 170-202 of growing children, 196, 197 purine, 130-134, 136-137 Metaproteins, 53, 405 Methyl glyoxal, 105, 106, 107, 108, 109, 115, 126, 216 Methylpentoses, 4 Metschnikoff, intestinal bacteria, 96 Metschnikoff and Woolman, intestinal putrefaction, 103 Michaelis, hydrogen ion concentration, 283 Milk, 146, 241. 256, 269, 289, 302, 353, 386, 387, 388, 389, 390, 391. 392, 394. 397> 398, 416, 424, 430 INDEX 447 Milk sugar, see Lactose Millet, 42s Millon reaction, 71 MiUs, injection of fatty oils, 117 Mince meat, 417 Mineral elements, see Ash constituents, also Inorganic elements function of, 236 Mineral metabolism, 234-3og Mitchell, feeding isolated amino acids, 67 Molasses, 3S6, 387, 388, 389, 390, 391, 392, 417, 425, 430 Molecular weights of proteins. 50 Monaminodicarboxylic acids, 44 Monaminomonocarboxylic acids, 43 Monosaccharides, 2, 4, 5, 6, 8, 17-18, 79, 104 Monosaccharoses, 4, 6-8, 17-18, 79, 104 Moore and Bergin, reaction of intestinal contents, 92 Morgulis, influence of feeding on metab- oHsm, 202 Moro, intestinal bacteria, 96 Moulton and Trowbridge, composition of beef fat, 30, 40-41 Mucilages, 5 Mucins, 53 Mulder, on protein, 42 Mujik, storage of food fat in the body, 32 Murlin, energy requirement in preg- nancy, 199 nutritive value of gelatin, 233 respiration incubator for study of energy metabolism, 202 Murlin and Bailey, energy requirement of new-born, 195 protein metabolism in pregnancy, 233 Murlin and Greer, heart action and energj' requirement, 175 Murlin and Hoobler, metabolism of chil- dren, 202 Murlin and Lusk, influence of ingestion of fat, 202 Muscular work, 179-188, 226-229 Mushrooms, 417, 425 Muskmelons, 417, 425, 430 Mustard, 425 Mutases, 76 Mutton, 417, 425, 430 Myosin, 49, 52, 240, 247 Myristic acid, 22, 116 Nectarines, 417 ^ Nef, behavior of sugars, 7, 17 Nencki, formation of fatty acids, 114 Nelson, phosphorus content of starch, 13, 18 Nelson and Vosburgh, kinetics of in- vertase action, 103 Nelson and WilUams, calcium output, 264, 283 Neumann, dietary study, 150, 151 Neutrality, 77, 273-284 Nicotinic acid, 327 Nitrogen, balance experiments, 207, 208, 209, 210, 215 distribution of excreted, 135, 136 fate in protein metaboUsm, 128 in body, 234 metabolism, 130 see also Protein Northrup, phosphorus content of starch, 13, 18 Northrup and Nelson, phosphorus in starch, 244 Nothnagel, practical medicine, 309 Nucleic acids, 130-137 Nuclein, 131 Nucleoalbumins, see Phosphoproteins Nucleoproteins, 53, 130, 131, 243, 405 Nucleosides, 131, 132 Nucleotidases, 131, 132 Nucleotides, 132; see also Nucleic acid Nutritive ratio, 147, 148 Nutritive requirements, see Energj-, Food, and under the individual nutrients Nuts, 3S6, 387, 388, 389, 390, 391, 392, 395 Nuttall and Thierfelder, intestinal bac- teria, 95 Oatmeal, 146, 241, 256, 269, 302, 394, 417, 42s, 430 Octoic acid, 113, 114 (Edema, 324 Ohler, experimental polyneuritis, 329 Okra, 417, 425 Oleic acid, 23 Olein, 23, 30 Olives, 395, 417, 425, 430 Olive oil, 146, 394 Onions, 395, 417, 425, 430 448 INDEX Oppenhcimcr, enzymes, 103 Oranges, 146, 256, 269, 302, 395, 417, 42s. 430 Ornithine, 44, 129 Orj'z^nine. 324, 325 Osborne, chemical nature of diastase, 72, 73 ratio of nitrogen to sulphur, 240 plant proteins, 60, 61, 68 structure of proteins, 47, 49 sulphur in proteins, 283 Osborne and Mendel, acceleration of growth after retardation, 358 amino acids in nutrition and growth, 358 bacteria in feces, 103 cottonseed flour, 354 efBciency of individual proteins, 233 experiments with isolated food sub- stances, ss-68, 224, 357 experiments with restricted amounts of adequate proteins, 340 gliadin in nutrition, 358 growth upon diets of isolated food substances, 358 growth-promoting effect of protein- free milk, 332 influence of butter fat and other fats on growth, 39, 358 nutritive factors in animal tissues, 358 nutritive properties of proteins, 55- 68, 224-226, 233, 339-342, 358 problem of protein minimum, 358 relation of growth to chemical con- stituents of diet, 55-68, 224-226, 233, 339-346, 358 relative efficiency of proteins, 55-68, 225, 226, 358 resumption of growth after long con- tinued failure to grow, 358 soy bean as food, 358 stability of growth-promoting sub- stance of butter fat, 358 suppression of growth, 57, 63, 64, 224, 341 vitamines, role of, in diet, 329 zein in growth, 66, 224, 340 Osborne, Mendel, and Ferry, effect of retardation of growth upon breed- ing period and duration of life, 35S Osborne, Van Slyke el al., products of hydrolysis of proteins, 68 Ovalbumin, 225 Ovovitellin, 49, 53, 61, 225, 243, 246 Oxidases, 75 Oxygen, 234 consumption, 181 Oxyhemoglobin, 49, 50, 53 Oxyproline, 61 Oxypurine, 132 Oysters, 417, 425, 430 Palmitic acid, 22, 116 Pancreatic juice, 90, 91 Paprika, 425 Parsnips, 395, 417, 425, 430 Passage of different foods through the digestive tract, 86, 87, 89, 92, 93 Paton, formation of complex phosphorus compounds, 246 Pawlow, digestion, 80, 87, 103 Peaches, 146, 395, 417, 425, 430 Peanuts, 146, 256, 269, 302, 395, 418, 425, 430 Pearl, effect of feeding pituitary sub- stance and corpus luteum on egg production and growth, 359 Pears, 395, 418, 425, 430 Peas, 241, 302, 395, 418, 425, 430 Pea soup, 417 Pecans, 395, 425, 430 Pectins, 5, 17, 18 Pekelharing, pepsin, 71, 72 Pentosans, 5, 11 Pentoses, 4, 132 Peppers, 418, 425, 430 Pepsin, 71, 73, 77, 80 Peptids, 45, 54, 406 Peptones, 54, 59, 73, 406 Perch, 430 Peristalsis, 85, 90-94 Persimmons, 418, 425, 430 Petit, pepsin, 78 Pfliiger, fat formation, 127 glycogen, 137 Phaseolin, 52, 60 Phenol, 98 Phenylalanine, 43, 45. 47, 60, 61, 125, 126, 403 Phlorizin diabetes, 118, 124 INDEX 449 Phosphates, 243-259, 276-283 ; sec also Phosphorus Phosphatids, 37, 38-39, 243, 244, 246, 322 ; sec also Phospholipins Phospholipins, 36, 38-39, 243, 244, 246, 322 Phosphoproteins, 243, 244, 246, 405 Phosphoric add, 39, 131, 132, 243-24S, 276-283 Phosphorus, 234, 271, 272, 383 amounts in dietaries, 255, 256, 257, 391-396 amounts in food materials, 256 comparative value of organic and inorganic, 250, 252 compounds, classified, 243 effect of deficiency, 343, 344 excretion, 253 metabolism, 242, 244, 254, 255 requirement, 252, 253, 255, 383, 391- 396 Photosynthesis, i, 2 Phycetoleic acid, 23 Phytates, 244, 246 Phytic acid, 244 Phytin, 322, 323 Phytosterol, 37 Phytosynthesis, i, 2 Pies, 418 Pignolias, 418 Pineapple, 146, 395, 418, 425, 430 Pine nuts, 418 Pistachios, 418 Pitcairn, triturating action of stomach, 70 Playfair, dietary standard, 363 Plimmer, constitution of proteins, 68 metabolism of organic phosphorus compounds, 259 Plums, 146, 395, 418, 42s, 430 Polyneuritis. 318, 321, 327 Polypeptids, 46, 54, 403 Polysaccharides, 4, 5, 11-18 Polysaccharoses, 4, 5, 11-18 Pomegranates, 418, 426 Pork, 418, 426, 430 Portions, Standard or loo-Calorie, of foods, 144-146, 410-420 Potassium, 234, 237, 271, 272 Potatoes, 146, 241, 256, 269, 302, 395, 418, 426, 430 2 G Pottcvin, reversion of enzyme action, 79 Poullr>', 386, 387, 388, 389, 390. 391. 392 Prausnitz, composition of feces from different diets, 99 Primary protein derivative, 53, 405, 406 Prohne, 44, 47, 56, 60, 61, 126 Protamins, 53, 404 Proteans, 53, 405 Proteases, 74, 75 Proteid, 403 Protcin(s), 42-68, 403 absorption of, 119 acid-, 54 alcohol-soluble, 404 alkali-, 54 allowance, 376-380, 381 classification, 51-54, 403-406 coagulated, 54, 406 complete, 224, 225 composition of, 4S-50 conjugated, 53, 405 derivatives, 53, 54, 405-406 derived, 53, 405 energy value of, 142, 143 general properties, 42-51 hydrolysis of, 46, 47, 118 incomplete, 225, 340, 341 injection of, 1 20 in growth, 55-68, 339-34°. 355-358 in neutraUty, 278 metabolism, 1 18-137, 203-233, 339- 342, 373-382 in fasting, 203, 204 influence of body fat, 205, 206 molecular weights, 50 opinions upon Uberal diet, 374, 375, 376, 377 partially incomplete, 225 primarj' derivatives, 53, 405 properties, of individual, 54-67 physical, 51 putrefaction of, 97 requirement, 217-220, 339-340, 373- 382, 383 determining factors, 203 effect of muscular exercise, 227 influence of choice of food, 222, 223 relation to age and growth, 229, 230, 231 results of experiments, 220 versus protein standard, 220-222 450 INDEX Protcin(s) — Continued respirator>' quotient, no, in secondarj' derivatives, 406 simple, 52, 403, 404 sparing, 210-217 standard, 220-222, 273, 274, 373- 382, 383 for children, 381, 382-383 for families, 382-383 utilization in tissues, 122, 123 value of high intake, 375, 378, 379 Proteolytic enzyme, see Proteases Proteoses, 54, 59, 73, 406 Prunes, 146, 256, 269, 302, 395, 419, 426, 430 Psychic factors in digestion, 80-83, 88 Ptyalin, 73, 76, 79, 83, 86 Pumpkins, 419, 426, 430 Purines, 130, 131, 327 Putrefaction, 98 Putrefactive bacteria, 97, 98 Pylorus, 83, 84, 8s, 86 Pyridines, 326, 327 Pyrimidines, 131, 133, 323, 327 Pyruvic acid, 108, 109, 114, 125, 216 Pyruvic aldehyde, 124, 125; see also Methyl glyoxal Radishes, 395, 419, 426, 430 Raffinose, s Raisins, 146, 395, 419, 426, 430 Raper, normal octoic acid, 114 Raspberries, 419, 426, 430 Rate of passage of foods through the digestive tract, 86, 87, 89, 92, 93 Reaumur, gastric digestion, 70 Reductases, 75 Regulation of body temperature, 191- 193 Reichert, differentiation and specificity of starches, 12, 18 Relation of height and weight, 367-370, 372, 373 Rcnnin, 75, 78 Requirements, see Food Requirements; see also Metabolism ; also Stand- ard Resorption, 93 Respiration experiments, 151; see also Calorimeter work of, 168 Respiratory quotient, 109, no. ni, 152. 153. 154. 187 Rettger, influence of milk feeding on mortality and growth, 359 Ribose, 4, 132 Rice, 146, 256, 269, 302, 350, 394, 419, 426, 430 protein, products of hydrolysis, ref.. 68 Richardson (A. E.), and Green, cotton- seed flour, 353, 359 Richardson (W. D.), chemical character- istics of lard, 41 Riche, adiabatic bomb calorimeter, 140 Rhamnose, 4 Rhubarb, 419, 426, 430 Robertson, chemical mechanism main- taining neutrality, 283 growth, and growth-controlling sub- stances of pituitary body, 359 Roentgen rays, 84, 86, 92, 93 Rohmann, phosphoproteins versus in- organic phosphates, 247 Romaine (salad), 426 Rona, absorption of amino acids, 1 20 Rose, creatinuria, 137 Rose and Cooper, potato nitrogen, 224, 233 Rubner, fenergy metabolism, 169, 170 fuel values of food constituents, 143 influence of food on metabolism, 189, 190 relation of body surface to metabo- lism, 171 specific dynamic action of foodstuffs, 189-190 Rubner and Heubner, storage of food for growth, 194 Rutabagas, 426, 430 Rye, 426, 430 Saccharose, see Sucrose Salivary digestion, 80, 82-84 Salmon, 146, 419, 426, 430 Salt, craving for, 237-239 effect upon metabolism, 239 Saponification, 19 Sapota, 426 Sausage, 419 Scallop, 61 Schaumann, beriberi, 322, 329 Schlossmann, phosphorus in milk, 259 INDEX 451 Schondorfl, distribution of glycogen in the body, 15 Schmidt, medicinal iron in hemoglobin formation, 296 Schmidt and Strassburger, composition of feces; ref. 103 Schottelius, bacterial action in digestion, 95. 96 Schryver and Haynes, pectins, 18 Schulze and Reineke, composition of fat of different mammals, 30 Schwann, pepsin, 71 Score value, 392-395 Scurvy, 310-318, 327-329 Secalose, 5 Secondary protein derivatives, 54 Secretin, 91, 92 Sedoheptose, s Seeds, deficiency as sole food, 352, 353 Seegen, formation of carbohydrate from protein, 123 Seidell, antineuritic vitamine, 325, 329 Serine, 43, 45. 47, 60, 61, 126 Serum globulin, 49, 52, 240 Sex, relation to food requirement, 199, 264-265, 300, 371, 372 Shad, 419 Shaffer, nitrogen output during rest and work, 229 Sherman, iron in food and nutrition, 298-309 Sherman and Baker, starch, 13, 18 Sherman and Gettler, balance of acid- forming and base-forming ele- ments, 279-280, 283 chemical nature of enzyme prepara- tions, 103 Sherman and Gillett, adequacy and economy of city dietaries, 401 Sherman, Mcttler and Sinclair, calcium, magnesium, and phosphorus in food and nutrition, 259 Sherman and Schlesingcr, pancreatic amylase preparation, 78, 103 Shredded wheat, 419, 426, 430 Shrimp, 426 Silicon, 234 Sitosterol, 37 Siven, protein requirement, 233 Size, relation to metabolism, 170-175; see also Age; Children Sjostrom, influence of temperature on carbon dioxide output, 202 Skatol, 98 Skraup and Behler, structure of gela- tin, 47 Smedley, formation of fat from carbo- hydrate, 41, 114, 115 Snell, bomb calorimeter, 139 Socin, experiments with organic and inorganic iron, 288, 309 Soderstrom, Meyer, and DuBois, com- parison of metabolism of men flat in bed and sitting in steamer chair, 202 Sodium, 234, 271, 272 Soluble starch, 14 Sonden and Tigerstedt, energy metabo- lism, 157 Sorbose, 5 Sorensen, hydrogen ion concentration, ■ 77 Soup, 426 Spallanzani, gastric juice, 70, 71 Specific dynamic action of foodstuffs, 189-191, 201, 202 Spinach, 146, 302, 395, 419, 426, 430 Squash, 395, 419, 426, 430 Stachyase, 5 Standards, dietary, 361-367, 382, 383, 385 for calcium, 267, 382, 383 for energy, 183, 186, 187, 196-197, 360-373 for iron, 299-300, 382, 383 for phosphorus, 255, 382, 383 for protein, 220-222, 229-233, 373- 383. 385 Starch, 5, 12-14, i7. 18, 73, 83, 142 Starch sugar, see Glucose Starling, hormones, 88, 89 physiology of digestion, 103 secretion of bile, 91 Steapsin, 90 Stearic acid, 22, 116, 216 Stearin, 216 Steenbock and Hart, calcium require- ment of animals, 265, 284 Steenbock, Nelson, and Hart, acidosis, 284 Steinitz, phosphoproteins, 246 Stepp, lipoids, 39-40 452 INDEX Sterols, 36, 37-38 Stevens, experiments with gastric juice, 70 Stockman, absorption of inorganic iron, 289, 290 iron requirement, 298 Stoeltzner, significance of calcium in growth of bone, 284 Stoklasa, iron-protein compound of onion, 306 Stomach, 82-89 Strawberries, 419, 426, 430 Substrate, 76 Sued, metaboUsm during fasting, 206 Succotash, 419 Succus entericus, 89 Sucrase, 77, 79, 92, 103 Sucrose, s, 8-10, 142 Sugar, 2, 146, 386, 387, 388. 389. 390. 391, 392, 394, 419; see also Su- crose double, 4 references, 17, 18 simple, 2 Sulpholipins, 36 Sulphur, 234-242, 271, 272 elimination, 242 metabolism, 239, 240, 241 proportion in protein, 49, 240, 241 Suzuki, Shamimura, and Odakc, or>'za- nine, 329 Swartz, utiUzation of cellulose, 16, 18 galactans, 17, 18 mannans, 16, 18 pentosans, 11, 18 Sylvius, fermentation and digestion, 69 Symonds, tables of heights and weights, 368 Syntonin, 54 Tagatose, 5 Takaki, beriberi, 319 Talbot, energy requirement of infants, 202 Tallquist, protein-protecting powers of fat and carbohydrate, 212-214 Talose, 5 Tamarind, 426 Tangl, metaboUsm of an artificially fed child, 284 Tapioca, 426, 430 Tartakowsky, assimilation of inorganic iron, 295, 309 Tashiro, carbon dioxide production in nerve, 177, 202 Taylor, diet of prisoners of war in Germany, 401 digestion and metabolism, 103 fats and lipoids in body, 35 Temperature, sec Regulation Terminolog>' of hydrolj'tic enzymes, 76 Tetrahexoses, 5 Tetranucleotides, 131 Tetrasaccharides, 5 Tetrasaccharoses, ', Tetroses, 4 Thioamino acid, see Cystine Thomas (A. W.), constitution of starch, 13. 18 phosphorus content of starch, 13, 18 Thomas (K.), nutritive efficiency of proteins, 223 Threose, 4 Thrombin or thrombase, 75 Thymine, 131, 132, 133 Thymonucleic acid, 53 Thymus, 132 Thyroid, 178 Tigerstedt, ash content of ordinary dietary, 284 estimates of food requirements, 186, 187 metaboUsm at various ages, 194 metaboUsm during fasting, i8g Tomatoes, 146, 395, 419, 426, 430 Toruline, 324, 325 Transportation, effect upon prices, 398, 390 Trehalose, 5 Triglycerides, simple and mixed, 24-27 Trigonelline, 327 Trihexoscs, 5 Triolein, 79 Trioses, 4 Trioxypurine, 132 Tripeptids, 46 Trisaccharides, 5 Trisaccharoses, 5 Triticonucleic acid, 53 Truffles, 426 Trypsin, 77, 80, 90, 92 Trypsinogen, 92 INDEX 453 Tryptophane, 44, 45, 47, 48, 60, 61, 62, 63, 71- 120, 127, 224, 339, 341 Tuberin, 52 Tubular glands, 91 Tuna, 419 Turanose, 5 Turkey, 419 Turnips, 146, 256, 269, 302, 395, 420, 426, 430 Tyrosine, 43, 45, 47, 60, 61, 125 Underhill, metabolism of ammonium salts, 137, 284 Uracil, 131, 132, 133 Urea, 129, 142 Uric acid, 130-134, 281 Uricolysis, 134 Uridine, 132 Uridine nucleotide, 132 Urine, acidity, 281 Valine, 43, 47, 48, 60, 61, 120, 126 Van Slyke, amino acids in intermediary metabolism, 1 1 9-1 2 2 amino acids in physiology and pathol- ogy, 137 Van Slyke et al., fate of protein digestion products, 1 19-122, 137 Van Slyke, Cullen, Stillman, and Fitz, acid excretion and alkaline re- serve, 284 Van Slyke and Meyer, absorption and distribution of amino acids, 119- 120 Von Hehnont, digestion, 69 Von Hosslin, relation of size to heat production, 171 Von Noorden, metabolism dependent upon build of body, 174 metaboHsmand medicine, 169, 202, 233 need for high protein intake, 375, 376 nitrogen equiUbrium, 207-210 use of vegetables in feeding children, 308 Von Wendt, dicalcium phosphate, 247, 309 iron requirements, 298, 309 Veal, 420, 427, 430 Vedder, beriberi, 330 Vegetables, 386, 387, 388, 389, 390, 391, 392, 394. 397, 398 Vegetable soup, 420 Venous radicles, 90 Vernon, intracellular enzymes, 103 Vicilin, 60 Vignin, 52, 60 Vinegar, 427, 430 Vitamines, xii, 323, 324, 325, 345 Voegtlin, vitamines, 330 V'oegtUn and White, can adenine acquire antineuritic properties, 330 Voit, calcium in animal nutrition, 284 dietary standard, 362 effects of insufficient calcium, 262 fat production from protein, 127 food requirement, 180, 362 iron metabolism in dogs, 288, 289 nitrogen elimination in fasting, 204 phosphorus metabolism during fast- ing, 253 Walnuts, 256, 269, 302, 395, 420, 427, 430 Water cress, 427, 430 Watermelon, 420, 427, 430 Waters, capacity of animals to grow under adverse conditions, 359 experiments with energy-deficient diets, 336, 337 influence of nutrition on animal form, 359 Water soluble B, xiii, S33, 345. 347. 383, 384, see. also Growth Waxes, 36 Weight, relation to height and age, 197, 368, 369, 371, 372 Wells, nucleoproteins, 131 Wheat, 146, 241, 256, 269, 302, 420, 427, 430 embryo, 349, 350 kernel, 349 Whey, 427, 430 White bean, 351, 352 Whitefish, 420 W'hortleberries, 427, 430 Willcock and Hopkins, feeding experi- ments with zein and amino acids, 55 Williams, chemical nature of vitamines, 330 relation of chemical structure to anti- neuritic action, 326, 327 454 INDEX Williams and Salecby, treatment of human beriljeri, 330 vitamine preparation, 325 Williams and Seidell, vitamine of yeast, 327 Wilson, nitrogen metabolism during pregnancy, 233 Wine, 427, 430 Withers and Carruth, gossypol, 353 Wolffberg, formation of carbohydrate from protein, 123 Woltering, experiments with inorganic iron, 290, 309 Woodyatt, carbohydrate metabolism, 137 Work, influence on metabolism, 179- 188, 226-229 Wright, scurvy, 313 Xanthine, 132, 133 Xanthoprotein test, 71 Xylans, 5 Xyloketose, 4 Xylose, 4 Yeast, 75, 132 Yoshikawa, Yana, and Menals, beri, 330 beri- Zadik, phosphoproteins, 246 Zein, 47, 48, 49, 50, 52, 55, 57, 61, 63, 65, 66, 224, 225, 240, 339 Zuntz, metabolism experiment with ergometer, 185 respiration mask, 151 work, and consumption of oxvgen, 181 Zuntz and Morgulis, influence of under- nutrition on metabolism, 202 Zuntz and Schumberg, energy values, 153 Zwieback, 420 Zymase of yeast, 75 Zymogen, 76 Priated in the United States of America 'T^HE following pages contain advertisements of books by the same author or on kindred subjects Food Products By henry C SHERMAN Professor of Food Chemistry in Columbia University $^.40 A comprehensive, descriptive text-book on the general subject of foods. The first and second chapters deal with the principal constituents and functions of food and with food legislation; then follow chapters on: Milk; Cheese and Other Milk Products ; Eggs, Meats and Meat Prod- ucts; Poultry, Fish and Shellfish; Grain Products; Vege- tables, Fruits and Nuts; Edible Fats and Oils; Sugar, Molasses, Sirups and Confectionery; and Food Adjuncts. The plan of the book makes it easy for the teacher to fol- low a different order of topics if desired. " Admirable." — Science. "A really excellent text-book." — Educational Review. "A valuable contribution to the hterature of foods as well as a scientific presentation of many new facts regard- ing food values." — Insurance. "A scientific study of great value." — New York Times. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York Methods of Organic Analysis By henry C. SHERMAN, Ph.D. Professor of Food Chemistry in Columbia University. Author of "Chemistry of Food and Nutrition." Illustrated, cloth, 8vo, $3.4.0 Some Reviews That a substantial and somewhat diversified course of laboratory practice in the methods of organic analysis should form part of the training of every professional chemist is now a generally recognized fact. The successful conduct of a large and increasing proportion of our chemical industries, as well as the protection of the consumer, de- mands all the aid which a skillful application of these methods is able to bring. Professor Sherman's book contains the well-ordered mate- rial for such a course of instruction. — Technology Quarterly. A feature of the book that commends itself, is the general presenta- tion of a subject in one chapter, that on carbohydrate for example, followed by a chapter upon the special methods of analysis. When it is impracticable to give all the methods for the analysis of the various compounds considered, references are made to standard works upon the subject; these are often supplemented by copious foot-notes making the book encyclopedic in scope. — Journal of the American Chemical Society. Commendable features are: the free use of references in the form of both foot-notes and bibliographical compilations; the carefully worked out procedures; the clear and pertinent notes and discussions. The isolated student or casual worker in methods of organic analysis will find the book of especial value in pointing out original and often scattered sources of information. — Science. While this book is not primarily intended for medical students or physicians, it presents a concise yet complete treatment of the subject of organic analysis including the analysis of food products such as milk, cereals, butter, etc. It will be especially useful to students in advanced courses or in post-graduate work who are fitting themselves for the position of food experts or as agricultural chemists. — Medical Record. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York A Laboratory Manual of Foods and Cookery By EMMA B. MATTESON Instructor in Home Economics, George I'cabody College for Teachers AND ETHEL M. NEWLANDS Director of Home Economics, The Buffalo Technical High School Cloth, 12)110, xi -\- J2J pages, $i.jO This volume has been designed to meet the need which has been felt for a book approaching the study of cookery through experimental work upon the chemical, physical, bacteriological, and biological properties of food. The sub- ject matter is arranged topically according to the usual classification of food materials, each topic being dev^eloped by means of a series of experiments which will give a first- hand acquaintance with the leading characteristics of the different types of food and give such a grasp of the prin- ciples involved as may enable the student to work without recipes or develop original recipes. The Laboratory Manual was worked out through the correlated experience of the authors at Pratt Institute, Brooklyn, and Simmons College, Boston, and is amplified by their individual experiences in The School of Education of The University of Chicago, and The Diet School of Johns Hopkins Hospital. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York Feeding the Family By MARY SWARTZ ROSE Illustrated, $2.10 This is a clear and concise account in simple every-day terms of the ways in which modern knowledge of the science of nutrition may be appHed in ordinary life. The food needs of the different members of the typical family group — men, women, infants, children of various {Iges — are discussed in separate chapters, and many concrete illustrations in the form of food plans and dietaries are included. The problems of the housewife in trying to reconcile the needs of different ages and tastes at the same table are also taken up, as are the cost of food and the construction of menus. A final chapter deals with feeding the sick. ''The volume is so simply and entertainingly written that it cannot but be enjoyed by anyone interested in the planning or preparation of household meals, and it would be difficult to imagine a more helpful book to put into the hands of a reader desiring information along such lines." — Trained Nurse. THE MACMLLIAN COMPANY Publishers 64-66 Fifth Avenue New York A Laboratory Hand-book for Dietetics By MARY SWaRTZ ROSE, Ph.D. Assistant Professor, Department of Nutrition, Teachers College, Columbia University Cloth, Svo, $1.10 Investigations into the quantitative requirements of the human body have progressed so far as to make dietetics to a certain extent an exact science, and to emphasize the importance of a quantitative study of food materials. This little book explains the problems involved in the calculation of food values and food requirements, and the construction of dietaries, and furnishes reference tables which will minimize the labor involved in such work without limiting dietary study to a few food materials Only brief statements of the conditions affecting food requirements have been made, the reader being referred to general textbooks on the subject of nutrition for fuller information, but such data have been included as seem most useful in determining the amount of food foi any normal individual under varying conditions of age and activity. TABLE OF CONTENTS Part I Pood Values and Food Requirements. The Composition of Food Materiai-s. The Functions of Food. Food as a Source of Energy. Food as Building Material. Food in the Regulation of Body Processes. Food Requirement. The Energy Requirement of Normal Adults. The Energy Requirement of Children. The Energy Requirement of the Aged. The Protein Requirement. The Fat and Carbohydrate Requirement. The Ash Requirement. Part II Problems in Dietary Calculations. Studies in Weight, Measure, and Cost of Some Common Food Materials. Relation between Percentage Composition and Weight. Calulation of the Fuel Value of a Single Food Material. Calculation of the Weight of a Standard or loo-Calorie Portion. Food Value of a Combination of Food Materials. Distribution of Foodstuffs in a Standard Portion of a Sinu;le Food Material. Calculation of a Standard Portion of a Combination of Food Materials. Analysis of a Recipe. Modification of Cow's Milk to a Required Formula. Calculation of the Percentage Composition of a Food Mixture. The Calculation of a Complete Dietary. Scoring of the Dietary. Reference Tables. Refuse in Food Materials. Conversion Tables — Grams to Ounces. Conversion Tables — Ounces to Grams. Conversion Tables — Pounds to Grams. Food Values in Terms of Standard Units of Weight. Ash Constituents in Percentages of the Edible Portion, Ash Constituents in Standard or loo-Calorie Portions. Appendix The Equipment of a Dietetics Laboratory. THE MACMILLAN COMPANY Publishers 64-66 Fifth Avenue New York