in nutrition which has been a much debated

In

problem for fifty years. Indeed, interest in this goes back to the days of the epochmaking publications of Liebig on the relation of organic chemistry to physiology and pathology, issued in the early forties. these he developed his idea of the functions of various foods in the nutrition of man and laid particular stress on the importance of protein as the source of muscular energy. According to this early Liebig view our foods may be divided into plastic or tissue-forming, on the one hand, and heat-producing, on the other. The production of heat appeared as an end in itself and the fats and carbohydrates served for this purpose. The protein substances are built up into tissues and in the oxidation of the latter, it was held, we have the sole source of muscular energy. The name of Liebig was all-potent in science in those days and his nutrition theory held sway for twenty years or more without question. It will not be necessary to recount the steps in the opposition which finally developed, but it may be well to recall the famous experiment of Fick and Wislicenus in which, in an ascent of the Faulhorn, in 1866, they calculated the work done and the protein oxidized, as measured by the urea excretion. The protein combustion was found to be far too little to account for the expenditure of work in the climb, which result confirmed the theoretical objections urged, especially by J. R. Mayer, of Heilbronn. Other important investigations followed in the same direction, and almost without exception they have gone to show that while the protein oxidation. may furnish a part of the muscular energy of the body, or even all of it under certain extreme circumstances, the fats and carbohydrates are the usual sources of such energy in man, and that heat production is only incidental, not an end, but an unavoidable accompaniment. A few recent experi

ments which have seemed to support the Liebig contention have been made largely with carnivorous animals and have no real bearing on the problem as far as man is. concerned.

As a necessary consequence of the Liebig theory it was held that our protein consumption must be high, and hence the large amounts of nitrogenous substances insisted upon in the older dietaries. But after a time physiologists naturally began to inquire into the real uses of protein, if it is not called for in the work of the muscles; if, as appeared evident, it is used mainly in the repair of waste tissues, why metabolize so much, since in this metabolism an enormous amount of extra work is thrown on the oxidizing and excreting organs of the body. It certainly can not be assumed that the disposal of the katabolic products of proteins can be accomplished without using up a considerable amount of energy, and without a great strain on the liver and kidneys. What, then, is the amount of protein actually needed for the normal body? Numerous answers have been given to this question and in late years several investigations have apparently brought the daily protein down to 25 to 40 grams, or even lower. But it has been urged against all the experiments leading to such results that they were of too short duration to actually prove anything of value. ample, Siven carried out a 32-day test in which the protein metabolized daily was about 38 grams; during a part of this time the body was kept in nitrogen equilibrium. by about 25 grams daily. Hirschfeld somewhat earlier had made numerous observations in which the protein consumption through about two weeks was 35 to 45 grams, but fats and carbohydrates brought the diet up to an equivalent of 3,750 to 3,900 calories.

For ex

The importance of the subject is worthy of the fullest investigation, and such a

study has finally been carried out by Chittenden through experiments, first on himself, and then on groups of men engaged in various occupations. In the first of these remarkable experiments, which have recently been described in book form under the title 'Physiological Economy in Nutrition' the distinguished Yale scientist determined in his own case how far he could safely reduce the protein of his diet and still retain the body in nitrogen equilibrium. To do this close watch was held on the food and excreta through a year, November, 1902, to October, 1903, and under varying external conditions of work and temperature. As a result of these systematic tests Chittenden found that he could live very comfortably, and in perfect health, on a diet containing 35 to 40 grams of protein daily, with fats and carbohydrates sufficient to yield 1,500 to 1,600 calories. These valuable personal experiments were regarded as preliminary only. Later, systematic observations were made with three groups.of men, the work being carried through periods of five to nine months for each group.

The first group comprised colleagues of the author of the experiments, Yale professors and instructors. The average protein metabolism here was about 46 grams. The second group was composed of soldiers from the hospital corps of the United States army who were detailed for the purpose of the study. Of the twenty who began, thirteen followed the tests through the whole period of over six months. Those who deserted, or were dropped, had much to say through the newspapers about starvation diet, but this was a curious misnomer, since, as the records show, the men who remained were kept in perfect nitrogen equilibrium and found themselves in far better physical condition at the end of the experiments than at the beginning. Through all this time they had plenty of

work to perform, with constant and rather severe requirements on the muscular system. The average protein consumption daily was not far from 55 grams.

Finally, eight Yale athletes showed themselves willing to work through the training and competing season on the restricted protein diet. The results here were equally remarkable, in fact probably the most remarkable, as the work done by these men was of a character to call for very high protein diet according to all of our old standards. The experiments were carried out through a period of five months, February to June, 1904, and through the last two months a very close record was kept of diet, excretion, weight and various other factors concerning the men. Through this sixty-day period, when the muscular exertion was, perhaps, the most taxing, protein equilibrium was maintained on an average of 8.81 grams of nitrogen metabolized for each man daily, corresponding to about 55 grams of protein. All these men took high rank in athletic work, several of them being prize winners. The reproductions of photographs, published in the book, show them to be men of excellent physique, and even of remarkable muscular development in some cases. While the protein diet of these men was low the fat and carbohydrates were generous but not excessive, the calorific value of the whole being seldom over 3,000 calories.

For all these men under examination in these three sets of tests, professional men, soldiers, athletes, complete statistics for each day are published, from which the reader may derive the fullest possible information. Painstaking accuracy is evident in every page, and from the standpoint of logical requirement in experimental proof the tables meet any reasonable objection.

This Chittenden investigation then must

be regarded as of fundamental importance, as it demonstrates beyond cavil just what is possible in protein restriction under ordinary conditions. The periods of investigation chosen were long enough to answer objections to the results of some of the earlier tests, and the values obtained for the soldiers and athletes of about 55 grams of protein metabolized daily will have to be taken as practical standards. It doubtless remains true that for men at severe work at low temperatures a large number of calories are required in the food. instructive example of such dietaries is given in the recent publication by C. D. Woods on the diet of Maine lumbermen,

An

where it is shown that the heat value of the food consumed daily by men in the lumber camps may amount to 6,000 or 8,000 calories. It would be interesting to experiment in such cases on the replacement of a good share of the protein by fat and carbohydrates.

A study of the Chittenden series of experiments on men shows very clearly that as far as the human organism, at any rate, is concerned the old Liebig notion of the source of muscular energy is without foundation. As suggested above, experiments with carnivorous animals do not apply to man; it would be as justifiable to discuss the food value of pentoses for man from experiments on the feeding of straw to cattle. It is true that for short periods, or under special conditions, proteins may serve man as the main or only source of muscular energy, but evidently this is not usually or normally the case.

When the far-reaching importance of the whole question is realized, and when it is further remembered that considerable internal work must be done to remove, especially, the products of protein metabolism, I believe it will be granted that I am right in placing this work of Chittenden

among the most important recent achievements in physiological chemistry.

The next topic of which I wish to speak very briefly deals with a problem even older than that of the Liebig theory of the source of muscular energy. Some years before the organic chemistry of Liebig was published Mulder had introduced the term protein, and had even announced the essential composition of what he considered the protein nucleus. His positive statements led to extended investigations on the part of others, and the work of many chemists soon disclosed the fact that no one simple nucleus may be assumed to exist in these molecules and that they must be enormous

ly complex. Ever since the early forties the problem has been an extremely interesting one, but it is only recently that it has been seriously attacked from the second side possible in such investigations. Up to a period within five years the work done on the protein question has been largely in the way of analysis or disintegration, but now we have the beginning of attempts at synthesis or reconstruction of large groups. Glycine and leucine had been known since about 1820 as decomposition products of glue and other bodies by action of acid. Nearly thirty years later tyrosine was added as obtained in about the same way, and soon a few other individual substances were listed among the products which could be secured in various decompositions of proteins. In the seventies systematic methods of hydrolysis by alkalies and acids were worked out, especially by Schützenberger and Hlasiwetz and Habermann. Numerous products were recognized, but at first these attracted no great attention, as there remained always the possibility that the amino acids and other compounds found might be results of secondary reactions.

We can not infer much regarding the structure of soft coal from the presence of methane in the gas, or of benzene, tolu

ene and naphthalene in the tar, and an analogous proposition may be true for the proteins. Later, however, this important generalization was reached; it matters little how the decomposition is effected, the products of protein destruction are essentially the same as long as brought about in the presence of water, and all seem to be in the nature of hydrolytic cleavages. The action of boiling acid or alkali, steam under pressure, pepsin and hydrochloric acid, trypsin and weak alkalies, all lead to nearly the same resultant products, and among these certain a-amino acids are always the most abundant. The conclusion follows, therefore, almost of necessity, that these are the true nucleus groups and the question naturally suggests itself, is it possible to put these things together and build up anything like a true protein. An answer to the question has been slow in coming, but a beginning has been made, and especially through the experiments of Curtius and Fischer. The condensation method followed by the former is a general one, through which a large number of amino groups have already been combined. depends first on the production of hydrazides, then azides, which are very reactive, and take on additional amino groups with loss of hydronitric acid. For example, the ethyl ester of hippuric acid condenses with hydrazine hydrate to form the hydrazide: C.H.CO.NHCH,CO.NH.NH;

this with nitrous acid gives the azide CH.CO.NHCH,CO.N.

It

By treatment with glycine under certain conditions this reaction follows:

C.H.CO.NHCH,CO.N + NH,CH,COOH=

NgH+CH.CO.NHCHCO.NHCHCOOH.

In other words, we have started with benzoylglycine and have obtained benzoylglycylglycine. This in turn may be used as a new starting point. A silver salt is made,

this turned into ester with ethyl bromide, and then a new hydrazide and azide to be combined with glycine, as before. These steps will lead to benzoyl diglycylglycine, and by using alanine, leucine or other amino acid it will be seen that by repeat-. ing the processes extremely complex groups may be finally built up. Curtius has carried the reaction to the formation of benzoyl hexaglycylglycine,

CH.CO.(NHCH,CO)NHCH,COOH.

Hippuryl and three alanine groups have also been condensed to form benzoyl glycyldialanylalanine.

Fischer has worked from a different standpoint. The study of various hydrolytic products from proteins, already referred to, and the isolation of certain other groups by Fischer himself, led to the belief that the complex molecules in the proteins must be built up by the union of amino acid groups. Various attempts had been made to condense some of the simple amino acids in anhydride form, but without much. success, until the first experiments were made by Fischer in 1901. The starting point of the series of condensations was found in the product obtained from the glycine anhydride described in 1888 by Curtius and Goebel. This may be looked upon as formed by the union of two molecules of glycine with loss of two molecules of water, and when digested with strong hydrochloric acid suffers a peculiar decomposition and yields a body to which Fischer has given the name glycylglycine.