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SCIENCE. 



[]Sr. S. Vol. XXII. No. 553. 



in nutrition which has been a much debated 

 problem for fifty years. Indeed, interest 

 in this goes back to the days of the epoch- 

 making publications of Liebig on the rela- 

 tion of organic chemistry to physiology and 

 pathology, issued in the early forties. In 

 these he developed his idea of the functions 

 of various foods in the nutrition of man 

 and laid particular stress on the impor- 

 tance 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 produc- 

 tion 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 ob- 

 jections urged, especially by J. E.. Mayer, 

 of Heilbronn. Other important investiga- 

 tions 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 carbo- 

 hydrates are the usual sources of such en- 

 ergy in man, and that heat production is 

 only incidental, not an end, but an unavoid- 

 able 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 con- 

 sumption 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 in- 

 quire 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 metab- 

 olize 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. For ex- 

 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 some- 

 what earlier had made numerous observa- 

 tions 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. 



The importance of the subject is worthy 

 of the fullest investigation, and such a 



