NUTRITION OF MAN 137 



[On the basis of some very thorough studies of the composition of normal 

 human urines following different diets, Folin has worked out a theory of proteid 

 metabolism which has received such extensive notice as to deserve mention in 

 this connection. Folin finds that in order to explain the changes in the com- 

 position of the urine with reference to nitrogen and sulphur, it is necessary to 

 assume that the proteid metabolism is of two kinds. " One kind is extremely 

 variable in quantity, the other tends to remain constant. The one yields chiefly 

 urea and inorganic sulphates, no creatinin and probably no neutral sulphur. The 

 other, the constant metabolism, is largely represented by creatinin and neutral 

 sulphur and to a less extent by uric acid and ethereal sulphates." 



The variable metabolism is conceived as consisting of a series of hydrolytic 

 splittings of food proteid (cf. Chap. VII), begun in the intestinal wall and com- 

 pleted in the liver, which result in the elimination of the proteid nitrogen as 

 urea. This is called exogenous metabolism. The constant metabolism repre- 

 sented by creatinin (and uric acid) is regarded as a true index of that destruc- 

 tion of living substance necessary to the continuation of life, and is therefore 

 called endogenous metabolism. In Foliii's view, only that amount of proteid 

 necessary for the endogenous metabolism is really needed by the body. The 

 greater part of the proteid in ordinary diets, i. e., that amount representing the 

 exogenous metabolism, is not needed, or at least its nitrogen is not needed. 



This theory agrees with Voit's theory as stated above in regarding the liv- 

 ing substance as relatively stable, but differs from it in regarding the more ready 

 dissolution of ingested food-proteid not as a matter of preference on the part 

 of the cells, but as a specially developed means for removing the unnecessary 

 nitrogen of the proteid ingested. The carbonaceous part of the proteid mole- 

 cule, which alone is conceived as undergoing true oxidations similar to those 

 which fats and carbohydrates undergo, is thereby rendered available. 



The theory seems to explain the facts of proteid metabolism as stated by 

 the author in this chapter quite as well as Voit's theory, and in addition seems 

 to place a new physiological significance on the portal circulation. ED.] 



SECOND SECTION 



NUTRITION OF MAN 



If the diet contains in sufficient quantities and in the proper proportion 

 all those substances which the body needs, it constitutes what Voit calls a 

 " food/' As applied to a healthy adult man, this ration is that quantity of 

 foodstuffs which is necessary to keep the body in an equilibrium of substance. 

 For growing children as well as for adults in a poor state of nutrition, the 

 ration must be more plentiful so that a part of it can be retained in the body. 



In this section we have to study the nutritive requirements of man and 

 some of the circumstances affecting them. Naturally we cannot go into de- 

 tails here; important as they are, they belong to hygiene and dietetics, rather 

 than to the physiology of nutrition. 



The nutritive requirements of a man are represented by those quantities 

 of the different foodstuffs which must be added to the body from the intestine 

 every day. But inasmuch as we do not commonly eat pure foodstuffs, but 

 meals prepared from various articles of food, the question may be raised. 



