CHAP, v.] NUTRITION. 4/53 



Moreover it is possible that the rapid proteid metabolism indicated 

 by the great increase of urea which follows upon a meal rich in 

 proteids, may be, as we have hinted, merely a destructive digestion 

 of the proteids while they still are retained in the alimentary 

 canal. 



We may however call attention to a possible analogy between 

 the history of proteids and that of fats and carbohydrates. The 

 uniform composition of the blood, which the body seems ever 

 striving to maintain, probably applies to its proteids as well as 

 to its other constituents. We have seen that* a surplus of non- 

 nitrogenous materials in the blood is withdrawn from the circula- 

 tion and stored up as fat or glycogen, and it is possible that an 

 excess of proteids might similarly be stored up in some tissue or 

 tissues, though from the facts previously mentioned it is obvious 

 that the power of storage is far less than in the case of fats 

 and carbohydrates. Such a store of proteid matter would re- 

 present a sort of circulating proteid, but nevertheless for its final 

 metabolism might have to form an integral part of some living 

 tissue unit. 



The Effects of Fatty and of Carbohydrate Pood. Unlike 

 those of proteid food, the effects of fats and carbohydrates cannot 

 be studied alone. When an animal is fed simply on non-nitro- 

 genous food, death soon takes place ; the food rapidly ceases to be 

 digested, and starvation ensues. We can therefore only study the 

 dietetic effects of these substances when they are taken together 

 with proteid material. 



When a small quantity of fat is taken, in company with a fixed 

 moderate quantity of proteid material, the whole of the carbon of 

 tha food reappears in the egesta. No fat is stored up.; some even 

 of the previously existing fat of the body may be consumed. As 

 the fat of the meal is increased, a point is soon reached at which 

 carbon is retained in the body as fat. So also with starch or sugar; 

 when the quantity of this is small, there is no retention of carbon; 

 as soon however as it is increased beyond a certain limit, carbon is 

 stored up in the form of fat or, to a smaller extent, as glycogen. 

 Fats and carbohydrates therefore differ markedly from proteid food 

 in that they are not so distinctly provocative of metabolism. This 

 is exceedingly well shewn in the results of Lawes and Gilbert, for 

 in the pig previously mentioned 472 parts of fat were laid on for 

 every 100 parts of fat taken as such in the food (which consisting 

 of barley-meal, &c. contained a very small amount of actual fat), 

 while of every 100 parts of the total dry non-nitrogenous food 

 including fat, starch, cellulose, &c. no less than 21 '2 parts were 

 retained in the body in the form of fat. No clearer proof 

 than this could be afforded that fat is formed in the body out of 

 something which is not fat. 



As one might imagine, the presence of fat or carbohydrates in 

 the food is found to decrease the amount of proteid metabolism 



