NORMAL METABOLISM. 95 



spoiids to the intake. Having once gained an equilibrium, we 

 may raise its level by gradually increasing the protein intake. 

 During this progressive raising of the protein intake, it will be 

 found, at least in the carnivora (cat and dog) that a certain 

 amount of nitrogen is retained by the body for a day or so imme- 

 diately following each increase in protein intake. The excre- 

 tion of nitrogen, in other words, does not immediately catch up 

 on the intake. The amount of nitrogen thus retained is too great 

 to be accounted as a retention of disintegration products of pro- 

 tein; it must therefore be due to an actual building up of new 

 protein tissue, that is, growth of muscles. 



Such results undoubtedly obtain in the cat, and less markedly 

 in the dog. But they do not do so in man and the herbivorous 

 animals. In these, we can never give a sufficiency of protein 

 alone to maintain nitrogen equilibrium; there will always be an 

 excess of excretion over intake. But indeed it scarcely requires 

 any experiment to prove this, for it is self-evident when we con- 

 sider that there are only 400 C. in a pound of lean meat, and 

 there are few who could eat more than 4 pounds a day, an amount 

 which however would only furnish about half of the required 

 calories. A person fed exclusively on flesh is therefore being 

 partly starved, even although he may think that he is eating 

 abundantly and be quite comfortable and active. This fact has 

 a practical application in the so-called Banting cure for obesity, 

 which consists in almost limiting the diet to flesh and green 

 vegetables, allowing only a very small quota of carbohydrates 

 or fats. 



Very different results are obtained when carbohydrates or fats 

 are freely given with the protein. Nitrogen equilibrium can 

 then be regained on very much less protein, so we speak of fats 

 and carbohydrates as being "protein sparers." Carbohydrates 

 are much better protein sparers than fats ; indeed they are so effi- 

 cient in this regard that it is now commonly believed that carbo- 

 hydrates are essential for life, so that when the food contains no 

 trace of carbohydrates, a part of the carbon of protein has to be 

 converted into this substance. This important truth is supported 

 by evidence derived from other fields of investigation (e. g., the 



