612 METABOLISM. NUTRITION AND DIETETICS 



case the excretion of nitrogen would not necessarily be altered ; the 

 proteins which, in the absence of work, would have been oxidized 

 within the muscular substance or elsewhere, their energy appearing 

 entirely as heat, may, when the call for protein to take the place of 

 that broken down in muscular contraction arises, be diverted to this 

 purpose. 



In any case, there is no doubt that a dog fed on lean meat may 

 go on for a long time performing far more work than can be yielded 

 by the energy of fat and carbo-hydrates occurring in traces in the 

 food, or taken from the stock in the animal's body at the beginning 

 of the period at work. A large portion, and perhaps the whole, of 

 the work, must in this case be derived from the energy of the pro- 

 teins (Pfliiger). On the other hand, it is well established that when 

 fats and carbo-hydrates are present in sufficient quantity in the 

 tissues or the food, they constitute the main source of the energy 

 of muscular contraction (p. 772), and there is some evidence that of 

 the two classes of food materials carbo-hydrates in the form of 

 dextrose (or glycogen) is the material of election. 



The outcome, then, of this famous controversy is essentially a 

 compromise. Everybody now admits that the muscular machine 

 can and does utilize predominantly any one of the great groups 

 of food substances, be it carbo-hydrate, fat, or protein, when the 

 dietetic conditions are such that only one of these is offered to it 

 in large amount, the others being either absent or offered in small 

 amount. To be sure, amino-acids are not the first choice, but if 

 it must do so the muscle can make shift with them, and can indeed 

 make them serve excellently well. When all the food substances 

 are present in abundance, carbo-hydrate is favoured above fat, and 

 fat above protein. 



Experience has shown that the minimum quantity of nitrogen 

 required in the food of a man whose daily work involves hard 

 physical toil is higher than the minimum required by a person lead- 

 ing an easy, sedentary life. This is evidently in accordance with 

 the view that protein is actually used up in muscular contraction ; 

 but it is not inconsistent with the opposite view. For the body of a 

 man fit for continuous hard labour has a greater mass of muscle 

 to feed than the body of a man who is only fit to handle a composing- 

 stick, or drive a quill, or ply a needle ; and the greater the muscular 

 mass, the greater the muscular waste. But if an animal just in 

 nitrogenous equilibrium on a diet of lean meat when doing no work 

 is made to labour day after day, it will lose flesh unless the diet 

 be increased. This must mean that some of the protein is being 

 diverted to muscular work, and that the balance is not sufficient 

 to keep up the original mass of ' flesh ' (see p. 628). 



Relative Value of Different Proteins in Nutrition Synthesis of 

 Amino-Acids. The fact that the various proteins differ quantita- 



