METABOLISM, NUTRITION AND DIETETICS 539 



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 of work. A large portion, and 

 perhaps the whole, of the work, must in this case be derived 

 from the energy of the proteins (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 contrac- 

 tion (p. 669). 



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 

 leading an easy, sedentary life. This is evidently in accordance 

 with the view that protein is actually used up in muscular con- 

 traction ; 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 mus- 

 cular work, and that the balance is not sufficient to keep up the 

 original mass of ' flesh ' (see p. 548). 



(2) Income and Expenditure of Carbon. This division of 

 the subject has been necessarily referred to in treating of the 

 nitrogen balance-sheet, and may now be formally completed. 



Carbon Equilibrium. A body in nitrogenous equilibrium 

 may or may not be in carbon equilibrium. It has been re- 

 peatedly pointed out that the continued loss or gain of carbon 

 by an organism in nitrogenous equilibrium means the loss or 

 gain of fat ; and, since the quantity of fat in the body may vary 

 within wide limits without harm, carbon equilibrium is less 

 important than nitrogen equilibrium. It is also less easily 

 attained when the carbon of the food is increased, for, the 

 consumption of fat is not necessarily increased with the supply 

 of fat or fat-producing food, and there is by no means the 

 same prompt adjustment of expenditure to income in the case 

 of carbon as in the case of nitrogen. 



Carbon equilibrium can be obtained in a flesh-eating animal, 

 like a dog, with an exclusively protein diet ; but a far higher 

 minimum is required than for nitrogenous equilibrium alone. 

 Voit's dog required at least 1,500 grammes of meat in the twenty- 

 four hours to prevent his body from losing carbon. For a man 

 weighing 70 kilos, the daily excretion of carbon on an ordinary 



