SECTION III 



THE EFFECT OF FOOD ON THE METABOLISM 



OF THE BODY 



A MARKED contrast exists between protein and the other two 

 classes of food-stuffs in relation to nutrition. Whereas it is pos- 

 sible in the case of many animals to maintain life with a diet 

 consisting of proteins, salts, and water, such as is contained in 

 the leanest possible meat, a diet of pure fat or carbohydrate, or 

 a mixture of the two, is almost equivalent to an absolute abstinence 

 from food, the animal on such a diet surviving only a few days 

 longer than during complete starvation. The primary importance 

 of proteins in nutrition therefore indicates that it is advisable to 

 deal first with the effects on metabolism of a diet consisting of this 

 food-stuff alone. In an animal which has been starved for five or 

 six days the nitrogenous output of the body has attained a prac- 

 tically constant level, varying with the size of the animal and with 

 the relative content of its body in fat. Let us assume that such 

 an animal is excreting 5 grm. of nitrogen daily, corresponding to 

 a protein metabolism of 31-25 grm. of protein. It might be 

 thought that this loss of protein to the body would be met if we 

 administered to the animal as food a similar amount, i.e. 31-25 grm. 

 of protein. On trying the experiment, however, we find the effect 

 of giving protein food is to increase largely the nitrogenous out- 

 put of the body, so that after receiving this amount of protein 

 the animal's nitrogenous excretion will amount to nearly 10 grm. 

 The waste of tissue-protein in the body therefore proceeds. In 

 order to stop this waste, i.e. to ensure that the animal does not lose 

 more nitrogen than it receives in its food, we must give an amount 

 of protein corresponding to between two and a half and five times 

 the amount of protein which undergoes disintegration during starva- 

 tion. The reason for this is obvious on reference top. 701. There 

 we see that a man on the fifth day of starvation excreted 11-44 grm. 

 of nitrogen, corresponding to 71-5 grm. of protein. This protein 

 metabolism did not, however, represent the sole source of the energy 

 output of the body. The total energy output was 1979 calories. Of 

 this amount only 293 calories could be obtained from the combustion 

 of the 71 grm. of protein, the balance being due to the oxidation of 



706 



