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 possible 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 pro- 

 teins 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 practically 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 ex- 

 periment, however, we find the effect of giving protein food is to increase 

 largely the nitrogenous output 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 be- 

 tween two and a half and five times the amount of protein which undergoes 

 disintegration during starvation. The reason for this is obvious on reference 

 to p. 627. There we see that a man on the fifth day of starvation excreted 

 1144 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 fat stored up in the body. 

 This signifies that only one-fifth of the total energy requirements of the body 

 were supplied at the expense of protein. We cannot therefore expect to 

 stop loss of body substance by giving an amount of protein food which would 

 correspond only to one-fifth of the energy requirements. In most cases, 



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