THE EFFECT OF FOOD ON METABOLISM 707 



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. The same ratio between 

 protein metabolism and total energy output of the body is shown 

 in the Table, p. 710, from experiments on animals. In most cases, 

 if we are dealing with an animal with a considerable store of fat in 

 its body, nitrogenous equilibrium, i.e. an equivalence between income 

 and output of nitrogen, is attained with a quantity of protein in the 

 food which is less than five times the amount lost during starvation. 

 In such a case the total energy requirements of the body are met 

 not only at the expense of the protein food but also at the expense 

 of the fat of the tissues. The animal will continue to lose weight 

 and to become thin, although he is in a state of nitrogenous equilibrium. 

 The protein taken in with the food on a pure protein diet has a 

 twofold function to perform. In the first place, every functional 

 activity of the living tissues is probably associated with a certain 

 amount of wear and tear, and results in the production of disintegration 

 products which are not in a condition to be resynthetised into living 

 working protoplasm. We know, for instance, that from every 

 mucous surface dead cells are being continually cast of! and that a 

 constant disintegration of red blood corpuscles goes % on, resulting 

 in the production of the bile pigments ; and we are warranted in 

 extending the operation of these changes of which we have ocular 

 evidence to the case of other cells, such as those of the liver and 

 of the muscles, where direct proof of destruction of tissue during 

 normal metabolism is more difficult to obtain. We shall have occasion 

 to discuss later on the extent of this process of disintegration. It is 

 certain that some portion of the nitrogen excreted during complete 

 starvation must come from this source, and that one of the functions 

 of protein food is the replacement of tissue which has been lost in 

 this way. When, however, we are feeding an animal on a pure protein 

 diet, by far the larger portion of the food is utilised for meeting the 

 energy requirements of the body. In this function protein food, 

 apart from accidents of digestibility and structural adaptation of 

 the animal's digestive arrangements to its habits of life, presents no 

 apparent advantages over the other two classes of food-stuffs. Its 

 value to the animal is represented by its physiological heat- value. 

 It may be represented therefore numerically as 4-1, and is equivalent 

 to the value of carbohydrate* and is far inferior to the value of fats 

 with a heat equivalent of 9-3. If, instead of giving to the starving 



* This may be expressed by saying that protein is isodynamic with an 

 equal weight of carbohydrate. 



