632 PHYSIOLOGY 



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 off 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. 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 repre- 

 sented 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 animal a pure protein diet, we administer a mixed diet 

 containing a sufficient quantity of fat or carbohydrate, or of both substances, 

 to meet the normal energy requirements of the body, we can restrict the 

 utilisation of protein more nearly to the replacement of tissue waste in the 

 body, and are therefore able to attain nitrogenous equilibrium with a much 

 smaller proportion of protein than is possible when this substance furnishes 

 the whole diet. In carnivora, which have the habit of supplying a large 

 proportion of their energy needs at the expense of protein, the amount of 

 carbohydrate and fat which must be added to protein in order to attain nitro- 

 genous equilibrium with a nitrogenous output equal to that in starvation is 

 very large and corresponds to an energy- value in excess of the total energy 

 expenditure during starvation. In omnivora, such as man, it is easy to attain 

 .nitrogenous equilibrium on a mixed diet with a smaller nitrogen turnover 



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

 of carbohydrate. 



