578 THE ANIMAL BODY AS A MACHINE 



namic values to the protein constituents of the dietary, arises from the 

 fact that an increase of protein in diet actually stimulates the total 

 metabolism, so that more food is burnt and more heat evolved on a diet 

 high in protein than upon a diet which contains less protein. This 

 phenomenon, which Rubner terms the Specific Dynamic Action of pro- 

 teins, is very well displayed by the effect of administering protein to a 

 starving animal. One might suppose that if a starving animal is losing 

 a certain amount of tissue-protein daily, the administration of this 

 amount of protein daily would suffice to balance the nitrogenous input 

 and output. This is not the case, however, for on increasing the 

 nitrogenous input an increase of nitrogenous output also occurs and 

 the balance remains negative. A further increase of nitrogenous input 

 calls forth a still greater metabolism of protein until, on an exclusively 

 protein diet, a balance between intake and output is attained with an 

 output of nitrogen no less than three and one-half times that which is 

 observed in the starving animal. In man the quantity of protein thus 

 required to obtain nitrogenous equilibrium is greater than he can 

 conveniently consume, and even when nitrogenous equilibrium has 

 been attained the carbon balance remains negative, since not only the 

 nitrogenous metabolism, but the metabolism of fats and carbohydrates 

 is stimulated by protein. The effect of protein is therefore to greatly 

 increase the heat-evolution of the body, and the replacement of fat or 

 carbohydrate by protein in a diet which is just sufficient to maintain 

 equilibrium results in rendering the diet inadequate to replenish the 

 tissue-loss. The proteins cannot, therefore, replace fats or carbo- 

 hydrates in isodynamic proportions. 



The origin of the specific dynamic action of the proteins has been 

 sought by Lusk, who investigated the effects of individual Amino- 

 acids upon the heat-output in starving dogs. He found that while 

 glycocoll and alanine greatly increase the production of heat, and 

 leucine and tyrosine slightly, glutamic acid is devoid of action. A 

 mixture of 5.5 grams each of glycocoll, alanine, glutamic acid and 

 tyrosine produced as much increase of heat-output as 100 grams of 

 meat. 



THE PROTEIN REQUIREMENT IN THE DIETARY. 



From the preceding considerations it must be evident that the 

 proteins are the most wasteful constituents of the dietary, since they 

 increase the consumption of other constituents as well as that of pro- 

 tein itself. The proteins are also the most expensive foodstuffs from 

 a commercial point of view and this is particularly true of the proteins 

 of animal origin, for while there is little wastage of energy or materials 

 in the growth of the vegetable constituents of the diet, a very large 

 wastage occurs in the synthesis of animal proteins for human consump- 

 tion. An ox or sheep may, for our immediate purpose, be regarded as 

 an ambulatory factory of protein. In order to supply this factory with 

 raw materials, vegetable proteins, carbohydrates and fats must first 



