110 THE PROTEIN ELEMENT IN NUTRITION 



cannot be maintained at anything like the same high level of 

 nutrition as when the supply of protein is liberal. That this 

 view is correct would appear probable from the results of observa- 

 tions on the effects of establishing an animal on the lower limits 

 of nitrogenous equilibrium the carbonaceous constituents of 

 the diet being abundant and then causing an increase in the 

 work performed. The animal will soon pass from its condition 

 of nitrogen balance, and rapidly lose weight from the disintegra- 

 tion of its own tissues. In this experiment the heat value of the 

 diet may be exceedingly abundant, and yet the extra work 

 determines a disintegration of protein that cannot be met by the 

 amount present in the food, nor by the makeshift arrangement 

 of using over again the damaged building materials of the 

 tissue cells. In this way the increasing demands of the body 

 for protein during severe muscular exertion can be explained, 

 and the importance of a high level of nitrogenous interchange 

 in maintaining the tissues of the body in an efficient and 

 economical condition can be understood. Until more is known 

 concerning the changes that go on within the body from the 

 digestion of protein until its elimination, it is impossible to say 

 how much, or in what way, nitrogen is made use of in the work 

 of muscle or in the other nitrogenous tissues. 



As Stewart has pointed out in connection with the fate of 

 protein within the body : " The fact that very soon after the 

 introduction of proteins (as well as other food substances) into 

 the blood the increased metabolism of them begins, is not of 

 itself sufficient to show that they are destroyed without being 

 built up into the protoplasm. For the protoplasm may have 

 the power of rapidly assimilating the protein in the presence of 

 an abundant supply, and of rapidly breaking down at the same 

 time."* 



It may be concluded, therefore, that so far as evidence afforded 

 by the dietaries of mankind assists in determining the most 

 advantageous level of protein metabolism, it points to at least 

 100 grammes of protein daily as the limit below which, on the 

 average, nitrogenous interchange should not fall ; that when hard 

 muscular labour is performed, amounts considerably in excess 

 of this are necessary ; that the carbonaceous material of a 

 dietary being sufficient, the fact that a larger amount of protein 

 is required to maintain the body in nitrogenous equilibrium 



* Stewart, " A Manual of Physiology," p. 470. 



