104 THE PROTEIN ELEMENT IN NUTRITION 



The fact that approximately ten times this amount of nitrogen 

 is necessary in a dietary in order to maintain the body in 

 nitrogenous equilibrium, when the caloric value of the diet is 

 sufficient, would show that Folin's deductions regarding the 

 creatinine-nitrogen as a measure of endogenous metabolism do 

 not cover the facts, or, if it be such a measure, then there must 

 be a large demand for protein in the body beyond that required 

 to repair tissue waste and make good its loss. 



From an analysis of the more recent work on creatinine ex- 

 cretion, we have shown in our introduction that the small and 

 constant quantity eliminated daily cannot be made use of as 

 evidence to support the arguments of those who consider that 

 the protein requirements of the body would be covered by the 

 presence of a very limited amount of nitrogen in the daily food. 



It is just possible that creatinine is a partial measure of the 

 nitrogenous interchanges going on within muscular tissue, and 

 that 0-6 gramme of creatinine-nitrogen represents to some 

 extent the degree of this special type of metabolism. This would 

 leave out of consideration the interchanges taking place between 

 the cells of all other tissues and the lymph, and the large supply 

 of fluid protein not actually incorporated into the material of the 

 living cell the " labile protein " of physiologists. 



In the light of recent researches on the constitution of the 

 protein molecule and on the demands of the tissues, not for 

 nitrogen merely, but for nitrogen in certain combinations, or 

 complexes of molecules, it is possible that, in the providing of 

 even 0-6 gramme of nitrogen in the form required by the muscle 

 cells, large quantities of protein taken in the food will require 

 to be broken down into their different units in order that the 

 suitable " building-stones " may be available in sufficient amount. 

 However, without an accurate knowledge of the total nitrogenous 

 requirements, and lacking any reliable method of determining 

 them, these possibilities are merely interesting speculations. 

 The fact that in the case of the fasting subject the excretion of 

 nitrogen is far in excess of the amount which, according to Folin, 

 corresponds to disintegrated tissue protein, would show clearly 

 that his deductions regarding the significance of creatinine in 

 the urine as a measure of the protein requirements of the tissues 

 cannot be correct. 



An attempt has been made to arrive at the true protein 

 requirements of man by studying the conditions that obtain 



