86 THE PHYSIOLOGY OF PROTEIN METABOLISM 



pointed out that even when the organism was in a state of nitrogenous 

 equilibrium a certain amount of the protein ingested was required to 

 replace broken down tissue protein. A certain amount was also re- 

 tained in the organism to replace store or labile protein which had 

 been utilized. In other words, the nitrogen excreted on a particular 

 day did not wholly represent the nitrogen ingested on that day, part 

 came from the food protein and part from the protein already present 

 in the organism. Magnus-Levy, therefore, subtracted from the 80 parts 

 of the nitrogen said to be disintegrated by Gruber on the day of inges- 

 tion the amount which would be required to replace the effete tissue 

 protein. He estimated this amount as 30 parts. He devised two 

 tables, one of which presumed a moderate protein intake with a cor- 

 responding small retention of store protein, and the other a large in- 

 take of protein and a corresponding increase in the amount of labile 

 protein retained. The table dealing with the moderate intake is given 

 here : 



This regularity of storage, or excretion, only occurs when there are 

 no disturbing factors present. If, for instance, the subject had under- 

 gone a period of nitrogen starvation previous to his nitrogenous intake 

 experiment then there would be very marked retention of nitrogen, pre- 

 sumably for purposes of repair. 



All proteins are not broken down with equal readiness, and probably 

 as a result there is considerable variation in the period in which nitrogen 



