THE PROTEIN REQUIREMENTS OF MANKIND 111 



when at work than when resting, points indisputably to the 

 importance of protein as a factor in the provision of energy for 

 muscular contraction. The investigations of Paton, Stockman, 

 Frentzel, and others afford further evidence of the important 

 part played by protein in muscular work, and show that severe 

 and excessive exercise is accompanied and followed by an 

 increased elimination of nitrogen in the urine. 



Further, we have attempted to demonstrate that, when an 

 abundant supply of protein is available, the tissues, and par- 

 ticularly the muscular tissues, are in a position to replace dis- 

 integrated tissue with new material derived from the products 

 of tryptic digestion an opportunity not afforded them when 

 the level of nitrogenous metabolism is low, in which case it is 

 not improbable that the tissue cells have to make use of their 

 old or damaged building-stones until such time as fresh material 

 is forthcoming. It is not difficult to imagine that under such 

 conditions the efficiency and economical working of the body 

 may suffer, as compared with the more advantageous position 

 which obtains when the protein metabolism is pitched at a high 

 level ; and this condition would be all the more marked and 

 exaggerated when, in combination with a low level of nitrogenous 

 metabolism, a high standard of functional activity is demanded, 

 as in severe muscular exercises. Lastly, we have referred to the 

 view that only a few grammes of nitrogen daily is needed, as the 

 speedy elimination of the nitrogen taken as food is suggested 

 to mean that the great proportion of it never reaches the tissues, 

 being split off as ammonia, changed into urea in the liver, and 

 rapidly excreted in the urine. While this, in all probability, 

 does take place to a certain extent, it is by no means proved to 

 occur with the greater part of the protein, and the weight of 

 available evidence would locate the metabolism of proteins, 

 as well as other food substances, within the tissue cells. It 

 would appear much more probable that the bioplasm possesses 

 the power of rapidly assimilating the products of tryptic digestion 

 in the presence of an abundant supply, and thus being able to 

 replace at once any worn, damaged, or fatigued elements, in 

 this way maintaining the whole mass of the protoplasmic tissues 

 at a high level of nutrition and efficiency. Considering the weight 

 of the total nitrogenous tissues of an average man, and making 

 allowance for a certain proportion of nitrogen probably made 

 use of in the form of ammonia to maintain the intracellular 



