16 THE PKOTEIN ELEMENT IN NUTRITION 



quickly built up into the living protoplasm, only to be accom- 

 panied, or immediately succeeded, by a similar degree of 

 katabolism. As this explanation did not appear very probable, 

 in the light of Nencki's rinding of a large excess of ammonia in 

 the blood of the portal vein during digestion, this view is not 

 regarded now as sufficient to cover the facts. It is considered 

 much more probable that the ammonia split off from the amino- 

 acids of tryptic digestion is transformed into urea in the liver, 

 and that the nitrogen represented by that urea never reaches 

 the protoplasm of the cells, but is quickly got rid of in the urine. 



The constancy in the output of creatinine for each individual 

 has been suggested to mean that this substance is a product of 

 true tissue metabolism ; the one factor determining its amount 

 being the weight of the true tissue elements of the body. Further, 

 creatinine has been looked upon to some extent as a measure of 

 the extent of the nitrogenous interchanges within the body ; and 

 as the total daily output rarely exceeds 0-6 gramme nitrogen, 

 it has been used as an argument to support the view that only a 

 small amount of protein is needed in the diet. The line of 

 reasoning from this point of view is quite clear. Shortly, it is as 

 follows : 



The products of tryptic digestion lose a large proportion of 

 their nitrogen on absorption ; this exogenous katabolism appears 

 to have no physiological meaning, and the extent to which it 

 takes place in the ordinary individual on a liberal protein diet 

 no justification. The constant splitting off of nitrogen appears 

 to serve no useful purpose, as the organism can neither use it 

 nor store it up for future use. The carbohydrate moiety of the 

 protein molecule is made use of as a source of body heat and 

 potential energy, but this could easily be replaced by carbo- 

 hydrates given as such in the daily food. Having the further 

 information that the nitrogenous requirements of the true 

 cellular tissues as measured by the products of endogenous 

 metabolism are very small, the question naturally arises, 

 Would it not be greatly to the benefit of the organism to lessen 

 the protein element in the diet to an amount closely approxi- 

 mating that of the true needs of the body ? This question the 

 advocates of a low protein dietary answer in the affirmative. 

 The true answer can only be arrived at by a very careful con- 

 sideration of the principles involved in all their bearings, and 

 particularly by an investigation to determine the effects of a 



