78 THE PHYSIOLOGY OF PROTEIN METABOLISM 



As regards the individual amino acids it has already been pointed 

 out that Levene and Meyer (255) and Miss Bostock (71) among others 

 have demonstrated that within a few hours (eight to nine) about 90 per 

 cent, of the ingested nitrogen of alanine and glycine is excreted, for the 

 most part, in the form of urea. Here again, as in the case of the protein, 

 there is some variation in the rapidity of excretion depending on the 

 amino acid used. For instance, in the case of leucine only about 54 

 per cent, of the nitrogen appears in the first twenty-four hours. In a 

 series of experiments, not yet published, which I have carried out 

 recently, where various forms of nitrogenous material were introduced 

 directly into the small intestine, and the urine was collected directly 

 from the ureters for a period of several hours, it was found in some 

 cases, for example with a simple amino acid like alanine, that about 

 70 per cent, of the nitrogen introduced could be recovered from the 

 urine of the animal within five hours. The rapidity with which the 

 nitrogen was eliminated again, depended largely on the nature of the 

 substance tested, and on the rate of its absorption. In all my ex- 

 periments the endogenous output of nitrogen was estimated and sub- 

 tracted from the total nitrogen excreted, so that the figures obtained 

 represented the actual degree of catabolism of the absorbed protein or 

 amino acid. 



The Effect of the Partition of the Diet on the Output of the 



Nitrogen. 



As Voit pointed out long ago the nitrogen output depends for the 

 most part on the amount of nitrogen contained in the food consumed 

 The rate of this output seems to be influenced, however, by the partition 

 of the food the giving of the food in several amounts instead of at a 

 single meal. Adrian (51, 52) found that this partition sometimes in- 

 creased and sometimes decreased the amount of nitrogen excreted. 

 The variation he observed was probably due to faulty methods. Munk 

 (295) also with poor methods found a slight increase in the nitrogen 

 output, but Krummacher (233), on the other hand, using a fairly reliable 

 method, showed that the division into meals brought about a slight de- 

 crease due to the fact that there was a constant succession of maxima 

 of absorption one supply of food coming in before the previous supply 

 was completely dealt with, with the result that there was some slight 

 decrease in the absolute amount of material metabolized. Gebhardt 

 (151) also found that the partition of the food into separate meals 



