PROTEIN REQUIREMENTS 



What is the Cause of the Retention ? 



The cause of the retention is the need of the organism for nitrogen, 

 but the laws which govern this demand and the nature of this reten- 

 tion and which regulate the rate and the manner of the output are not 

 fully understood. 



As an explanation of the nature of the retention Ehrstrom (in) 

 has suggested that there are certain " affinities " in the tissues to which 

 the absorbed material attaches itself in a more or less labile union, 

 this union lasting for very variable periods. The duration of the re- 

 tention, he believes, is absolutely independent of the chemical form 

 assumed by the retained material. On the other hand, Gruber (160), 

 without entering into any vague theorizing, sums up his opinion in 

 the sentence "that this temporary retention of protein is simply the 

 result of the superposition of the hourly curves ". Assuming that the 

 hourly curve of the output of nitrogen of one day is identical with the 

 next (always if the protein be provided in the same amount and 

 fashion), and that as a result the nitrogen from the daily protein cata- 

 bolism is excreted in the same proportion in each 24, 48, 72 and 96 

 hours, Gruber has put forward an extremely interesting hypothesis 

 which offers a very probable explanation, of the step-like increase or 

 decrease in the output of nitrogen produced by variations in the protein 

 intake (see Table). He suggested that 80 per cent, of the food nitrogen 

 was catabolized on the day of ingestion, 1 3 per cent, on the day fol- 

 lowing, 5 per cent, on the third day, and 2 per cent, on the fourth. 

 Thus, after an increase in the intake of protein there would be a 

 steady step-like rise in the output of nitrogen until equilibrium of intake 

 and output was reached. The same explanation holds good with a 

 decreased nitrogen intake. 



Magnus-Levy (III.) has improved on this table of Gruber. He 



