PROTEIN METABOLISM 



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of undetermined nitrogen which may include traces of purine bases, such as 

 xanthine and hypoxanthine, traces of amino-acids, small amounts of pig- 

 ment and of nucleo-protein from the wall of the bladder. The relative pro- 

 portions in which these bodies occur are not invariable, but differ according 

 to the nature of the protein foods taken and also according to the proportion 

 which the protein metabolism bears towards the total energy requirements 

 of the body. In the following Tables (Folin) are given the average composi- 

 tion of two specimens of urine from the same individual, one on a diet con- 

 taining the ordinary proportion, and the other on a diet containing only a 

 minimal amount of protein : 



TABLES I AND II 

 DISTRIBUTION OF NITROGEN IN URINE ON VARIOUS DIETS 



In dealing with the metabolism of .the body as a whole we saw reason to 

 believe that the proteins taken in with the food might be regarded as having 

 a twofold destiny. One part, and under normal circumstances the greater 

 part, is applied to the production of energy, in this respect discharging a 

 function which might equally well be performed by the fats and carbohy- 

 drates of the food. In its second function protein cannot be replaced by any 

 other food-stuff, since it alone contains the necessary elements as well as the 

 groupings of these elements which are essential for the building up of the 

 living tissues. We saw reason to believe that this tissue metabolism only 

 accounted, however, for a small part of the nitrogen of the food. On this 

 account it is possible to ensure health and a condition of nitrogenous equili- 

 brium with amounts of protein in the diet of man which might vary between 

 40 and 200 grm. per diem. The more protein that is taken in with the food 

 the greater is the relative amount which is applied to the energy needs of 

 the body. If therefore we would attempt to find out what are the end- 

 products of the tissue metabolism we should confine the energy metabolism 

 of proteins within the smallest possible limits by reducing 'the quota of pro- 

 tein in the diet to its minimum. Folin has shown that if we compare'the com- 

 position of the urine obtained under these two conditions, namely, on a diet 

 containing a normal quantity of protein and on a diet containing a minimal 



