PROTEIN METABOLISM 



855 



appear in the urine, and to determine, where possible, their pre- 

 cursors, and the conditions which determine their formation in the 

 body. The chief nitrogenous constituents of urine are urea, ammonia, 

 uric acid, creatinine, hippuric acid. There is a small residue of 

 undetermined nitrogen which may include traces of purine bases, 

 such as xanthine and hypoxanthine, traces of amino-acids, small 

 amounts of pigment and of nucleo- protein from the wall of the 

 bladder. The relative proportions 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 meta- 

 bolism bears towards the total energy requirements of the body. In 

 the following Tables (Folin) are given the average composition of two 

 specimens of urine from the same individual, one on a diet containing 

 the ordinary proportion of protein, and the other on a diet containing 

 only a minimal amount of this food-stuff : 



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 the body, in this respect discharging a function which might 

 equally well be performed by the fats and carbohydrates 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 meta- 

 bolism only accounted, however, for a small part of the nitrogen of 

 the food. On this account it is possible to ensure health and a 



