RATIO OF PROTEIN" TO TOTAL KATABOLISM. 11 



Protein. — Balance experiments, however, while confirming the 

 conclusion that the loss of tissue in fasting usually consists chiefly 

 of fat together with some carbohydrates, show that there is also a 

 continual breaking down of body protein and a corresponding ex- 

 cretion of urinary nitrogen. While the energy supply of the fasting 

 animal is chiefly derived from the breaking down of nonnitrogenous 

 material, the functional activity of the tissues necessarily involves the 

 katabolism of a certain amount of protein. 



RATIO OF PROTEIN TO TOTAL KATABOLISM. 



Qualitatively, then, the katabolism of the fasting animal is substan- 

 tially a katabolism of fat and of protein, and it becomes of interest 

 to consider the quantitative relations between the two. Such a com- 

 parison is best made on the basis of the amounts of energy liberated in 

 the body in the katabolism of protein and of fat respectively. This 

 aspect of the subject has been treated especially in an article by E. 

 Voit 2 in which the results of a considerable number of fasting ex- 

 periments are compiled and discussed. While some of Voit's com- 

 putations are based on estimates, they are sufficiently accurate to 

 outline definitely the main features of the fasting katabolism. In- 

 cluding only experiments on animals well nourished at the beginning, 

 he obtained the following averages for the percentage of the total 

 energy liberated which was supplied by the katabolism of protein in 

 the case of a number of different species. The results of the first day 

 or two of fasting are not included in the averages. 



Proportion of energy derived^ from protein in fasting — E. Voit, 



Kind of animal. 



Live 

 weight. 



Protein 

 katabolism 

 in per cent 



of total 

 katabolism. 





Kilos. 

 115.0 

 63.7 

 f 28.6 

 \ 18.7 

 I 7.2 

 2.7 

 .6 

 3.3 

 2.1 



Per cent. 



7.3 

 15.6 

 13.2 

 10.7 

 13.5 

 16.5 

 10.8 



7.4 

 10.0 





Dog 



Rabbit 



Guinea pig 



Goose 



Hen 





While both the total and protein katabolism naturally showed a 

 wide range as to absolute amount, whether per head or per unit of 

 live weight, the ratio of protein to total katabolism proved notably 

 uniform with only two exceptions. The experiments upon dogs, 27 

 in number, included in the foregoing table furnished the basis for 

 the following comparison, showing that in 74 per cent of the cases 

 the ratio ranged from 10 to 17 per cent. 



1 Zeitscbrift fur Biolo^ie, vol. 41, p. 167. 



