THE FASTING KATABOLISM 255 



animal with liberal amounts of non-nitrogenous nutrients, but 

 no protein, the protein katabolism might not be reduced to an 

 amount even smaller than that observed in the absence of all 

 feed. Experiments by C. Voit and by Rubner on dogs and by 

 Landergren, Folin and Cathcart on man have shown this to 

 be the case with these species. 



Comparisons of this sort on farm animals are not readily 

 made, especially with herbivora, and none have yet been re- 

 ported, but McCollum and Steenbock 1 have shown that the 

 protein katabolism of the pig may be reduced by long continued 

 feeding on a non-nitrogenous diet (starch) to an amount materi- 

 ally less than appears to be necessary in the feed of the animal 

 under ordinary conditions to maintain nitrogen equilibrium 

 (417), the average of all of their experiments being equivalent 

 to 0.28 Ib. of protein per 1000 Ibs. live weight. 



340. Functions of protein in fasting. The fact that a 

 certain minimum katabolism of body protein persists even in 

 the presence of the most abundant supply of non-nitrogenous 

 nutrients has generally been interpreted in the past as showing 

 that a certain amount of the protein of the cell is necessarily 

 broken down in the performance of its physiological functions. 

 This necessary minimum has been somewhat vaguely compared 

 to the wear of a machine, Rubner especially designating it as 

 the " wear and tear " quota of the protein katabolism. More 

 recent investigations, however, have suggested the possibility 

 of another explanation. 



The actual nitrogenous nutriment of the body cells is not 

 proteins as such, but substantially the simple amino acids out 

 of which they are built up. As required, these amino acids 

 may be synthesized to protein (226, 232), but thqre appears to 

 be some reason for believing that they may also be necessary 

 for other purposes in the body ; that certain of them may, for 

 example, as was suggested by Willcock and Hopkins, be essential 

 to the production of the various internal secretions and hormones 

 which apparently play so large a part in metabolism. If, how- 

 ever, the normal performance of the body functions calls for 

 a supply of some particular amino acid, tryptophan e.g., this 

 can be derived, in the fasting animal, only from the cleavage of 

 body protein, since there is no evidence that tryptophan can 



1 Wis. Expt. Sta., Research Bui. No. 21, p. 55. 



