16 MAINTENANCE EATIONS OF FARM ANIMALS. 



fasting animal is supplied mainly by the katabolism of protein and 

 fat. In the intermediate stages of fasting, as has been shown, the 

 katabolism is largely that of fat, but the ratio between fat and 

 protein katabolizecl may differ widely according to circumstances. 

 In other words the protein requirement, or at least the amount of 

 protein used, may vary, while the energy requirement remains nearly 

 constant. The fasting organism requires a definite quantity of 

 energy, but seems more or less indifferent as to its source. 



THE ENERGY REQUIREMENT EOR MAINTENANCE. 



In the fasting animal the store of potential energy in the body 

 is diminished daily by the amount required to carry on the vital ac- 

 tivities, this amount being, as just shown, relatively constant. In 

 order to prevent such a loss and maintain the store of body energy, 

 it is evident that a corresponding quantity of energy must be sup- 

 plied in the feed and that a maintenance ration is one which supplies 

 this requisite quantity. 



REPLACEMENT OF NUTRIENTS. 



For this purpose experiments have shown that the various di- 

 gestible nutrients may replace each other or the ingredients of the 

 body through a very wide range. 



FEED FAT AND BODY FAT. 



Fat fed to a previously fasting animal diminishes or suspends the 

 loss of body fat. The following averages of Pettenkofer and Voit's 

 experiments, 1 computed from Atwater and Langworthy's digest, 2 

 may serve to illustrate this substitution of feed fat for body fat : 



Replacement of body fat by feed fat — Pettenkofer and Voit. 



Food. 



Number 

 of experi- 

 ments. 



Gain or loss by 

 body. 



Nitrogen. 



Fat. 



None 



5 

 2 

 1 



Grams. 

 -6. 64 

 -4.90 

 -7. 70 



Grams. 



- 97.76 



- 16.25 

 + 113.60 



100 grams fat 







The smaller amount of fat not only diminished the protein katabolism but 

 also largely reduced the loss of fat from the body. While the larger amount 

 of fat showed a tendency to increase the protein katabolism, it not only sus- 

 pended the loss of body fat but caused a storage of fat in the organism. Of 

 course there is no means of distinguishing in such a case between feed fat and 



1 Zeitschrift fur Biologie, vol. 5, p. 370. 



2 U. S. Department of Agriculture, Office of Experiment Stations, Bulletin 45. 



