76 



MAINTENANCE RATIONS OF FARM ANIMALS. 



processes, since the latter go on for a considerable time in a sub- 

 stantially normal manner. The question at once arises whether this 

 fasting katabolism represents the amount of digestible protein which 

 must be supplied in the feed in order to maintain the protein tissues 

 of the body. 



INFLUENCE OF NONNITROGENOUS MATERIALS. 



In the first place, it is to be remarked that, as just shown, the 

 protein katabolism during fasting is by no means a fixed and definite 

 quantity, but may var}^ even in the same individual within quite 

 wide limits both absolutely and as regards the proportion of the 

 total energy requirement which is supplied by it. From the results 

 cited on pages 12-13, it is evident that a most important factor 

 influencing the fasting katabolism is the stock of fat in the body 

 and that when the latter is reduced protein is katabolized for the 

 sake of its energy. In other words, a lack of readily available non- 

 nitrogenous material in the body tends to increase the protein 

 katabolism above its minimum value. Evidently, then, in seeking 

 to determine the minimum amount of protein required for main- 

 tenance, the food given should contain a liberal supply of non- 

 nitrogenous nutrients to supply the necessary energy for the animal, 

 since otherwise there is danger that the protein will be katabolized 

 for this purpose, resulting in an apparent increase of the maintenance 

 requirement. 



RELATION TO FASTING KATABOLISM. 



In the early experiments upon this subject, especially those of Voit, the full 

 significance of this fact had not been recognized. His experiments, in which 

 increasing amounts of protein alone were fed (compare p. 79), showed that 

 protein equal to two and a half to three times the fasting katabolism was 

 necessary to reach nitrogen equilibrium, and this result was generalized and 

 passed current for a considerable time. 



Munk 1 seems to have been the first to challenge this view and to claim not 

 only that an amount of protein equal to that katabolized during fasting is 

 adequate, but that with an abundant supply of nonnitrogenous material, espe- 

 cially carbohydrates, in the feed a notably smaller amount of protein is suffi- 

 cient to maintain the nitrogen balance. Munk's experiments either include no 

 comparison with the fasting katabolism of the same animal or a comparison 

 not in all respects satisfactory, but they show clearly that nitrogen equilibrium 

 was maintained on a supply of protein less than that usually found to be 

 katabolized in similar fasting animals. 



On the other hand, extensive experiments by Voit and Korkunoff 2 on dogs 

 led these experimenters to an opposite conclusion. Starting with a ration 

 deficient in protein but containing a very liberal supply of nonnitrogenous 

 nutrients, the protein of the feed was gradually increased until an amount was 



Wirchow's Archiv fur Pathologische Anatomie und Phj T siologie und fur Klinische Medi- 

 zin, vol. 101, p. 91 ; vol. 133, Supp. ; vol. 132, p. 91. Archiv fur (Anatomie und) Pbysi- 

 ologie, 1896, p. 183. 



2 Zeitschrift fur Biologie, vol. 32, p. 58 



