316 NUTRITION OF FARM ANIMALS 



between mixed proteins from different sources as regards their 

 value for maintenance have been shown to exist, they appear 

 in many cases to be less than might be anticipated from the 

 known differences in their chemical constitution. As a matter 

 of practical necessity, then, pending the further investigations 

 so greatly to be desired, the only course open for the present 

 seems to be to follow established custom and deal with the pro- 

 tein of feeding stuffs and rations as a whole, with the conscious- 

 ness that it is of unequal nutritive value in different materials 

 but in the belief that such differences are not in all probability 

 so great as to seriously invalidate the general usefulness of the 

 results. 



Influence of feed supply on protein katabolism 



401. The minimum of feed protein. The physiological 

 minimum (339) below which the protein katabolism of the 

 fasting animal cannot be reduced evidently constitutes a lower 

 limit to the necessary supply of feed protein, but what surplus, 

 if any, above this minimum must be supplied in order to secure 

 actual protein maintenance is still an unsettled question. That 

 the amount of feed protein necessary for maintenance is rel- 

 atively small has been fully demonstrated. It appears to be 

 well established also that on a diet containing an abundance 

 of non-nitrogenous nutrients, especially of carbohydrates, a 

 supply of protein materially less than the protein katabolism 

 during complete fasting is sufficient to meet the needs of the 

 organism, while it is possible that an amount little or no greater 

 than that katabolized when abundance of carbohydrates is 

 consumed will suffice. 



Fats appear to be distinctly less efficient than carbohydrates in 

 keeping the protein katabolism at the minimum. Precisely why this 

 is the case has not been fully made out, although Landergren l has 

 advanced the explanation that a minimum of carbohydrates is essen- 

 tial to the chemical processes of metabolism and that when a sufficient 

 amount is not supplied in the feed, protein is katabolized for the sake 

 of producing carbohydrates, with the result that on a low protein 

 diet nitrogen katabolism is increased. 



1 Jahresber. Tier. Chem., 32 (1903), 685. 



