78 



MAINTENANCE .RATIONS OF FARM ANIMALS. 



while at a subsequent period of the day there inay be a deficiency which will 

 be made good by a draft upon the proteins of the tissues. 



For the purpose of this discussion, it is unnecessary to pursue 

 further the somewhat complicated question of the absolute pro- 

 tein minimum and its relations to the fasting protein katabolism. 

 especially in view of the fact that, as has been shown, the latter is 

 itself more or less variable. It appears well established that on a diet 

 containing an abundance of carbohydrates a supply of protein 

 equivalent to the fasting protein katabolism is sufficient to meet the 

 needs of the organism, while it is possible that a less amount 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 1 

 has advanced the explanation that a minimum of carbohydrates is 

 essential 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. In any case, it 

 is clear that the protein requirement upon a mixed ration sufficient 

 in quantity is comparatively small. 



EFFECT OF SURPLUS OF PROTEIN. 

 INCREASES PROTEIN KATABOLISM. 



But while a relatively small quantity of digestible protein is suffi- 

 cient, in the presence of an abundant supply of fuel material, to main- 

 tain the body in nitrogen equilibrium, an increase of the feed protein 

 above this minimum does not result in any large or long- continued 

 gain of protein tissue by the mature animal, but causes a correspond- 

 ing increase in the protein katabolism, as is shown by the prompt 

 increase in the amount of nitrogen excreted in the urine. 



This fact was demonstrated more than 50 years ago by C. Voit, in collabora- 

 tion at first with Bischoff 2 and later alone and with Pettenkofer, 3 in experiments 

 on carnivorous animals, and almost innumerable subsequent investigations have 

 shown that it is true not only of these animals, but of man and of herbivorous 

 animals as well. The protein katabolism is determined chiefly by the supply 

 of digestible protein in the feed, and the body comes quite promptly 

 into equilibrium with any amount above the maintenance requirement which 

 can be consumed, the nitrogen of the excreta substantially equaling that of the 

 feed. This is well illustrated by the following selection from Bischoff and 

 Voit's results upon a dog, 4 arranged in the order of the amount of protein 

 eaten. 



1 Jahresbericht iiber die Forschritte der Tier-Chemie, vol. 32, p. 685. 



2 Gesetze der Ernahrung des Fleischfressers, 1860. 



8 Published chiefly in the Annalen der Chemie und Pharmacie and the Zeitschrift fur 

 Biologie. See also Voit : " Physiologie des Stoffwechsels," in Hermann's Handbuch der 

 lMiysiologie. 



4 Voit's compilation, Zeitschrift fur Biologie, vol. 3, p. 5. 



