MAINTENANCE REQUIREMENTS OF MATTER 331 



the stock of protein in the body and katabolizing the remainder 

 as fuel material. An increase of the protein supply above the 

 minimum causes, after a relatively short time, the main- 

 tenance of the body protein at a higher level (411). The prac- 

 tical question in actual maintenance is far less as to the least 

 amount of protein which may be used than as to the most 

 advantageous level of protein nutrition; that is, as to the 

 optimum of protein. 



This question has been warmly debated in connection with 

 human nutrition, having been brought to the fore especially 

 by the investigations of Chittenden and his associates. 1 



On the whole it cannot be said that a considerable surplus 

 of protein over the minimum requirement for maintenance 

 that is, the maintenance of protein nutrition on a high plane 

 -has been proved to be of any material advantage in the 

 maintenance either of men or of domestic animals during 

 periods covering several months. Whether a continued low 

 protein diet through years or generations would show a different 

 result is at present largely a matter of speculation. It is to be 

 remarked, however, that the particular point under discussion 

 is the protein requirement of the mature organism. That a 

 deficiency of protein in the diet of a growing animal may have 

 disastrous results is clear. If, however, the habitual food supply 

 of a race of men or a group of animals is low in protein, the 

 young are likely to share this deficiency with the mature, and 

 it seems not impossible that this is an important factor in the 

 alleged physical inferiority of certain races of men living on a 

 low protein diet. This consideration warns us to exercise care 

 in this respect in the management of the breeding herd. 



420. Digestibility of low protein rations. In the actual 

 maintenance feeding of farm animals, the matter of the digest- 

 ibility of the ration must also be considered. It has been 

 shown (723, 724) that a relative deficiency of protein in the 

 ration tends to depress the apparent digestibility of both the 

 protein and non-nitrogenous nutrients, especially in the case of 

 ruminants. A maintenance ration for these animals containing 

 the minimum amount of protein together with the quantity 

 of non-nitrogenous nutrients required to maintain the energy 

 supply, would have a nutritive ratio (709) , computed in the ordi- 



1 Physiological Economy in Nutrition. 



