RELATIVE VALUES OF PROTEINS. 



99 



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. 



In the actual maintenance feeding of farm animals, the matter of 

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

 shown that a relative deficiency of protein in the ration tends to 

 depress the apparent digestibility of both the protein and nonnitro- 

 'genous nutrients, especially in the case of ruminants. A maintenance 

 ration for these animals containing the minimum amount of protein, 

 together with the quantities of nonnitrogenous nutrients required to 

 maintain the energy supply, would have a nutritive ratio, computed 

 in the ordinary way, of approximately 1 : 12. On such a ration, there 

 would, in all probability, be some loss of digestibility. An increase 

 of its protein by 50 per cent would very probably effect a gain in 

 digestibility which would more than offset the increased cost, if any. 

 Indeed, unless feeds especially poor in protein are used, it may often 

 be difficult, even if desirable, to reduce the protein content of a main- 

 tenance ration to the low level of absolute necessity. 



RELATIVE VALUES OF PROTEINS. 



In the discussions of the foregoing paragraphs, following the usual 

 practice, the word protein has been used as if it designated a single 

 chemical individual. In reality, of course, this is very far from 

 being the case. The protein of the body or of the feed in this con- 

 ventional sense includes a large number of distinct and in some re- 

 spects, widely differing proteins. The studies of the chemical struc- 

 ture of the protein molecule made in recent years, beginning with the 

 fundamental investigations of Emil Fischer, have shown marked dif- 

 ferences in the proportions of the various "building stones" (amino- 

 acids, etc.) contained in different proteins, while studies in immunity 

 have led to the recognition of marked specific and individual bio- 

 logical differences in animal proteins, although these have not been 

 definitely correlated with differences of chemical constitution. It is 

 pertinent to inquire, therefore, whether we are justified in discussing 

 the nutritive functions of feed protein as a group or whether we must 

 consider each individual protein by itself. In other words, are there 

 recognizable differences in nutritive value between individual pro- 

 teins ? 



DIFFERENCES IN CONSTITUTION OF PROTEINS. 



In discussions of this question, the chief emphasis has been laid 

 upon the demonstrated differences in the proportions of the various 

 cleavage products yielded by the different proteins when subjected 



