THE PRODUCTION VALUES OF FEEDING STUFFS 685. 



protein nutrition, however, it has become increasingly evident 

 that many of these earlier results are of little real significance 

 and that the question of the nutritive value of non-protein must 

 be approached from a different standpoint. It has become 

 evident, for example, that attempts to replace proteins com- 

 pletely by a single amino acid or even by two or three of them 

 must necessarily fail, since the formation of body protein re- 

 quires the presence of all its constituent building stones in 

 proper proportions. For the same reason the addition of an 

 amino acid to a ration can be effective only if the proteins of 

 that particular ration happen to be deficient in that one con- 

 stituent. 



Furthermore, experiments with ingredients of the non-protein 

 which do not form part of the protein molecule are of question- 

 able significance. For example, asparagin, which has been a 

 favorite subject of investigation for reasons of convenience, is 

 not found among the cleavage products of the proteins but be- 

 longs to the class of acid amides. So far as appears, it could 

 contribute to the formation of protein only after conversion into 

 the related aspartic acid (47) and it has not yet been shown 

 that the body can undo the amide linkage' of nitrogen. More- 

 over, as appeared in Chapter I (60-67), the non-protein in- 

 cludes, in addition to acid amides like asparagin, a great 

 variety of nitrogenous substances which are but remotely re- 

 lated chemically to the proteins and whose nutritive value is 

 at best doubtful. 



It would appear that the value of the non-protein of a feeding 

 stuff as a source of body protein must be determined by pre- 

 cisely the same thing which is believed to measure the value of 

 an individual protein or of the mixed proteins of feeding stuffs, 

 viz., the kinds and proportions of amino acids which it can 

 yield, since there is no evident reason why an amino acid ex- 

 isting ready formed in a feeding stuff should differ in value from 

 the same substance split off from protein in the process of di- 

 gestion. If this be admitted, however, the distinction made in 

 recent years between protein and non-protein in feeding stuffs 

 becomes rather meaningless. If the value of each is measured by 

 its amino acid content, then what is needed to fix the produc- 

 tion values of feeding stuffs as regards protein is a knowledge of 

 the kinds and amounts of these compounds which the feeding 



