THE PRODUCTION VALUES OF FEEDING STUFFS 679 



be synthesized in the body. In place of a single requirement for 

 protein, it would appear that there must be substituted a num- 

 ber of separate amino acid requirements, a deficiency as regards 

 any one of which may constitute a limiting factor. 



781. Incomplete and unbalanced proteins. As appeared 

 in Chapter I (50) certain vegetable proteins may be classed 

 as incomplete proteins in the sense that they lack entirely 

 one or more of the amino acids characteristic of proteins in 

 general. The classic example of an incomplete protein is 

 gelatin, which lacks ty rosin and tryptophan and which has 

 long been known to be incapable by itself of maintaining the 

 stock of body protein in an animal. A similar case among the 

 vegetable proteins which has been much discussed is the zein 

 of maize, which yields neither lysin, glycin nor tryptophan on 

 hydrolysis and which is incapable of supporting either main- 

 tenance (399) or growth (465). Still another instance is afforded 

 by the gliadin of wheat (465), which lacks lysin and which, 

 while adequate for maintenance, is unable to support growth. 

 Furthermore, the proteins of the cereal grains in general, while 

 not incomplete in the sense of absolutely lacking certain amino 

 acids, may, from the standpoint of animal nutrition, be called 

 unbalanced in that, as compared with the body proteins, they 

 are relatively rich in glutamic acid and therefore correspondingly 

 deficient in other constitutents, including those ingredients 

 which, like lysin in particular, appear to be essential to growth. 

 It appears evident that in the conversion of a unit weight of 

 such a protein into body protein, a considerable portion of the 

 amino acid present in excess must undergo deaminization (233) 

 and be substantially a waste product so far as the protein re- 

 quirement of the body is concerned, although it may of course 

 serve as a source of energy. Quantitative results as to the 

 maximum percentage utilization of individual proteins, how- 

 ever, are not yet available. 



782. Application of results. But while the general validity of 

 the newer point of view seems well established, it does not appear 

 possible as yet to utilize it in establishing net protein values for 

 feeding stuffs comparable to the net energy values discussed in 

 2 of this chapter. For this there are three principal reasons. 



First, sufficient knowledge of the proteins of feeding stuffs 

 is lacking. Although the constituents of a considerable number 



