230 ANNUAL, REPOET SMITHSONIAN INSTITUTION, 1923 



for children and convalescents. It is perhaps no exaggeration to 

 say that the importance of animal proteins in the diet resides as 

 much in their capacity of supplementing cereal and legume proteins 

 as in their own excellence. 



As mentioned above, our nitrogen balance data indicate a better 

 utilization of protein the lower the level at which it is fed, until at 

 the maintenance level proteins, with few exceptions, are all well 

 utilized. Confirmation of this view may be found in the work of 

 Osborne and Mendel. A practical corollary following from this 

 statement is that with adult animals, to which high protein diets 

 are not essential, the kind of protein consumed would seem to be 

 largely a matter of indifference. For men, the protein minimum is 

 something like 30 grams per day for a man of average weight. 

 Such a man consumes, however, close to 100 grams of protein per 

 day, so that even though the digestibility and the quality of the 

 dietary protein were relatively poor, there would be little danger of 

 a protein deficiency if the values given above are correct. If the 

 man were consuming even much less protein than 100 grams, there 

 would be a tendency for the smaller amounts to be digested more 

 completely and subsequently utilized more completely in metabolism, 

 so that here also the danger of a protein deficiency would seem to 

 be remote. The theory that pellagra is a disease involving pri- 

 marily a deficiency of protein, therefore, finds no support from 

 such considerations. These statements assume, of course, that the 

 satisfaction of protein requirements is simply a matter involving 

 the chemical adequacy of the dietary proteins and the amount 

 consumed. 



Obviously, with growing children and nursing mothers, the qual- 

 ity and quantity of protein consumed is a matter of far greater 

 moment, and until reliable information as to protein requirements 

 in such cases is at hand, the safe procedure would be to provide 

 liberally with proteins of good quality and of good supplementing 

 capacities. Such proteins are contained in milk, meat, fish, and 

 eggs. The presence of these foods in the diet will permit the use 

 of considerable amounts of other foods whose proteins are poor in 

 quality and low in concentration, without reducing the net value of 

 the mixed proteins of the diet to the danger point. The legumes, 

 with the exception of soy beans, while high in protein, are not suit- 

 able foods for this purpose, on account of the relatively low value 

 of their proteins, both as regards digestibility and subsequent utiliza- 

 tion. With beans, again excepting the soy bean, the amino acid 

 cystine seems to be the factor limiting the biological value. One 

 would expect, therefore, that meat proteins or egg proteins, com- 



