142 THE AMINO ACIDS 



of a well-nigh doubled life, and lose, instead of gaining, 

 weight, the utilization of the constituent would seem to 

 be of some direct and specific nature." These words 

 give the first definite suggestion that individual amino 

 acids may play a specific role in the maintenance of 

 nutritional rhythm. 



The failure of zein as a suitable source for the 

 essential nitrogen requirement leads to the query 

 whether any single protein will suffice in this respect. 

 Attempts to answer this question have been many and 

 it is only recently that a satisfactory positive reply has 

 been given. In many of the older experiments lack of 

 success has been attributed to various factors other 

 than the character of the protein, and where appar- 

 ently successful results have been obtained criticism 

 has been pertinent in that, in most instances, the protein 

 or proteins employed have not been free from impuri- 

 ties. The general impression gained from this type 

 of investigation has been that sooner or later animals 

 die when kept for a prolonged period upon a con- 

 stant diet even though an abundance of energy 

 producing material may be present. A so-called 

 "pure" diet has been deemed impracticable. Lunin, one 

 of the early investigators of the problem, fed mice 

 with mixtures of casein, fat, cane sugar, and milk 

 ash. On this artificial diet death occurred in from 

 twenty to thirty days, a survival period greater than 

 when the ash of milk was omitted. Mice fed dried 

 milk were alive at the end of two months. Hall with 

 mice and Steinitz with dogs obtained comparable 



