FACTORS INFLUENCING B VITAMIN REQUIREMENTS 279 



out reducing growth below optimum levels, regardless of the methionine 

 present) was about 0.1 per cent of the diet (containing 0.55 per cent 

 L-cystine) and the essential level of methionine was 0.5 per cent of the 

 diet. 



Thus, it seems generally true that the choline requirement is quite 

 dependent upon methionine and serine. A further discussion of these inter- 

 relationships is found in a later chapter. 



Effect of Tryptophan on the Nicotinic Acid Requirement. While in 

 reality a subject belonging to the discussion of the previous section, the 

 effect of tryptophan on the nicotinic acid requirement is of such far- 

 reaching importance that it merits entirely separate treatment. For the 

 sake of clarity of discussion, however, many of the aspects of the closely 

 associated effect of corn on the nicotinic acid requirement will be treated 

 in greater detail in a later section dealing specifically with inhibitory 

 effects. 



Despite the fact that by 1938 the identity of nicotinic acid as the 

 pellagra-preventive vitamin was well established, it was still apparent 

 that other factors were involved in the etiology of this condition. Aykroyd 

 and Swaminathan 72 observed that in Moldavia the staple corn diet sup- 

 plied 15 mg of nicotinic acid daily and that there was endemic pellagra, 

 whereas in southern India the 5 mg/day of nicotinic acid derived from 

 rice diets produced only rare cases. A variety of similar observations led 

 to the conclusion that corn in some manner antagonized the utilization 

 of nicotinic acid. Krehl et al. 73 showed, moreover, that in the rat, which 

 normally is able to synthesize its own supply of nicotinic acid, corn grits 

 almost completely prevented growth, but that this effect was entirely 

 reversed by the addition of 1 mg per cent of nicotinic acid to the diet. 

 Moreover, the nicotinic acid requirement of dogs on purified diets con- 



