FACTORS INFLUENCING B VITAMIN REQUIREMENTS 281 



of tryptophan is as a metabolic precursor of nicotinic acid. The mode of 

 conversion is by no means known at this time, however, nor is there 

 unequivocal reason to select the indole-nitrogen over the a-amino-nitrogen 

 as the precursor of that nitrogen atom which occurs in niacin (see p. 356) . 



Recent studies have indicated that the tryptophan-nicotinic acid rela- 

 tionship holds with a variety of other species. The lack of growth of 

 guinea pigs on a corn-soybean oil meal-alfalfa ration is reversed by 

 nicotinic acid or tryptophan. 83 Chicks on a 10 per cent gelatin diet exhibit 

 depressed growth rates which are corrected by 5 mg of nicotinic acid or 

 200 mg of DL-tryptophan per 100 gm of diet. 84 Krehl has found 77 that 

 for rats 1 to 1.5 mg of nicotinic acid are equivalent to 50 mg of tryptophan 

 per 100 gm of diet. Monkeys, however, develop a deficiency on such diets, 

 which is corrected by neither nicotinic acid nor tryptophan, nor both, but 

 by liver powder or by lyophilized liver. 85 Various other workers have also 

 recently produced a variety of evidence to show that man does actually 

 convert tryptophan into nicotinic acid 8687 (see p. 354). Present indica- 

 tions are that neither indole, anthranilic acid, nor indoleacetic acid may 

 serve in lieu of tryptophan as a nicotinic acid precursor. 88 Additional 

 data bearing on the problem of the conversion of tryptophan to nicotinic 

 acid will be found in the discussion of nicotinic acid metabolism in Chap- 

 ters V A and IV C. Without further consideration it seems fair to assume 

 that, for most higher animals at least, the nicotinic acid requirement can 

 be stated only in the light of some estimate of the tryptophan intake. 

 Frazier and Friedemann 89 have shown that on corn-free diets the human 

 requirement may be as low as 4 mg/day of nicotinic acid, but that this 

 is increased to about 5 mg/day by corn when there is a high vitamin and 

 protein level, and to 7.5 mg/day when vitamin and protein levels are 

 lower. 



Of considerable interest is the recent report 90 that bacteriologically 

 sterile Drosophila melanogaster require both tryptophan and niacin, and 

 that the niacin requirement is increased by higher tryptophan levels in 

 the diet. This indicates that there may be wide variation in the animal 

 kingdom with regard to this interrelationship, and to some extent tends to 

 suggest the theory that there is at least contributory intestinal synthesis 

 of nicotinic acid by bacteria. Other recent work showing that sulfonamides 

 decrease the excretion of nicotinic acid metabolites 91 has further brought 

 this earlier hypothesis into prominence again. Nutritional studies with 

 "germ-free animals" (p. 300) may do much to resolve this dilemma, and 

 it seems probable that both factors may eventually be shown to be 

 involved. 



Vitamin Interrelationships. Strangely enough, little is as yet known 

 about the effects of vitamins on the requirements of other vitamins. As 



