The effect of gelatin on the transformation of tryptophan to niacin in rats on low casein diets.

It was demonstrated by Elvehjem and collaborators (l-4) that, if to a niacin-free diet containing 9 per cent casein and permitting moderate growth there was added an excess of tryptophan-deficient proteins (corn, gelatin, acid hydrolysates of proteins) or certain amino acids (glycine, threonine, phenylalanine), growth of young rats was impaired or stopped. Furthermore, this effect could be abolished equally well by the addition to the diet of approximately 1.5 mg. per cent of niacin or of 50 mg. per cent of tryptophan. This observation appears to imply that the superimposition of certain amino acids upon a diet containing barely adequate amounts of complete protein resulted in a niacin deficiency. The question which still remained to be solved was whether the added amino acids raised the quantitative requirements for niacin, or, since tryptophan in the casein is the only known source of niacin in this diet, whether the transformation of tryptophan to niacin is impaired under these conditions. This question is partly answered by the data of Elvehjem et al. (4) who showed that the growth of rats receiving the basal 9 per cent casein diet amounted to 12 gm. per week, while the gain in rats receiving the same diet plus 1.5 mg. per cent of niacin was 16 gm. per week. Other data in the published studies of the Wisconsin group and our own unpublished data support the view that niacin added to a low protein diet enhances growth to a small extent, probably sparing a part of the food tryptophan as well as supplying the vitamin. This is further borne out by the growth data on the rats receiving 9 per cent casein plus added amino acids in which the addition of 1.5 mg. per cent of niacin restored, or even improved, the rate of growth, as compared with animals receiving the basal diet and the added niacin only ((4) Table I).