Effect of Dietary Tryptophan Imbalance upon the Metabolism of Tryptophan and Nicotinic Acid in Man*

Growth of animals receiving diets low in protein and nicotinic acid may be retarded by the addition of corn products (l-4), of certain amino acids (5-9), or of proteins deficient in tryptophan (8-11). This inhibition, which is obtained with diets relatively deficient in tryptophan, can be overcome by the administration of nicotinic acid or of tryptophan (l-11). It has been suggested that this dietary amino acid imbalance is an important pellagragenic factor (5-l 1). In the present experiments a dietary amino acid imbalance (tryptophandeficient) was created in man by the addition of gelatin or glycine to an otherwise constant diet, and the effect upon the excretion of nicotinic acid and tryptophan metabolites was studied. Since these experiments have been carried out, other workers have shown that the addition of gelatin to the casein diet of rats had little effect upon N1-methylnicotinamide (Nl-Me) excretion, although the dietary change inhibited the growth of the animals (12, 13). Previous work on man has shown that the replacement of part of the wheat in a diet by corn products resulted in a decreased excretion of N1-Me (14, 15). It could not be determined from the data whether this was due in part to the presence in corn of an inhibitory agent (16) which interfered with the metabolism of tryptophan or nicotinic acid, or whether it was due entirely to the lower tryptophan and protein content of the corn diet compared with the wheat diet. In the present studies on the effect of corn products, the tryptophan and protein levels of the corn and wheat diets were maintained at more comparable levels.