Niacin-Tryptophan Relationships in Man and Niacin Requirement

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Abstract
Pellagra has been known to be associated with diets high in corn and low in animal protein since it was first described two centuries ago. Recent research had elucidated this relationship, clarified the etiology of pellagra and made possible the estimation of human niacin requirement. Niacin was shown to be the pellagra-preventive vitamin in 1937. Some years later, the amino acid, tryptophan, was found to be a precursor of niacin in many animal species and in man. Administration of tryptophan is followed by an increase in urinary excretion of niacin metabolites. Tryptophan, when given in large doses, is effective in the treatment of pellagra. The pathway of conversion of tryptophan to niacin was elucidated by studies in neurospora; intermediate substances were found to be kynurenine, 3-hydroxykynurenine and 3-hydroxyanthranilic acid. Proof of a similar pathway of conversion in mammals was demonstrated by experiments with C14 labeled tryptophan. Studies in normal adults have shown that an average of 3.3 per cent of administered tryptophan is converted to niacin compounds. On a molar basis, approximately 60 mg of dietary tryptophan appears to be equivalent to 1 mg of niacin. The minimum human requirement for niacin is about 9 to 12 mg daily in adults, including that formed from tryptophan. Requirement appears to be related to body size and to caloric intake. The close association of pellagra with diets high in corn can be explained in large part by the low niacin and tryptophan content of this cereal. Other factors may have a role in the pellagragenic effect of corn but appear to be of minor importance.

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