Intakes of less than 7.50 mg niacin per day have been associated with occurrence of pellagra but these studies were made before the contribution of tryptophan to the formation of niacin was fully acknowledged (WHO, 1967). It has been shown that 60 mg of tryptophan can be converted by the human body into approximately 1 mg of nicotinic acid (Goldsmith et al., 1961). In recognition of the contribution of tryptophan to niacin nutriture the term “niacin equivalent” was introduced, which permitted the calculation of the combined effects of both niacin and tryptophan.

Essentially all the information used in estimating niacin requirements comes from studies conducted more than 30 years ago on adult men and women who were fed diets deficient in niacin equivalents but which were otherwise complete (Goldsmith et al., 1952; Horwitt et al., 1956). The studies compared the intake of niacin equivalents to the incidence of pellagra and showed that 4.4 niacin equivalents per 1000 kcal or 9.2 to 12.3 niacin equivalents daily for 38 to 87 weeks was the minimum requirement for the prevention of the clinical deficiency in adults (Annex, Table A).

Depletion-repletion studies further showed that an intake of 5.5 niacin equivalents per 1000 calories per day was an intake at which no clinical signs were observed and at which some of the subjects showed an increase in urinary excretion of niacin metabolites. The inclusion of a factor to account for individual variation led to a recommended intake of 6.6 niacin equivalents per 1000 kcal per day.