On the basis of depletion/repletion studies in which the urinary excretion of niacin metabolites was measured after feeding tryptophan or preformed niacin, the average requirement for niacin is 1.3 mg of niacin equivalents/MJ energy expenditure, and reference intakes are based on 1.6 mg/MJ.
Average intakes of tryptophan in Western diets will more than meet requirements without the need for a dietary source of preformed niacin.
Although the nicotinamide nucleotide coenzymes function in a large number of oxidation and reduc-tion reactions, this cannot be exploited as a means of assessing the state of the body’s niacin reserves, because the coenzymes are not firmly attached to their apoenzymes, as are thiamin pyrophosphate, riboflavin, and pyridoxal phosphate, but act as cosub-strates of the reactions, binding to and leaving the enzyme as the reaction proceeds. No specific meta-bolic lesions associated with NAD(P) depletion have been identified.
The two methods of assessing niacin nutritional status are measurement of the ratio of NAD/ NADP in red blood cells and the urinary excretion of niacin metabolites, neither of which is wholly satisfactory.