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Deficiencies of other micronutrients (e.g., pyridoxine and iron) required to convert tryptophan to niacin may also contribute to the appearance of pellagra. Because pellagra is a late and serious manifestation of deficiency, it was determined that the average requirement must exceed the amount required to prevent pellagra.

FACTORS AFFECTING THE NIACIN REQUIREMENT

Bioavailability

Niacin in mature cereal grains is largely bound and thus is only about 30 percent available; alkali treatment of the grain increases the percentage absorbed (Carpenter and Lewin, 1985; Carter and Carpenter, 1982). Niacin in the coenzyme nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NAD/ NADP) form in meats appears to be much more available. Niacin added during enrichment or fortification is in the free form and thus highly available. Foods that contain niacin in the free form include beans and liver. Quantitative data are not available on which to base adjustments for the bioavailability from different types of foods.

The conversion efficiency of tryptophan to niacin, although assumed to be 60:1 (Horwitt et al., 1981), varies depending on a number of dietary and metabolic factors. The requirement for preformed niacin is increased by factors that reduce the conversion of tryptophan to niacin (McCormick, 1988), including low tryptophan intake; carcinoid syndrome in which tryptophan is preferentially oxidized to 5-hydroxytryptophan and serotonin; prolonged treatment with the drug Isoniazid, which competes with pyridoxal 5'-phosphate, a vitamin B6-derived coenzyme required in the tryptophan-to-niacin pathway; and Hartnup’s disease, an autosomal recessive disorder that interferes with the absorption of tryptophan in the intestine and kidney.

The efficiency of the conversion of dietary tryptophan to niacin is decreased by deficiencies of other nutrients (see “Nutrient-Nutrient Interactions”). Conversion efficiency may increase with some dietary restrictions because of changes in activities of pathway enzymes including tryptophan oxygenase, quinolinate phosphoribosyltransferase, and picolinate carboxylase.

The requirement for preformed niacin as a proportion of the total niacin requirement tends to be lower with higher tryptophan intakes (a greater proportion of tryptophan is available for conversion to NAD once protein synthesis needs are met) and pregnancy



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