Since 1998, at least one large population study (O’Brien et al., 2001) on intake reports that a sizable segment of the population (12.5 percent of men and 20.6 percent of women) had intakes below the Estimated Average Requirement (EAR) in Ireland.

Hustad and coworkers (2000) show that riboflavin is inversely associated with homocysteine concentration in blood. The authors propose that individuals who have a SNP in the flavin-requiring enzyme methylene tetrahydrofolate reductase (MTHFR) are sensitive to riboflavin concentrations. With higher intakes of riboflavin, the homocysteine concentration in their blood decreases.

Flavin-adenine dinucleotide (FAD)-dependent glutathione reductases have been used as the markers for riboflavin sufficiency in setting the EAR for riboflavin. Some work suggests a new functional measure for riboflavin status. In particular, a clinical trial (Jacques et al., 2005) reports a reduction of age-related lens opacification in humans treated with riboflavin supplements. Cataract production was an end point that was not fully considered when setting the EAR for riboflavin, but it probably is of interest in the consideration of revisions to riboflavin requirements.

Research gaps Research gaps particular to niacin include (1) increased niacin requirements secondary to oxidant exposure, (2) the identification of a better method for determining niacin status other than the urinary excretion currently used, and (3) improvement of nutrient databases to differentiate the forms of niacin—specifically the naturally occurring niacin content of foods and niacin added as a fortificant.

Progress made Some progress has been made in addressing niacin research gaps since 1998:

• A toxicology panel was convened; in 2005, it reported on the toxicity and potential toxicity of higher dose niacin (Cosmetic Ingredient Review Expert Panel, 2005). That report discusses possible end points and markers, and it would be useful in reconsidering Tolerable Upper Intake Levels (ULs) for niacin.