Folate bioavailability

Abstract

Bioavailability can be defined as the proportion of an ingested nutrient that is absorbed and becomes available for metabolic processes or storage. Natural food folates are a mixture of reduced forms of the vitamin, typically involving one-carbon substitution of the pteridine ring (most predominantly 5-methyltetrahydrofolate [5-methyl-THF]) and usually in the polyglutamylated form containing a variable number of glutamate residues. The synthetic form, folic acid, is a monoglutamate found in the human diet only in fortifi ed foods and supplements. Whereas natural food folates are labile and prone to oxidative cleavage at the C9-N10 bond [1], folic acid is a fully oxidized molecule and as such is stable and relatively resistant to destruction [2]. These structural differences have a major impact on folate bioavailability, and there is widespread agreement that the bioavailability of folates from natural food sources is incomplete when compared with folic acid. However, published estimates from controlled longterm feeding trials of the extent of lower bioavailability of food folates compared with folic acid (relative bioavailability) vary greatly, ranging between 30% and 98% [3-7]. Thus, there remains uncertainty regarding folate bioavailability [8] and controversy regarding the extent to which the bioavailability of folate from natural food sources is reduced relative to folic acid.