The folate in foods consists of a mixture of the different one-carbon substituted derivatives, with varying numbers of conjugated glutamyl residues. The biological availability of these vitamers differs and is consistently lower than that of free folic acid (pteroyl monoglutamate), which is the compound that
has been used in depletion/repletion studies to determine folate requirements, and is the form used in food fortification. In order to permit calculation of folate intakes in terms of both naturally occurring mixed food folates and added folic acid, 1 fg of dietary folate equivalent has been defined as the sum of fig of food folate + 1.7 x fg of folic acid (Institute of Medicine, 1998).
Most of the dietary folate consists of polyglutamates; a variable amount may be substituted with various one-carbon fragments or be present as dihydrofo-late derivatives. There is little information on either the distribution of folate vitamers in foods or their relative biological activities (Gregory, 2001). Unsub-stituted reduced folates in foods are chemically unstable and readily undergo cleavage to p-aminobenzoic acid and the pteridine; between 50% to 75% of the folate in food may be lost during processing and storage (Scott, 1999).
The growth responses of the microorganisms used for bioassay are different for the different vitamers, and polyglutamates are used by bacteria only after enzymic hydrolysis with conjugase. Endogenous conjugase in foods may cause breakdown of polyglutamates during extraction and sample preparation, whereas autoclaving and the addition of preservatives will also hydrolyze some of the vitamers and may oxidize tetrahydrofolates, so that chromatographic analysis also may not reflect the true distribution of vitamers in foods.
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