Digestion and Absorption of Folates

Because conjugase is a zinc metallo-enzyme, zinc deficiency can impair the absorption of conjugated food folates, but not folate monoglutamate. 

Conjugase responds rapidly to zinc depletion and repletion, and it has been suggested that the absorption of a test dose of folate polyglutamates may provide a sensitive index of zinc nutritional status (Canton and Cremin, 1990). The absorption of folate monoglutamates (from pharmaceutical preparations or foods) is not affected. 

Free folate, released by conjugase action, is absorbed by a carrier-mediated mechanism in the jejunum. However, the folate in mUk is mainly bound to a specific binding protein (which has been used in radioligand binding assays for folate) the protein-folate complex is absorbed intact, mainly in the ileum, by a mechanism that is distinct from the j ejunal transport system for free folate. The biological availability of folate from miLk, or of folate from diets to which mUk has been added, is considerably greater than that of unbound folate, whereas that of folate from cereal foods, or of free folic acid taken with cereal foods, is lower. 

Most of the dietary folate undergoes reduction and methylation within the intestinalmucosa and what enters theportal bloodstream is alargely 5-methyl-tetrahydrofolate. Single doses of more than about 200 /xg of folic acid saturate the intestinal dUiydrofolate reductase, so that free folic acid is absorbed and circulates in the bloodstream. It can be taken up by tissues, reduced to tetrahy-drofolate, and utilized. 

There is considerable enterohepatic circulation of folate, equivalent to about one-third of the dietary intake. Methyl-tetrahydrofolate is secreted in the bUe, then reabsorbed in the jejunum together with food folates. In experimental animals, bUe drainage for 6 hours results in a reduction of serum folate to 30% to 40% of normal (Steinberg et al., 1979). There is very litde loss of folate jejunal absorption is very efficient, and the fecal excretion of 450 nmol (200 /xg) of folates per day largely represents synthesis by intestinal flora and does not reflect intake to any significant extent. 

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