Food folates mainly consist of reduced polygluta-mates, which are hydrolyzed to monoglutamates in the gut prior to absorption across the intestinal mucosa. The conjugase enzyme that hydrolyzes dietary folates has been found on the luminal brush border membrane in the human jejunum and has equal affinity for polyglutamates of various chain lengths. Transport is facilitated by a saturable carrier-mediated uptake system, although changes in luminal pH and the presence of conjugase inhibitors, folate binders, or other food components can adversely affect the rate of hydrolysis and intestinal absorption. Such factors account for the wide variation in the bioavailabil-ity of the vitamin from foods of plant and animal origins. Some metabolism of the resultant monoglutamate, mainly to 5-methylTHF, appears to occur during the absorption process, though this may not be necessary for transport across the basolateral membrane of the intestinal mucosa into the portal circulation. The degree of metabolic conversion of dietary folic acid depends on the dose; pharmacological amounts are transported unaltered into the circulation.
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