Jejunum folic acid absorption

Unaltered folic acid is readily and completely absorbed in the proximal jejunum. Dietary folates, however, consist primarily of polyglutamate forms of N 5-methyltetrahydrofolate. Before absorption, all but one of the glutamyl residues of the polyglutamates must be hydrolyzed by the enzyme -1-glutamyl transferase ("conjugase") within the brush border of the intestinal mucosa. 

Occasionally, protein hinding of a vitamin on foods increases its absorption and hence its biological availability. For example, folate from milk is considerably better absorbed than that from either mixed food folates or free folic acid (Section 10.2.1). Folate bound to a specific binding protein in milk is absorbed in the Ueum, whereas free folate monoglutamate is absorbed in the (smaller) jejunum... 

ABSORPTION, DISTRIBUTION, AND ELIMINATION As with vitamin Bj, the diagnosis and management of folic acid deficiency depend on an understanding of the transport pathways and intracellular metabolism of the vitamin (Figure 53-10). Folates present in food are largely in the form of reduced polyglutamates, and absorption requires transport and the action of a pteroyl-glutamyl carboxypeptidase associated with mucosal cell membranes. The mucosae of the duodenum and upper part of the jejunum are rich in dihydrofolate reductase and can methylate most or aU of the reduced folate that is absorbed. Since most absorption occurs in the proximal small intestine, it is not unusual for folate deficiency to occur with jejunal disease. Both nontropical and tropical sprues are common causes of folate deficiency. 

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