Intestinal absorption of vitamins.

This article provides an overview of advances in understanding the cellular and molecular mechanisms and regulation of intestinal absorption processes of vitamins. The vitamins covered are the water-soluble vitamins folic acid, cobalamin (vitamin B12), biotin, pantothenic acid, and thiamine (vitamin B1) and the lipid-soluble vitamin A. For folate, significant advances have been made in regard to i) digestion of dietary folate polyglutamates to folate monoglutamates by the cloning of the responsible enzyme; ii) identification of the cDNA responsible for the intestinal folate transporter; iii) delineation of intracellular mechanisms that regulate small intestinal folate uptake; and iv) identification and characterization of a specific, pH-dependent, carrier-mediated system for folate uptake at the luminal (apical) membrane of human colonocytes. Studies on cobalamine have focused on cellular and molecular characterization of the intrinsic factor and its receptor. Studies on biotin transport in the small intestine have shown that the uptake process is shared by another water-soluble vitamin, pantothenic acid. Furthermore, a Na-dependent, carrier-mediated biotin uptake system that is also shared with pantothenic acid has been identified at the apical membrane of human colonocytes. This carrier is believed to be responsible for the absorption of the bacterially synthesized biotin and pantothenic acid in the large intestine. Also, preliminary studies have reported the cloning of a biotin transporter from the small intestine. As for thiamine intestinal transport, a study has shown thiamine uptake by small intestinal biopsy specimens to be via a carrier-mediated, Na-independent mechanism, which appears to be up-regulated in thiamine deficiency. Studies on vitamin A intestinal absorption have shown the existence of a receptor-mediated mechanism for the uptake of retinol bound to retinol-binding protein in the small intestine of suckling rats. Another study has shown that retinoic acid increases the mRNA level of the cellular retinol binding protein II and the rate of retinol uptake by Caco-2 intestinal epithelial cells. The study suggested that retinoids may play a role in the regulation of vitamin A intestinal absorption.