Both active and passive transport
Folic acid is absorbed in the proximal jejunum through both active and passive transport. First step in the absorption of folate involves hydrolysis of pteroyl polyglutamates into pteroyl monoglutamate by conjugase enzyme located in the intestinal mucosa. Next step involves absorption of pteroyl monoglutamate through active carrier mediated transport. Besides this, pharmacologic doses of folate are also absorbed passively by facilitated diffusion.
- After absorption from the lumen into the enterocyte intracellular enzymes reduce and methylate the monoglutamate to produce metabolically active form of folic acid. This form is then absorbed into circulation by simple diffusion.
- Most of the vitamins are absorbed passively. Cyanocobalamin is the only vitamin that that depends on active transport for its absorption. Folic acid and thiamine are the only other vitamins for which active mechanism of absorption is known.