4. Enteric Absorption of Vitamin D
Micelle-Dependent Passive Diffusion
Vitamin D is absorbed from the small intestine by nonsaturable passive diffusion that is dependent on micellar solubilization and, hence, the presence of fat39 and bile salts. The fastest absorption appears to be in the upper portions of the small intestine (duodenum and ileum); however, the greatest amount of vitamin D is probably absorbed in the distal region where food has a longer transit time. Like other hydrophobic substances absorbed by micelle-dependent diffusion in mammals, vitamin D enters the lymphatic circulation40 predominantly (90% of the absorbed amount) in association with chylomicra (or portomicra), with most of the balance being associated with α-globulins.41 The efficiency of enteric absorption of dietary vitamin D appears to be about 50%. Newly absorbed vitamin D is released by enterocytes into the lymphatic circulation in chylomicra. The polar metabolites (25-OH-D; 1,25-[OH]2-D) appear to be transported by DBP (vitamin D-binding protein) in the portal blood; only small amounts (13% of 25-OH-D; 1% of 1,25-[OH]2-D) appear in the lymph in chylomicra.
