Because it is fat soluble, dietary vitamin D (either D2 or D3) is absorbed with other dietary fats in the small intestine. How much vitamin D is absorbed depends upon the presence of fat in the lumen, which triggers the release of bile acids and pancreatic lipase. Then, bile acids initiate the emulsification of lipids. Pancreatic lipase hydrolyzes the triglycerides into monoglycerides and free fatty acids, and bile acids support the formation of lipid-containing micelles, which diffuse into enterocytes.
The products of vitamin D metabolism are excreted through the bile into the feces, and a small amount is eliminated through the urine. This is due to renal reuptake of vitamin D metabolites bound to DBP (vitamin D binding protein).
