Vitamin B12 absorption occurs in
Vitamin B12 is absorbed bound to intrinsic factor, a small glycoprotein secreted by the parietal cells of the gastric mucosa. Gastric acid and pepsin release the vitamin from protein binding in food and make it available to bind to cobalophilin, a binding protein secreted in the saliva. In the duodenum, cobalophilin is hydrolyzed, releasing the vitamin for binding to intrinsic factor. Pancreatic insufficiency can, therefore, be a factor in the development of vitamin B12 deficiency, resulting in the excretion of cobalophilin-bound vitamin B12. Intrinsic factor binds only the active vitamin B12 vitamers and not other corrinoids. Vitamin B12 is absorbed from the distal third of the ileum receptors that bind the intrinsic factor-vitamin B12 complex, but not the free intrinsic factor or free vitamin. There is considerable enterohepatic circulation of vitamin B12, with excretion in the bile, then reabsorption after binding to intrinsic factor in the ileum. Reference: Harper; 30th edition; Page no: 558
Ileum
Jejunum
Duodenum
Colon
vitamins