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Vitamin B 12 is absorbed by a B 12-specific transport proteins or via passive diffusion. [12] Transport-mediated absorption and tissue delivery is a complex process involving three transport proteins: haptocorrin (HC), intrinsic factor (IF) and transcobalamin II (TC2), and respective membrane receptor proteins. HC is present in saliva.
In pigs it is obtained from the pylorus and beginning of the duodenum; [12] in human beings it is present in the fundus and body of the stomach. [ 13 ] The limited amount of normal human gastric intrinsic factor limits normal efficient absorption of B 12 to about 2 μg per meal, a nominally adequate intake of B 12 .
The same cells in the stomach that produce gastric hydrochloric acid, the parietal cells, also produce a molecule called the intrinsic factor (IF), which binds the B 12 after its release from haptocorrin by digestion, and without which only 1% of vitamin B 12 is absorbed. Intrinsic factor (IF) is a glycoprotein, with a molecular weight of 45 kDa.
Iron is absorbed in the duodenum. Folate (Vitamin B9) is absorbed in the duodenum and jejunum. Vitamin B12 and bile salts are absorbed in the terminal ileum. Vitamin B12 will only be absorbed by the ileum after binding to a protein known as intrinsic factor. Water is absorbed by osmosis and lipids by passive diffusion throughout the small ...
One of the superpowers of vitamin B12 is that it is the only water-soluble vitamin that can be stored in the human body, Kitchens points out, and it can remain stored in the liver for several years.
Its main function is to absorb vitamin B 12, bile salts, and whatever products of digestion that were not absorbed by the jejunum. The ileum follows the duodenum and jejunum and is separated from the cecum by the ileocecal valve (ICV). In humans, the ileum is about 2–4 m long, and the pH is usually between 7 and 8 (neutral or slightly basic).
That includes folks with an autoimmune disorder called pernicious anemia, who are unable to absorb B12, and people who take a lot of over-the-counter stomach-acid reducers, like famotidine ...
Transcobalamin I (TC-1), also known as haptocorrin (HC), R-factor, and R-protein is encoded in the human by the TCN1 gene. TC-1 is a glycoprotein produced by the salivary glands of the mouth. It primarily serves to protect cobalamin (Vitamin B12) from acid degradation in the stomach by producing a HC-Vitamin B 12 complex.