In the less acidic environment of the small intestine, pancreatic enzymes digest the glycoprotein carrier and vitamin B12 can then bind to intrinsic factor. This new complex is then absorbed by the epithelial cells (enterocytes) of the ileum. Inside the cells, vitamin B12 dissociates once again and binds to another protein, transcobalamin II; the new complex can then exit the epithelial cells to be carried to the liver.
Intrinsic factor is secreted by parietal cells within the stomach, and so is present in the gastric juice as well as in the gastric mucous membrane. The optimum pH for its action is approximately 7. Its concentration does not correlate with the amount of HCl or pepsin in the gastric juice, e.g., intrinsic factor may be present even when pepsin is largely absent. The site of formation of the intrinsic factor varies in different species. In pigs it is obtained from the pylorus and beginning of the duodenum; in human beings it is present in the fundus and body of the stomach.
The limited amount of normal human gastric intrinsic factor limits normal efficient absorption of B12 to about 2 μg per meal, a nominally adequate intake of B12.
In most countries, intramuscular injections of vitamin B12 are used to treat pernicious anemia. Orally administered vitamin B12 is absorbed without intrinsic factor, but at levels of less than one percent than if intrinsic factor is present. Despite the low amounts absorbed, oral vitamin B12 therapy is effective at reducing symptoms of pernicious anemia. Vitamin B12 can also be given sublingually, but there is no evidence that this route of administration is superior to the oral route, and only Canada and Sweden routinely prescribe this route of administration.
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