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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. [14]
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.
The length of the small intestine can vary greatly, from as short as 3 metres (10 feet) to as long as 10.5 m (34 + 1 ⁄ 2 ft), also depending on the measuring technique used. [3] The typical length in a living person is 3–5 m (10– 16 + 1 ⁄ 2 ft).
The National Institutes of Health recommends that the average man consume a mere 2.4 micrograms a day, which is actually the lowest daily recommended dose of any vitamin. “Vitamin B12 ...
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.
Vitamin B 12 cannot be produced by the human body, and must be obtained from the diet. [2] The body normally gets enough vitamin B 12 from the consumption of foods from animal sources. [2] Inadequate dietary intake of animal products such as eggs, meat, milk, fish, fowl (and some types of edible algae) can result in a deficiency state. [135]
Because they are soluble in water, excess B vitamins are generally readily excreted, although individual absorption, use and metabolism may vary. [7] The elderly and athletes may need to supplement their intake of B 12 and other B vitamins due to problems in absorption and increased needs for energy production.
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.