Vitamin B12, also called cobalamin, is a water-soluble vitamin that has a key role in the normal functioning of the brain and nervous system, and the formation of red blood cells. It is one of eight B vitamins. It is involved in the metabolism of every cell of the human body, especially affecting DNA synthesis, fatty acid and amino acid metabolism.
No fungi, plants, nor animals (including humans) are capable of producing vitamin B12. Only bacteria and archaea have the enzymes needed for its synthesis. Proven sources of B12 are animal products (meat, fish, dairy products) and supplements. Some research states that certain non-animal products possibly can be a natural source of B12 because of bacterial symbiosis.
B12 is the largest and most structurally complicated vitamin and can be produced industrially only through a bacterial fermentation-synthesis. This synthetic B12 is used to fortify foods and sold as a dietary supplement.
Vitamin B12 consists of a class of chemically related compounds (vitamers), all of which show pharmacological activity. It contains the biochemically rare element cobalt (chemical symbol Co) positioned in the center of a planar tetra-pyrrole ring called a corrin ring. The vitamer is produced by bacteria as hydroxocobalamin, but conversion between different forms of the vitamin occurs in the body after consumption.
Vitamin B12 was discovered as a result of its relationship to the disease pernicious anemia, an autoimmune disease in which parietal cells of the stomach responsible for secreting intrinsic factor are destroyed; these cells are also responsible for secreting acid in the stomach. Because intrinsic factor is crucial for the normal absorption of B12, its lack in the presence of pernicious anemia causes a vitamin B12 deficiency. Due to impairment of vitamin B12 absorption during aging, people over age 60 are at risk of deficiency.
Foods having rich content of vitamin B12 include clams, organ meats (especially liver) from lamb, veal, beef, and turkey, fish eggs, mackerel, and crab meat.
B12 is produced in nature only by prokaryotes in the form of certain bacteria and archaea; it is not made by any multicellular or single-celled eukaryotes. It is synthesized by some gut bacteria in humans and other animals, but humans cannot absorb the B12made in their guts, as it is made in the colon which is too far from the small intestine, where absorption of B12 occurs.
Animals store vitamin B12 in liver and muscle and some pass the vitamin into their eggs and milk; meat, liver, eggs and milk are therefore sources of the vitamin for other animals, including people.
For humans, the bioavailability from eggs is less than 9%, compared to 40% to 60% from fish, fowl and meat.
Foods fortified with B12 are also dietary sources of the vitamin. Foods for which B12-fortified versions are widely available include breakfast cereals, soy products, energy bars, and nutritional yeast. The UK Vegan Society, the Vegetarian Resource Group, and the Physicians Committee for Responsible Medicine, among others, recommend that every vegan who is not consuming adequate B12 from fortified foods take supplements.
Vitamin B12 is an ingredient in multi-vitamin pills and in some countries used to enrich grain-based foods such as bread and pasta. In the U.S. non-prescription products can be purchased providing up to 5000 µg/serving, and it is a common ingredient in energy drinks and energy shots, usually at many times the recommended dietary allowance of B12. The vitamin can also be a prescription product via injection or other means.
The sublingual route, in which B12 is presumably or supposedly taken in more directly under the tongue, has not proven to be necessary or helpful, even though a number of lozenges, pills, and even a lollipop designed for sublingual absorption are being marketed. A 2003 study found no significant difference in absorption for serum levels from oral versus sublingual delivery of 0.5 mg of cobalamin. Sublingual methods of replacement are effective only because of the typically high doses (0.5 mg), which are swallowed, not because of placement of the tablet. As noted below, such very high doses of oral B12 may be effective as treatments, even if gastro-intestinal tract absorption is impaired by gastric atrophy (pernicious anemia).
Injection and patches are sometimes used if digestive absorption is impaired, but there is evidence that this course of action may not be necessary with modern high-potency oral supplements (such as 0.5–1 mg or more). Even pernicious anemia can be treated entirely by the oral route. These supplements carry such large doses of the vitamin that 1% to 5% of high oral doses of free crystalline B12 is absorbed along the entire intestine by passive diffusion.
If the patient has inborn errors in the methyltransfer pathway (cobalamin C disease, combined methylmalonic aciduria and homocystinuria), treatment with intravenous, intramuscular hydroxocobalamin or transdermal B12 is needed.
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