Also known as "5'-adenosylcobalamine" for obvious reasons :-). It is essential for making the amino acid methionine, among other things. It is a peculiar compound that was one of the first complex organic structures crystallised. The crystallographer was the famous Dorothy Crowfoot Hodgkin who also produced many protein structures, as well as her more famous role in the determination of DNA structure.

An unusual cobalt-porphyrin compound, it is used quite a lot by methanogenic bacteria (or, rather, analagous cofactors are used) to do tricky carbon bond chemistry. Said to be the only C-M bond in biology, it is also useful for isomerisation.

Also one of the only true hang-over cures. It works better than two gallons of Gatorade, is easier than eating 4 greasy cheeseburgers, and tastes better than bitters and soda.

note: I don't know why, but I have learned not to question certain things.

It is also the only vitamin needed by humans that, to the best of our knowledge, can be gotten only from animals (or pills). It is produced by bacteria present in milk, eggs, and meat. (However, inactive forms of it may also be found in nutritional yeast, soy products, etc.1.) It is needed in only very small amounts -- although the RDA of 2.4 micrograms is generally accepted to be much too low; a daily allowance of 10 micrograms is probably closer to a healthy level.

B12 deficiency was originally defined as levels low enough to cause pernicious anemia. But aside from being necessary to form healthy red blood cells, B12 is also necessary to maintain the sheath surrounding and protecting nerve fibers and is necessary for cellular metabolism in every cell in your body. It is now becoming recognized that healthy levels of B12 cannot be judged on preventing anemia alone.

B12 is a co-enzyme based on cobalamin; it is common to see B12 called cobalamin, but it is important to remember that not all cobalamin is a vitamin -- that is, there are a number of B12 analogues that do not provide a benefit to your body.

Many studies have found that nearly all vegans, vegetarians, and partial-vegetarians have low B12. This begins to have measurable negative effects within one to two months of beginning this sort of diet. In the past it was believed that the body stored B12 for a decade or longer, but it has since been found that this stored B12 is not accessible for all necessary functions. B12 deficiency is also common in the elderly, people on Metformin, and those who take frequent doses of antacids (the acids in the stomach are necessary to activate the B12 in your stomach).

There are no significant risks to taking megadoses of B122, but generally you should be wary of anyone recommending more than 500 micrograms a day (unless they are a doctor). Even that level is generally reserved for people with a B12 deficiency; a healthy young adult should probably aim for a daily intake of about 10 micrograms a day minimum, and perhaps a 250 a day as general maximum.


Footnotes:

1. Studies have found that forms of B12 from yeast and vegetable sources are not usable by most humans. There are however, some people who seem to be able to utilize these sources. It should be noted that these sources can increase vitamin B12 serum levels without reducing some effects of B12 deficiency.

2. Reports of skin conditions have been noted, but are rare.

Meat/dairy is not the only source of B12. In fact, animals do not even produce B12. Certain microorganisms found in soil and the intestinal tract of animals produce B12. The intestines then absorbe the B12 into the flesh for use in the body. Some people even have the right bacteria to produce their own B12 but it is apparently not sufficient to prevent B12 deficiency in most people.(1)

B12 production requires Cobalt (thus the "cobal" in its scientific name Cyanocobalamin)(2) so the presence of B12 producing microogranisms in the intestines may be linked to adequate cobalt in the diet.

There are vegetarians in India who actually have sufficient quantities of the right bacteria in their intestines to produce sufficient quantities of B12 for survival.(3)

The body has the ability to store B12 for future use. People who recently became vegan but ate meat previously often have a large reserve of B12. However, inadequate supply of B12 can cause serious health problems so it is not recommended to rely on this fact.

The quantity of B12 required (2.4mcg) is so little that you could feasibly get your required amount by just eating some good soil or by not washing your (hopefully organic) vegetables. However, there is a pretty severe trade-off with eating dirt since there can be other things in soil that you might NOT want to ingest such as anthrax, leprosy, cholera and other not so friendly microorganisms. For now I will just continue to eat my fortified nutritional yeast.

Organic farming and soil building techniques have been proven to increase the presence of B12 producing microorganisms and these vegetables have been shown to absorb some of the B12.(4) It is not clear if this would be sufficient to survive on but it would be a step up from eating dirt.

References
(1) Albert MJ, Mathan VI, Baker SJ. Vitamin B12 synthesis by human small intestinal bacteria. Nature 1980; 283: 781-782
(2) SOLSTICE magazine #34, Feb. '90 (http://www.sare.org/htdocs/hypermail/html-home/8-html/0278.html)
(3)http://www.mercola.com/2002/jan/30/vitamin_b12_a.htm
(4) Plant and Soil 167:305-311, 1994. Southern Medical Journal 84(1):4-10, 1991.

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