Vitamin B12

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 present in milk and eggs, and meat. (However, It may also be found in nutritional yeast, a Fungi.) It is needed in only very small amounts -- RDA is 2.4 micrograms.

Pernicious anemia is caused by a persons' body being unable to absorb enough B₁₂ to form healthy red blood cells. B₁₂ is also necessary to maintain the sheath surrounding and protecting nerve fibers.

B₁₂ is a co-enzyme based on cobalamin. There are two forms of B₁₂ that work as co-enzymes (adenosylcobalamin {also called 5'-deoxyadenosylcobalamin} and methylcobalamin), but our bodies can convert most cobalamins into one of these.

Vegan Vitamin B12 has even more information, although I would be very skeptical of any claim that humans 'evolved' as vegans in any way. (Maybe in the future....)

While what Tem42 said is true, that vitamin B12 is only found in animal products (and is therefore a stumbling block in pure vegan diets), the human body is extremely good as sequestering this stuff.

The average adult Westerner eating a normal diet will have several years worth of vitamin B12 stored in his/her liver and other places in their body. It will take several years for the body to deplete its stores of vitamin B12 after starting a strict vegan diet.

Deficiency of vitamin B12 is a serious condition which can result in all sorts of neurological problems including weakness and cognitive deficits. Vitamin B12 deficiency is also a differential diagnosis of macrocytic anaemia.

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.