Vitamin B12 is a very complex molecule. Besides cobalt, it also contains carbon, oxygen, phosphorus, and nitrogen. Cobalamin is stable to heat, though sensitive in heated acid or alkali solution, slightly sensitive to light, and destroyed by oxidizing and reducing agents and by some heavy metals.

Vitamin B12 was isolated in 1926 as the factor that treated a feared disease, pernicious anemia--termed "pernicious" because it could be fatal, most often from neurologic degeneration. But the substance cobalamin, when given orally (actually liver was used as the cure; it contains high amounts of B12), did not cure all of the people with the disease, and some people still developed pernicious anemia. It was later found that a mucoprotein enzyme produced by the stomach (by the parietal cells that also make hydrochloric acid) was also needed for vitamin B12 to be absorbed into the body from the intestines. This enzyme has been termed the "intrinsic factor," while vitamin B12 is the "extrinsic factor." Aging, stress, and problems with the stomach or stomach surgery weaken the body's ability to produce the "intrinsic factor"; also, some people appear to have a genetic predisposition that makes them more prone to pernicious anemia. Hydrochloric acid helps the absorption of B12; if acid production is weak, the absorption is lessened. Calcium and thyroid hormone assist as well. Pregnancy, absorb this important vitamin. Aging more likely lessens some of the many factors needed for ideal absorption of B12, so deficiency symptoms are more common in older people.

Cobalamin is absorbed primarily from the last part of the small intestine, the ileum. In the blood, it is bound to a protein globulin to be carried to the various tissues. The body actually stores vitamin B12, so any deficiencies may take several years to develop. The highest concentrations of B12 are found in the liver, heart, kidney, pancreas, brain, testes, blood, and bone marrow--all active metabolic tissues. The "red vitamin" is very important to the blood.

Cobalamin is made in nature by microbial synthesis--produced by bacteria in the intestinal tracts of animals and stored in their tissues. Some B12 is made during fermentation of foods as well. Cobalamin is the naturally occurring vitamin B12. Cyanocobalamin, as B12 is often known, is actually the commercial variety of B12 and contains a cyanide molecule attached to the cobalt. B12 is not synthesized but, like penicillin, must be grown in bacteria or molds and then processed. Other forms of B12 include hydroxycobalamin (technically, vitamin B12a), aquacobalamin (vitamin B12b), and nitrocobalamin (vitamin B12c).

Sources: Vitamin B12 is found in significant amounts only in the animal protein foods. B12 is also manufactured by bacteria in the human intestines, but it is not known how much we can naturally absorb and utilize from that source. In general, digestion and absorption must be good for adequate B12 to be obtained. Many laxatives and overuse of antacids can reduce absorption and deplete stores of B12.