Abstract
Vitamin B12 or cobalamin is a vitamin that can only be synthesized by microorganisms which in humans are localized in the colon where B12 cannot be taken up by the organism. Thus, cobalamin has to be taken up by the food and only animal derived food delivers sufficient amounts. In addition, absorption of B12 needs a cofactor from gastric parietal cells, the intrinsic factor. The coenzyme-forms of cobalamin are mainly involved in the degradation of the amino acid leucine, the degradation of propionic acid and the re-methylation of homocysteine to methionine. As high levels of homocysteine are regarded as a risk factor for several diseases, the latter function of B12 – together with folate and vitamin B6 – thus helps reducing this risk. Best sources are liver, mackerel and herring, but also some dairy products contain acceptable amounts of B12. Deficiencies are mostly due to a reduced intestinal resorption because of mainly gastric or intestinal malfunctions, symptoms are macrocytic hyperchromic anemia and funicular myelitis. A hypervitaminosis due to a supplementation of up to 10mgday−1 have not been found.
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