Cobalamins comprise a family of compounds which share a complex structure. Vitamin B12 is known as cj/anocobalamin because when originally isolated, an in-vitro artefact had placed a cyan group in the cobalt (3 position. Vitamin B)2 is an active cellular coenzyme essential for demethy-

lation of tetrahydrofolate and thus for DNA synthesis. Animals cannot synthesise cobalamin and so are directly or indirectly dependent upon microorganisms for it. Cobalamin is produced in nature only by cobalamin-producing microorganisms, and herbivores obtain their supply from plants contaminated with bacteria and faeces. Carnivores obtain their supply by ingesting the muscular and parenchymal tissues of these animals. Animal protein is the major dietary source of cobalamin in man. Although bacteria in the human colon synthesise cobalamin, it is formed too distally for absorption by the ileal transport system. Rabbits in the wild would suffer from B12 deficiency if they did not eat their own faeces.

In the presence of intrinsic factor about 70% of ingested cobalamin is absorbed, in its absence < 2% is absorbed. Some cyanocobalamin may be absorbed by passive diffusion, i.e. independently of intrinsic factor, though less reliably and only with large doses.

Dietary deficiency is virtually confined to people too impoverished to buy meat, and to Vegans, a sect of particularly uncompromising vegetarians.

Deficiency of vitamin B12 in the body leads to:

• Megaloblastic anaemia

• Degeneration of the brain, spinal cord (subacute combined degeneration) and peripheral nerves; symptoms may be psychiatric and physical

• Abnormalities of epithelial tissue, particularly of the alimentary tract, e.g. sore tongue and malabsorption.

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