Vitamins are vital organic micronutrients that are required in our diet because they provide essential enzyme cofactors, and animals have dispensed with the ability to synthesize them. Vitamin B12, or cobalamin, is the most complex of the vitamins, and the elucidation of its physiological role, its structure and its biosynthetic pathways have been the subject of impressive scientific endeavours over the years. Vitamin B12 deficiency leads to increased risk of cardiovascular disease, neurological symptoms and, in the most serious cases, pernicious anaemia. Cobalamin is synthesized only by prokaryotes, so we obtain it second-hand by eating other organisms that have accumulated the vitamin in their tissues. The richest dietary sources are liver, dairy products and also algae, many of which are like animals in that they require an exogenous supply of the vitamin for growth.

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