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vitamin B12one of the B-COMPLEX of water-soluble vitamins, containing cobalt and required for red blood cell formation. A deficiency of B12 can lead to pernicious anaemia especially amongst old people. The vitamin is synthesized by bacteria in the gut rather than being obtained from the diet, and is required in very tiny amounts. The richest food source is raw liver.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005