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vitamin b 12 deficiency

A nutritional condition produced by a deficiency of vitamin b 12 in the diet, characterised by megaloblastic anaemia. Since vitamin b 12 is not present in plants, humans have obtained their supply from animal products, from multivitamin supplements in the form of pills, and as additives to food preparations. A wide variety of neuropsychiatric abnormalities is also seen in vitamin b 12 deficiency and appears to be due to an undefined defect involving myelin synthesis.