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A vitamin is an organic chemical substance not produced by the body internally, but obtained from plant or animal dietary sources, which is required in small amounts for proper function of the enzymes of the body's chemical processes. (It is specifically not one of the required minerals like calcium, phosphorus, iron or zinc, which likewise are essential for enzymatic actions in the body but are differentiated from vitamins.) Lack of a vitamin typically causes a specific deficiency disease which is cured by restoration of normal vitamin levels by supplementation. There is no convincing evidence that very high doses of vitamins (so-called megavitamins) have any beneficial effect on humans. Some vitamins (e.g. vitamin A) are extremely toxic in overdose.