THE CONTEXT
Vitamins are defined by their small dietary requirement and their role in maintaining normal metabolic integrity, although some can be formed in the body.
Definition and deficiency
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A vitamin is an organic compound required in the diet in small amounts to maintain normal metabolic integrity.
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Vitamin deficiency causes a specific disease that is cured or prevented only by restoring the vitamin to the diet.
Exceptions to the strict definition
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Vitamin D can be formed in the skin from 7-dehydrocholesterol on exposure to sunlight.
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Niacin can be formed from the essential amino acid tryptophan.
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Because they can be formed in the body, vitamin D and niacin do not strictly comply with the stated definition.