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In order for a substance to be classified as a vitamin, its deficiency must produce a specific [[disease]]. For example, [[ascorbic acid]] is a vitamin ([[Vitamin C]]) in humans because its absence produces [[scurvy]], a lack of Vitamin D will result in [[rickets]], and [[thiamin]] is a vitamin (Vitamin B1) because its absence produces beriberi. There are exactly thirteen essential vitamins for humans.

In order for a substance to be classified as a vitamin, its deficiency must produce a specific [[disease]]. For example, [[ascorbic acid]] is a vitamin ([[Vitamin C]]) in humans because its absence produces [[scurvy]], a lack of Vitamin D will result in [[rickets]], and thiamin is a vitamin (Vitamin B1) because its absence produces beriberi. There are exactly thirteen essential vitamins for humans.

Other substances needed to sustain life are not vitamins because they are macronutrients (for example, the essential amino acids).<ref>Harrison's Principles of Internal Medicine 16th Edition by Dennis L. Kasper, Eugene Braunwald, Anthony Fauci, and Stephen Hauser, 2004</ref>

Other substances needed to sustain life are not vitamins because they are macronutrients (for example, the essential amino acids).<ref>Harrison's Principles of Internal Medicine 16th Edition by Dennis L. Kasper, Eugene Braunwald, Anthony Fauci, and Stephen Hauser, 2004</ref>