A vitamin is a chemical substance acting as a catalyst in the body's chemical reaction, commonly as part of metabolism. It must be obtained from dietary or other sources. Consuming vitamins can prevent disease:
|Substance||Disease it prevents|
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).
- ↑ 
- ↑ Harrison's Principles of Internal Medicine 16th Edition by Dennis L. Kasper, Eugene Braunwald, Anthony Fauci, and Stephen Hauser, 2004