Vitamins

What Vitamins Are

Certain organic compounds are needed by the body only in very small quantities, yet their absence has a very specific consequence: a deficiency of one particular vitamin produces one particular disease. This tight compound-to-disease link is what first led chemists to recognise vitamins as a distinct class of biomolecules, separate from carbohydrates, proteins, lipids, and nucleic acids in both structure and role.

The body is largely unable to make these compounds on its own — most vitamins must come from the diet. Plants, by contrast, can synthesise almost all of them, which is why vitamins are described as essential food factors: essential because the human body depends on an external supply, and food factors because that supply is normally the diet. There is one partial exception — some vitamins required by the body are actually produced by the bacteria that live in the gut, so not every vitamin need is met purely through food intake. In everyday balanced diets, all the vitamins the body needs are ordinarily available.

Vitamins do not share a common chemical backbone the way, say, carbohydrates share the sugar unit — they belong to a wide range of unrelated chemical classes, so it is not really possible to define "vitamin" in structural terms. Instead they are defined functionally, as organic compounds required in the diet in small amounts to carry out specific biological functions needed for an organism's normal growth and continued good health.

Naming of Vitamins

Vitamins are identified using letters of the alphabet — A, B, C, D, and so on — in the order in which they were historically discovered, rather than by any structural logic. Some of these letter-groups are further split into numbered sub-types, such as $B_1$, $B_2$, $B_6$, and $B_{12}$, each of which is a chemically distinct compound with its own specific role, even though all are grouped under the "B" umbrella. …