Vitamin

Vitamins are organic molecules that cells need in small quantities to work, grow and develop normally. The body cannot make most of them, or cannot make enough, so they must come from the diet. Many act as coenzymes, or are turned into them, helping enzymes carry out metabolic reactions; others work as antioxidants or as hormone-like signals. Humans need 13 vitamins: A, C, D, E, K and eight B vitamins.

Vitamins are grouped by how they dissolve. Vitamins A, D, E and K dissolve in fat, so they enter the body with dietary fat and are kept in the liver and fatty tissue, where very large doses can build up to toxic levels. The water-soluble vitamins, C and the B group, are not stored to any great extent and excess leaves in the urine, so they are needed regularly; vitamin B12 is an exception, since the liver can hold it for years. Vitamin D is unusual because skin makes it when exposed to sunlight.

Each vitamin has its own deficiency disease, such as scurvy without vitamin C or rickets without vitamin D. The Polish biochemist Casimir Funk coined the name vitamine in 1912, from Latin vita (life) and amine, believing all such substances were amines; the final e was dropped when many proved not to be.