A vitamin is an organic substance, necessary in small quantities (less than 100 mg / day - see table below) for the metabolism of a living organism, which cannot be synthesized in sufficient quantity by this organism. Each organism has specific needs: a molecule can be a vitamin for one species and not for another. This is the case, for example, with vitamin C which is essential for primates but not for most other mammals.

Organic molecules, the B vitamins are mainly precursors of coenzymes (molecules which participate in the active site of an enzyme) which contain one or more radicals essential for the synthesis of an enzyme or a hormone. They must be provided regularly and in sufficient quantity through the diet. In humans, two vitamins are synthesized by intestinal bacteria: vitamins K and B8. The other vitamins, for example vitamin D or vitamin C play completely different roles, acting as steroid hormone and anti-oxidant (redox reactions) respectively.

Insufficient intake or lack of vitamin respectively cause hypovitaminosis or avitaminosis which are the cause of various diseases (scurvy, beriberi, rickets, etc.). An excessive intake of fat-soluble vitamins (mainly A and D) causes hypervitaminosis, which is very toxic to the body, because the excess of fat-soluble vitamins cannot be eliminated quickly by the kidneys and is stored in the liver.

These vitamins were discovered by the Polish biochemist Kazimierz Funk who first isolated vitamin B1 from the rice husk in 1912.