The word vitamin derives from the ‘vital amine’ that was isolated at the turn of the twentieth century from brown rice by the researcher Casimir Funk. This substance could be used to cure individuals of the disease beriberi, which suggested that it was essential for human health. At about the same time, Elmer McCollum discovered a vital lipid soluble substance in the fat of dairy products and a vital water soluble substance in milk, which he called factor A and factor B, respectively. The term vitamin is now applied to all organic compounds required in small amounts for normal metabolic function in humans. A deficiency of one vitamin causes a specific disease which is reversible on reintroduction of that vitamin to the diet. In addition, vitamins may show pharmacological action which is not related to its vitamin function of maintaining a normal metabolic health.
The naming of the vitamins is not logical, because they have retained their historical names based on their time and method of discovery. Factor A and factor B where both found to be component of the fat soluble and water soluble parts of dairy respectively. These became known as vitamin A and vitamin B, with further studies showing that vitamin B was in fact a mixture of several compounds (one of which was thiamine from Funk’s brown rice) which were subsequently numbered B1, B2, B3, B4 and B5. However, some of these were compounds that already existed and had previously been discovered and named by research, or were not vitamins at all. There is no vitamin B3 or B5 because they was already known as nicotinic acid and pantothenic acid, respectively. Vitamin B4 was subsequently identified as the substance adenine which does not satisfy the definition of a vitamin as it is manufactured in humans.
Vitamin C, D and E were named in the order of their identification but vitamin K was named after the Danish for coagulation (koagulation). Vitamin F are the compounds now referred to as the essential fatty acids, α-linolenic acid and linoleic acid. Neither of these essential fats carry the vitamin designation anymore, although it could be argued that they should. Vitamin G was actually discovered to be riboflavin, already designated as vitamin B2. However, based on the strict definition neither vitamin D nor niacin should be included in group vitamins as the former is manufactured in the skin from the action of ultra violet light on cholesterol and the latter is synthesised in humans from tryptophan in adequate amounts for good health. Vitamins are generally designated as either fat soluble or water soluble depending on their partition coefficient value.
Subsequent research showed that some of the vitamins are actually collections of closely related compounds that share a biological activity in the body. Vitamin A is made up of the compounds retinol, retinaldehyde and retinoic acid as well as a variety of carotenes that can be cleaved in humans to form compounds with vitamin A activity. Vitamin E comprises of eight isomers divided into the tocopherols (α-, β-, γ- and δ-) and the tocotrienols (α-, β-, γ- and δ-), which all share the activity of α-tocopherol. Vitamin D comprises the animal synthesised cholecalciferol (vitamin D3) and the fungus and plant synthesised ergocalciferol (vitamin D2). Vitamin K comprises of the plant derived phylloquinones (vitamin K1) bacteria derived menaquinones (vitamin K2) and a synthetic form called menadione (vitamin K3), which must be activated by enzymes in tissues. Vitamin B12 is comprised of methylcobalamin, adenosylcobalamin and hydroxycobalamin.