Vitamer

A vitamer of a particular vitamin is any of a number of chemical compounds, generally having a similar molecular structure, each of which shows vitamin activity in a vitamin-deficient biological system.[1]

For instance, vitamin A₁ has at least six vitamer chemicals that all qualify as "vitamin A", each with slightly different properties. In such a system, "vitamin A" is termed the "generic descriptor" of the vitamin, which is defined by its biological properties in a vitamin-deficient organism, not by its chemical structure. In the "vitamin A"₁ system, four of those found naturally in plant origin foods are chemically carotenoids (three carotenes and one is a xanthophyll). However, the all-trans-retinol and retinal forms, which occur in animal-based foods, are several times as effective in humans, per microgram, than are the plant-based forms. All vitamers, in case of "vitamin A"₁ mediate the same biological functions serving as a precursor for retinal, for the visual cycle, or for all-trans-retinoic acid, functioning as a hormone-like substance and a ligand for the nuclear hormone receptors, RARs. In some cases these differences are extreme: for example the carotenoid forms of vitamin A₁ cannot be absorbed by cats or ferrets at all, and therefore do not have vitamin A activity in these species.

Typically, the vitamin activity of multiple vitamers is due to the body's limited ability to convert one vitamer to another, or many vitamers to the same enzymatic cofactor(s), which is active in the body as a form of the vitamin. Not all vitamers possess exactly the same vitamin potency, per mass. This is due to differences in absorption and interconversion of the various vitamers of a vitamin. Often for the same reason, the toxicity of vitamers varies by molecule, as is the case with vitamin E.[2]

A set of chemicals may be (but is not always) grouped under an alphabetized vitamin "generic descriptor" title, such as "vitamin A". Other examples of vitamers include cyanocobalamin, hydroxocobalamin, methylcobalamin, and 5-deoxyadenosylcobalamin (adenosylcobalamin—AdoB-12), which are all vitamers of B₁₂, and thus all possess "B₁₂ activity". Another example is that both niacinamide and nicotinic acid (niacin) have vitamin B₃ activity.