What is 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.
For instance, Vitamin A 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" system, four of the chemicals naturally found in foods are chemically carotenoids (three carotenes and one xanthophyll), found in certain plant foods like carrots. However, the retinol and retinal forms, which occur in animal-based foods, are several times as effective in humans, per microgram, as the carotenoid forms. In some cases these differences are extreme: for example the carotenoid forms of vitamin A 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 the important form of the vitamin.
As part of the definition of vitamin, the body cannot completely synthesize an optimal amount of vitamin activity from simple foodstuffs, without some minimal amount of a vitamer molecule as a basis.
Typically 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.
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.