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 those found naturally in plant origin foods are chemically carotenoids (three carotenes and one is a xanthophyll). However, the 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. 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. The retinoids are pharmaceutical forms of vitamin A.