vitamers

Vitamers are the chemical forms of a vitamin that can produce the same vitamin activity in a given organism. They are different molecular forms that may be interconverted in the body and can differ in bioavailability, stability, or biological effectiveness. The term highlights that a vitamin label or nutritional assessment may involve multiple related compounds, not just a single chemical species.

Examples by vitamin illustrate the concept. For vitamin A, active forms include retinol, retinal, and retinoic acid, while beta-carotene and other provitamin A carotenoids act as precursors that are converted to active forms in the body. For vitamin D, the main vitamers are ergocalciferol (D2) and cholecalciferol (D3), both of which are converted to the hormone calcitriol. Vitamin E comprises several vitamers across two families—tocopherols and tocotrienols—with alpha-tocopherol providing the most significant vitamin activity in humans. Vitamin K includes phylloquinone (K1) and menaquinones (K2); synthetic menadione (K3) is generally not used as a dietary vitamin due to safety concerns.

Many water-soluble vitamins also have multiple vitamers and coenzyme forms that participate in metabolism. For example, vitamin B2 (riboflavin) exists as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), and vitamin B6 includes several interconvertible forms such as pyridoxine, pyridoxal, and pyridoxamine. The concept of vitamers is important for nutrition science and dietary assessment because different forms can vary in absorption, conversion to active forms, and biological impact.