Vitamin A was the first fat-soluble vitamin identified (in 1913). Steenbock (1919) discovered the Vitamin A activity of carotenoids. Later he found that Vitamin A was present in the unsaponifiable fraction of fish liver oils.
A precursor to vitamin A is beta-carotene. (A precursor is a substance that, under the proper conditions, is converted to the active form of the vitamin.) Beta-carotene is relatively more stable.
The provitamin A carotenoids which include beta-carotene and others, are converted to retinoids with varying degree of efficiency. Retinoids are sensitive to heat, light, and oxidation by air.
The active form of vitamin A is retinol, which is present in foods of animal origin, including liver, egg yolks, fortified dairy products (e.g., vitamin A and D milk), margarine, and fish oil.
Vitamin A comprises the preformed retinoids, plus the precursor forms, the provitamin A carotenoids.