Vitamin A itself does not occur in plant products, but its precursors, carotenoids, do occur in several forms. These compounds, carotenoids, are commonly referred to as provitamin A, because the body can transform them into the active vitamin. The carotenoid pigments are widespread among diverse species, with over 600 different compounds estimated. About 60 of these have provitamin A activity. In practice, however, only five or six of these provitamin A compounds are commonly encountered in foods.
One IU of vitamin A is equivalent to 0.3 µg of all-trans-retinol, 0.344 µg all-trans-retinyl acetate, 0.55 µg all-trans-retinyl palmitate. Vitamin A is used in the feed industry as the ester forms of acetate, propionate, or palmitate.
