Vitamin A describes a group of fat-soluble metabolites that include preformed vitamin A molecules — retinols, retinals, retinyl esters, retinoic acids — and provitamin A molecules — carotenoids like beta-carotene — which are precursors that the body can convert into the other forms of vitamin A.[1][2][3] Vitamin A metabolites are involved in cell and tissue growth and in several bodily functions, including vision, bone metabolism, and immune function.[1][2]
Because they are fat soluble, vitamin A metabolites can be stored in the body (mainly in the liver) as a retinyl ester.[4][5][3][1] However, dietary intake is necessary because humans cannot synthesize vitamin A from scratch.[4][5][1][2] Humans obtain preformed vitamin A — retinols, retinals, retinyl esters, and retinoic acids — from animal products (e.g., fish, eggs, dairy, liver), and obtain provitamin A carotenoids (like beta-carotene) from plants (e.g., sweet potatoes, carrots, leafy greens); the latter can be converted to preformed vitamin A by the body.[6][3] In some countries, some foods — milk, margarine, cereals, etc. — are also fortified with vitamin A.[7][8][9][10]
Vitamin A is also taken as a dietary supplement. Such supplements typically contain preformed vitamin A (retinyl acetate or retinyl palmitate), provitamin A (beta-carotene), or a combination of both.[11] Vitamin A is also found in high levels in some fish oil supplements, such as cod liver oil, and in many multivitamins.[11]