When ingested, beta-carotene, a provitamin A carotenoid, is converted in the gastrointestinal system into a retinal form of vitamin A, which can be subsequently converted into retinol, retinoic acid, or retinyl ester forms of vitamin A.[1][2][3][4] Retinyl esters are the storage form of vitamin A, and are primarily stored in the liver.[1][2][3]
The metabolism of vitamin A is complex: there are many interconvertible forms, and each has a different role.[3][2][5][4] For example, different types of retinols, retinals, and retinyl esters are precursor molecules to the synthesis of rhodopsin, a pigment involved in vision;[2] retinols can act as cofactors in several enzymatic processes;[5] and retinoic acids directly regulate gene expression while also regulating vitamin A metabolism by, for example, modifying the intestinal absorption of beta-carotene.[2]