Vitamin A also functions in a very different role, as an irreversibly oxidized form retinoic
acid, which is an important hormone-like growth factor for epithelial
and other cells.
In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to an alcohol (retinol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal.