<P> Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids (most notably beta - carotene). Vitamin A has multiple functions: it is important for growth and development, for the maintenance of the immune system and good vision . Vitamin A is needed by the retina of the eye in the form of retinal, which combines with protein opsin to form rhodopsin, the light - absorbing molecule necessary for both low - light (scotopic vision) and color vision . Vitamin A also functions in a very different role as retinoic acid (an irreversibly oxidized form of retinol), which is an important hormone - like growth factor for epithelial and other cells . </P> <P> In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to retinol (chemically an alcohol) 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 . </P>

The retinoid that has the most important physiological role in the human body is