Vitamin A Deficiency

Vitamin A is linked to a family of similarly shaped molecules, the retinoids, which complete the remainder of the vitamin sequence. Its important part is the retinyl group, which can be found in several forms. 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. Vitamin A can also exist as an aldehyde (retinal). The acid (retinoic acid), a metabolite, has only partial vitamin A activity, and does not function in the retina. Precursors to the vitamin (provitamins) are present in foods of plant origin as three of the members of the carotenoid family of compounds. Carolyn Berdanier. 1997. Advanced Nutrition Micronutrients. pp 22-39

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached. This structure is essential for vitamin activity. The orange pigment of carrots - beta-carotene - can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group which give them some vitamin activity. None of the other carotenes have vitamin activity. The carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.

The retinyl group, when attached to a specific protein, is the only primary light absorber in visual perception, and the compound name is related to the retina of the eye. The retinyl group also functions in retinoic acid, which exhibits hormone-like activities in other parts of the body.

Vitamin A can be found in various forms:

*retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble, vitamin with importance in vision and bone growth. Since the alcohol form is unstable, the vitamin is usually produced and administered in a form of retinyl acetate or palmitate.

*other retinoids, a class of chemical compounds that are related chemically to vitamin A, are used in medicine. American Cancer Society: Retinoid Therapy

*The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as vitamin A in herbivores and omnivore animals, which possess the enzyme required to convert these compounds to retinal. Carnivores in general are poor converters of ionine-containg carotenoids, and pure carnivores such as cats and ferets lack beta-carotene 15,15'-monooxygenase and cannot convert any carotenoids to retinals (resulting in no carotenoids being forms of vitamin A for these species).