What is "vitamin a"

Retinol \Ret"i*nol\, n. [Gr. ??? resin + L. oleum oil.]

1. (Chem.) A hydrocarbon oil C32H16, obtained by the distillation of resin, -- used as a solvent, as an antiseptic, and in printer's ink.

2. One of the compounds which function as vitamin A. Called also vitamin A, vitamin A1 and vitamin A alcohol. See vitamin A1.

vitamin A \vitamin A\ n. any of several related fat-soluble vitamins (such as retinol) essential for normal vision; it also prevents night blindness or inflammation or dryness of the eyes.

Syn: antiophthalmic factor, axerophthol.

vitamin A1 \vitamin A1\ n. retinol ( C20H30O), one of the substances also called vitamin A. It is also called more specifically vitamin A alcohol. It is a required factor for human nutrition. The USP unit of activity is equal to 0.30 micrograms of retinol.

Syn: retinol.

vitamin A2 \vitamin A2\ n. dehydroretinol ( C20H28O), one of the substances also called vitamin A. It has about 40% of the bioactivity of vitamin A1.

Syn: dehydroretinol.

axerophthol \axerophthol\ n. 1. a vitamin essential for normal vision ( C20H30O); it prevents night blindness or inflammation or dryness of the eyes; same as vitamin A. One U.S.P. unit of vitamin A is equivalent to 0.30 micrograms of pure vitamin A alcohol.

Syn: vitamin A; vitamin A alcohol; antiophthalmic factor; biosterol; retinol; A.

n. (context vitamin English) Any of several closely related fat-soluble vitamins that are essential for normal vision; retinol.

n. any of several fat-soluble vitamins essential for normal vision; prevents night blindness or inflammation or dryness of the eyes [syn: antiophthalmic factor, axerophthol, A]

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.

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.

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are 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.

Vitamin A can be found in two principal forms in foods:

• Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
• 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 provitamin A in herbivores and omnivore animals, which possess the enzyme beta-carotene 15,15'-dioxygenase which cleaves beta-carotene in the intestinal mucosa and converts it to retinol. In general, carnivores are poor converters of ionone-containing carotenoids, and pure carnivores such as cats and ferrets lack beta-carotene 15,15'-dioxygenase and cannot convert any carotenoids to retinal (resulting in none of the carotenoids being forms of vitamin A for these species).