In the body, it exists as Retinol from retinal, Retinoic acid and retinyl phosphate. These molecules are altered by oxygen in the air, changes accelerated by light and heat.

Foods of animal origin (meat, dairy products and especially liver) contain retinol and retinyl esters, whereas plants contain predominantly carotenes which are precursors of retinol. One molecule of beta-carotene, by hydrolysis of the 15-15 ‘under the influence of carotenoid monooxygenase (??-carotene 15,15′ monooxygenase), gives two molecules of vitamin A. For cons, the two other Carotenes (alpha and gamma) give birth to only one molecule of vitamin A.

The need for vitamin A are estimated at about 5 000 IU per day. An international unit of vitamin A equals 0.3 mg of retinol. Retinol is considered the basic unit, retinol-equivalent, which compares the vitamin activity of various derivatives of vitamin A. The most useful form of vitamin A is retinol, although it can also be found in the form of retinal or retinoic acid. Retinol is a hydrophobic molecule synthesized from isoprene. Vitamin A is involved in bone growth and synthesis of pigments of the eye. Retinol is sometimes used to treat severe acne.

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