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),...
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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.
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...
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