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INFO: Full form for SS is Superficial Schizophrenic in Medical category
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INFO: Full form for RL is Recall Level in Medical category
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INFO: Full form for RL is Radio-Luminescence in Medical category
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INFO: Full form for RL is Right Lung in Medical category
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INFO: Full form for RL is Right Lobe in Medical category
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INFO: Full form for RL is Residents' Lounge in Medical category
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INFO: Full form for RL is Right to Left in Medical category
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INFO: Full form for RL is Right Leg in Medical category
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INFO: Full form for A is Blood Type A in Medical category
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Vitamin A is a group of unsaturated nutritional organic compounds that includes retinol, retinal, 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 for 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.

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: