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Vitamin A consists of a group of compounds that play an important role in vision, bone growth, reproduction, cell division and cell differentiation . It helps to regulate the immune system and may also help lymphocytes (meaning a type of white blood cell) better fight infection .
In general, there are two forms of vitamin A:
- Vitamin A found in foods that come from animals is called preformed vitamin A and is absorbed in the form of retinol, the most biologically active form of vitamin A.
- Vitamin A found in plants is called provitamin A carotenoid, which can be made into retinol in the body.
Vitamin A deficiency can lead to eye problems including dryness of the cornea and night blindness. Vitamin A plays an important role in maintaining the integrity of epithelial cells (the cell type that line hollow organs and glands and that make up the outer surface of the body). Consistent with this role, the incidence of diseases of the respiratory tract is increased during moderate vitamin A deficiency . Vitamin A deficiency has been associated with an increased risk of various epithelial cancers, including skin, lungs, stomach and breast cancer) . Additionally, an isomer (meaning a molecule with the same chemical formula but different structure) of retinol, all-trans-retinoic acid, has been shown to help cancer patients with metastatic renal cell carcinoma . As vitamin A is essential for the growth of bones, teeth and soft tissues, a deficiency of vitamin A can contribute to bone loss associated with osteoporosis .
Carotenoids are highly colored fat-soluable organic pigments that are naturally occurring in photosynthetic organisms. Common provitamin A carotenoids, which are also unoxidized carotenoids, such as alpha-carotene, beta-carotene and lycopene, are known as carotenes. In photosynthetic organisms, carotenoids participate in the energy transfer process or protect against auto-oxidation. In animals, carotenoids have been linked to oxidation-preventing mechanisms and are efficient free-radical scavengers, functioning as antioxidants.
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