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Vitamin A)
Retinol, the dietary form of vitamin A, is a fat-soluble, antioxidant vitamin important in vision and bone growth. It belongs to the family of chemical compounds known as retinoids.
Retinol is ingested in a precursor form; animal sources (milk and eggs) contain retinyl esters, whereas plants (carrots, spinach) contain carotenoids. The body converts these precursors to retinol, and then to either retinal or retinoic acid. Many of the non-vision functions of vitamin A are mediated by retinoic acid, which acts at intracellular retinoic acid receptors. Retinoic acid is used medicinally as a topical treatment for acne. The other main metabolite, retinal, is used in vision.
As can be seen from the structure, retinol is derived from isoprene.
Either deficiency or overdose of vitamin A can be harmful or fatal. Deficiency of vitamin A can cause night-blindness, and pale, dry skin. The body converts the dimerized form, carotene, into vitamin A as it is needed, therefore high levels of carotene are not toxic compared to the ester (animal) forms. The livers of certain animals, especially those adapted to polar environments, often contain amounts of vitamin A that would be toxic to humans. The first documented death due to vitamin A poisoning was Xavier Mertz, a Swiss scientist who died in January 1913 on an Antarctic expedition that had lost its food supplies and fell to eating its sled dogs. Mertz consumed lethal amounts of vitamin A by eating the dogs' livers. The liver of the polar bear also has enough vitamin A to kill a human being.
George Wald won the 1967 Nobel Prize in Physiology or Medicine for his work with retina pigments, including the role of vitamin A.
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