Vitamin A is a fat-soluble micronutrient that is required by all vertebrates to maintain vision, epithelial tissues, immune functions, reproduction, and for life itself. It was discovered in 1913 as a minor component in eggs, butter, whole milk, and fish liver oils. It soon became apparent that vitamin A exists in two chemically distinct yet structurally related forms. The first form to be characterized was retinol, a lipid alcohol that is present only in foods of animal origin. Retinol is also known as 'preformed vitamin A' because it can be metabolized directly into compounds that exert the biological effects of vitamin A. A second form of vitamin A, present in deep-yellow vegetables, was characterized as ^-carotene, which is synthesized only by plants but can be converted to retinol during absorption in the small intestines. These carotenoids are sometimes referred to as 'provitamin A.' The nutritional requirement for vitamin A can be met by preformed retinol, provitamin A carotenoids, or a mixture, and therefore it is possible to obtain a sufficient intake of vitamin A from carnivorous, herbivorous, or omnivorous diets.
Neither retinol nor the provitamin A carotenoids are directly bioactive. Retinol must be activated in a series of oxidative reactions, while the provitamin A carotenoids must first be cleaved to produce retinol. Of numerous metabolites of vitamin A, two are well recognized as crucial to its physiological functions. 11-cis-retinaldehyde (retinal) is a component of the visual pigment required for vision, rhodopsin. Reti-noic acid, an acidic derivative, is required for the regulation of gene expression in essentially all tissues.
Besides the natural forms of vitamin A, a large number of structurally related analogs of vitamin A have been synthesized as potential therapeutic
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