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  Sci. Lett. J. 2012, 1: 19  
  Research Article
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The biological significance of vitamin A in humans: A review of nutritional aspects and clinical considerations  
  Ramadhan Orucha, Ian F. Prymeb  
a School of Medicine, Department of Biochemistry and Molecular Biology, Najran University, Najran, Saudi Arabia
b Department of Biomedicine, University of Bergen, Bergen, Norway

  Vitamin A is an essential micronutrient for humans, meaning that it cannot be biosynthesized in the body and thus must be obtained from dietary sources. Retinol is its vitamer form in food and is converted in the body to 11-trans-retinal by an oxidative process where the hydroxyl group is converted into an aldehyde. 11-trans-retinal is subsequently isomerized into 11-cis-retinal, the functional isomer of the vitamin important in the physiology of vision. In addition, this fat-soluble essential metabolite and its derivatives play vital roles in the regulation of many metabolic and biological functions including growth and differentiation of cells, reproduction (spermatogenesis, oogenesis, placental development and embryonic growth), and the regulation of the immune system. This micronutrient is also an important antioxidant that performs a corner process necessary for protection against oxidative damage caused by free radicals generated in several metabolic processes in the human body. The scavenging of free radicals is fundamental for the body’s fecundity and well-being. In order to perform this, the vitamin needs to be supplied via natural dietary sources rich in this micronutrient. Hypervitaminosis A and toxicity, which usually occur as a consequence of the administration of large amounts of vitamin A preparations, usually for therapeutic purposes, can cause different serious discomforts and ailments through exertion of its oxidative properties. For vitamin A to perform as a friend or a foe is a dose-dependent issue. Thus the micronutrient should be provided in balanced amounts so that the body can deal with it safely and properly.  
  Retinol; Retinal; Retinoic acid; Retinoids; Carotenoids; Retinyl ester; Retinol recycling; Cell differentiation; Gene expression; Opsin; Obesity; Malnutrition  
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