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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. Tissue cells convert these precursors to retinol, and then to either retinal or retinoic acid. Chemical structure and functionMany different geometric isomers of retinol, retinal and retinoic acid are possible as a result of either a trans or cis configuration of the four double bonds found in the polyene chain. The cis isomers are less stable and can readily convert to the all-trans configuration (as seen in the structure of all-trans-retinol shown here). Nevertheless, some cis isomers are found naturally and carry out essential functions. For example, the 11-cis-retinal isomer is the chromophore of rhodopsin, the vertebrate photoreceptor molecule. Rhodopsin is comprised of the 11-cis-retinal covalently linked via a Schiff base to the opsin protein (either rod opsin or blue, red or green cone opsins). The process of vision relies on the light-induced isomerisation of the chromophore from 11-cis to all-trans resulting in a change of the conformation and activation of the photoreceptor molecule. One of the earliest signs of vitamin A deficiency is night-blindness followed by decreased visual acuity. George Wald won the 1967 Nobel Prize in Physiology or Medicine for his work with retina pigments (also called visual pigments), which led to the understanding of the role of vitamin A in vision. Many of the non-visual functions of vitamin A are mediated by retinoic acid, which regulates gene expression by activating intracellular retinoic acid receptors. The non-visual functions of vitamin A are essential in the immunological function, reproduction and embryonic development of vertebrates as evidenced by the impaired growth, susceptibility to infection and birth defects observed in populations receiving suboptimal vitamin A in their diet. Retinol can also be used in the treatment of acne in a topical cream. A form of retinoic acid, all-trans retinoic acid (ATRA) is currently used as chemotherapy for acute promyelocytic leukemia, a subtype of acute myelogenous leukemia. This is because this transformed cells of this subtype respond in most cases to agonists of the retinoic acid receptor (RAR). Dietary intakeDuring the absorption process in the intestines, retinol is incorporated into chylomicrons as the ester form, and it is these particles that mediate transport to the liver. Liver cells (hepatocytes) store vitamin A as the ester, and when retinol is needed in other tissues, it is de-esterifed and released into the blood as the alcohol. Retinol then attaches to a serum carrier, retinol binding protein, for transport to target tissues. A binding protein inside cells, cellular retinoic acid binding protein, serves to store and move retinoic acid intracellularly. Topical useAll forms of vitamin A are used in cosmetic and medical applications applied to the skin. Retinoic acid, retinyl palmitate, isotretinoin, tretinoin and retinol are all used medicinally as a topical treatment for acne and keratosis pilaris. In cosmetics, vitamin A derivatives are used as so-called antiaging chemicals- vitamin A is absorbed through the skin and increases the rate of skin turnover, and gives a temporary increase in collagen giving a more youthful appearance. Vitamin A deficiencyVitamin A deficiency is common in developing countries but rarely seen in developed countries. Approximately 250,000 to 500,000 malnourished children in the developing world go blind each year from a deficiency of vitamin A. Night blindness is one of the first signs of vitamin A deficiency. Vitamin A deficiency contributes to blindness by making the cornea very dry and damaging the retina and cornea. Vitamin A deficiency diminishes the ability to fight infections. In countries where children aren't immunized, infectious disease like measles have relatively higher fatality rates. Subclinical deficiency can also be a problem as it may increase children's risk of developing respiratory and diarrheal infections, decrease growth rate, slow bone development, and decrease likelihood of survival from serious illness. Iron deficiency can also affect vitamin A uptake. Excess alcohol consumption can deplete vitamin A, however a stressed liver may be more susceptible to vitamin A toxicity. People that consume large amounts of alcohol should seek medical advice before taking vitamin A supplements. Vitamin A overdoseToo much vitamin A can be harmful or fatal. 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, or enough to make even sled dogs very ill. Excess vitamin A has also been suspected to be a contributor to osteoporosis. The carotenoid beta carotene was interestingly associated with an increase in lung cancer when it was studied in a lung cancer prevention trial in male smokers. Closely related chemicals
Genetically engineered vitamin A enriched riceDue to the high prevalence of vitamin A deficiency in rice-eating societies, there are efforts to produce genetically modified rice rich in beta carotene. The golden rice project is one such effort, and is already undergoing trials. External links
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