| Retinol, the dietary form of vitamin A,
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| | target tissues.
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| is a fat-soluble, antioxidant vitamin
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| | A binding protein inside cells, cellular
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| important in vision and bone growth. It
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| | retinoic acid binding protein, serves to
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| belongs to the family of chemical
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| | store and move retinoic acid
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| compounds known as retinoids. Retinol is
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| | intracellularly. Carotenoid
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| ingested in a precursor form; animal
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| | bioavailability ranges between 1/5 to 1/
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| sources (milk and eggs) contain retinyl
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| | 10 of retinol's. Carotenoids are better
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| esters, whereas plants (carrots, spinach)
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| | absorbed when ingested as part of a fatty
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| contain pro-vitamin A carotenoids.
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| | meal. Also, the carotenoids in
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| Hydrolysis of retinyl esters results in
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| | vegetables, especially those with tough
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| retinol while pro-vitamin A carotenoids
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| | cell walls (e.g. carrots), are better
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| can be cleaved to produce retinal.
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| | absorbed when these cell walls are broken
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| Retinal, also known as retinaldehyde, can
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| | up by cooking or mincing.
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| be reversibly reduced to produce retinol
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| | Typical use All retinoid forms of vitamin
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| or it can be irreversibly oxidized to
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| | A are used in cosmetic and medical
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| produce retinoic acid. The best described
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| | applications applied to the skin.
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| active retinoid metabolites are
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| | Retinoic acid, retinyl palmitate,
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| 11-cis-retinal and the all-trans and
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| | isotretinoin, tretinoin and retinol are
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| 9-cis-isomers of retinoic acid.
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| | all used medicinally as a topical
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| Discovery In 1913, Elmer McCollum, a
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| | treatment for acne and keratosis pilaris.
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| biochemist at the University of
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| | In cosmetics, vitamin A derivatives are
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| Wisconsin, and colleague Marguerite Davis
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| | used as so-called antiaging chemicals-
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| identified a fat-soluble nutrient in
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| | vitamin A is absorbed through the skin
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| butterfat and cod liver oil. Their work
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| | and increases the rate of skin turnover,
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| confirmed that of Thomas Osborne and
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| | and gives a temporary increase in
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| Lafayette Mendel, at Yale, which
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| | collagen giving a more youthful
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| suggested a fat-soluble nutrient in
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| | appearance.
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| butterfat, also in 1913.
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| | Vitamin A deficiency Vitamin A deficiency
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| Vitamin A was first synthesized in 1947.
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| | is common in developing countries but
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| Chemical structure and function Many
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| | rarely seen in developed countries.
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| different geometric isomers of retinol,
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| | Approximately 250,000 to 500,000
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| retinal and retinoic acid are possible as
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| | malnourished children in the developing
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| a result of either a trans or cis
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| | world go blind each year from a
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| configuration of the four double bonds
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| | deficiency of vitamin A.
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| found in the polyene chain. The cis
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| | Night blindness is one of the first signs
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| isomers are less stable and can readily
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| | of vitamin A deficiency. Vitamin A
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| convert to the all-trans configuration
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| | deficiency contributes to blindness by
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| (as seen in the structure of
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| | making the cornea very dry and damaging
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| all-trans-retinol shown here).
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| | the retina and cornea.
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| Nevertheless, some cis isomers are found
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| | Vitamin A deficiency also diminishes the
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| naturally and carry out essential
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| | ability to fight infections. In countries
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| functions. For example, the
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| | where children are not immunized,
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| 11-cis-retinal isomer is the chromophore
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| | infectious disease like measles have
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| of rhodopsin, the vertebrate
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| | relatively higher fatality rates. As
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| photoreceptor molecule. Rhodopsin is
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| | elucidated by Dr. Alfred Sommer, even
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| comprised of the 11-cis-retinal
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| | mild, subclinical deficiency can also be
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| covalently linked via a Schiff base to
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| | a problem, as it may increase children's
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| the opsin protein (either rod opsin or
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| | risk of developing respiratory and
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| blue, red or green cone opsins). The
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| | diarrheal infections, decrease growth
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| process of vision relies on the
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| | rate, slow bone development, and decrease
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| light-induced isomerisation of the
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| | likelihood of survival from serious
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| chromophore from 11-cis to all-trans
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| | illness.
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| resulting in a change of the conformation
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| | In addition to dietary problems, there
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| and activation of the photoreceptor
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| | are other causes of vitamin A deficiency.
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| molecule. One of the earliest signs of
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| | Iron deficiency can affect vitamin A
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| vitamin A deficiency is night-blindness
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| | uptake. Excess alcohol consumption can
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| followed by decreased visual acuity.
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| | deplete vitamin A, and a stressed liver
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| George Wald won the 1967 Nobel Prize in
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| | may be more susceptible to vitamin A
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| Physiology or Medicine for his work with
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| | toxicity. People who consume large
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| retina pigments (also called visual
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| | amounts of alcohol should seek medical
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| pigments), which led to the understanding
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| | advice before taking vitamin A
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| of the role of vitamin A in vision.
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| | supplements.
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| Many of the non-visual functions of
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| | Treatment of vitamin A deficiency can be
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| vitamin A are mediated by retinoic acid,
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| | undertaken with both oral and injectable
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| which regulates gene expression by
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| | forms, generally as vitamin A palmitate.
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| activating intracellular retinoic acid
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| | Vitamin A overdose Too much vitamin A can
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| receptors. The non-visual functions of
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| | be harmful or fatal. The body converts
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| vitamin A are essential in the
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| | the dimerized form, carotene, into
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| immunological function, reproduction and
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| | vitamin A as it is needed, therefore high
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| embryonic development of vertebrates as
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| | levels of carotene are not toxic compared
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| evidenced by the impaired growth,
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| | to the ester (animal) forms. The livers
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| susceptibility to infection and birth
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| | of certain animals, especially those
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| defects observed in populations receiving
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| | adapted to polar environments, often
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| suboptimal vitamin A in their diet.
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| | contain amounts of vitamin A that would
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| Retinol can also be used in the treatment
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| | be toxic to humans. The first documented
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| of acne in a topical cream. A form of
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| | death due to vitamin A poisoning was
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| retinoic acid, all-trans retinoic acid
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| | Xavier Mertz, a Swiss scientist who died
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| (ATRA) is currently used as chemotherapy
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| | in January 1913 on an Antarctic
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| for acute promyelocytic leukemia, a
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| | expedition that had lost its food
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| subtype of acute myelogenous leukemia.
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| | supplies and fell to eating its sled
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| This is because this transformed cells of
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| | dogs. Mertz consumed lethal amounts of
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| this subtype respond in most cases to
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| | vitamin A by eating the dogs' livers.
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| agonists of the retinoic acid receptor
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| | The liver of the polar bear also has
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| (RAR).
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| | enough vitamin A to kill a human being,
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| Vision Vitamin A is required in the
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| | or enough to make even sled dogs very
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| production of rhodopsin, the visual
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| | ill.
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| pigment used in low light levels. This is
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| | Excess vitamin A has also been suspected
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| why eating foods rich in vitamin A is
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| | to be a contributor to osteoporosis.
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| said to allow you to see in the dark.
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| | This seems to happen at much lower doses
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| Epithelial Cells Vitamin A is essential
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| | than those required to induce acute
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| for the correct functioning of epithelial
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| | intoxication. Only preformed vitamin A
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| cells. In Vitamin A deficiency,
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| | can cause these problems, because the
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| mucus-secreting cells are replaced by
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| | conversion of carotenoids into vitamin A
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| keratin producing cells, leading to
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| | is downregulated when physiological
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| xerosis.
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| | requirements are met. An excessive uptake
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| Glycoprotein synthesis Glycoprotein
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| | of carotenoids can, however, cause
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| synthesis requires adequate Vitamin A
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| | carotenosis.
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| status. In severe Vitamin A deficiency,
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| | The carotenoid beta carotene was
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| lack of glycoproteins may lead to corneal
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| | interestingly associated with an increase
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| ulcers or liquefaction.
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| | in lung cancer when it was studied in a
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| Immune System Vitamin A is essential to
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| | lung cancer prevention trial in male
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| provide intact epithelial tissues as a
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| | smokers. In non-smokers, the opposite
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| physical barrier to infection; it is also
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| | effect has been noted.
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| involved in maintaining healthy
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| | Although cases of vitamin A toxicity have
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| lymphocytes and T-cells.
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| | been reported in arctic explorers and
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| Formation of red blood
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| | some people taking large doses of
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| cells(Haematopoiesis) Vitamin A may be
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| | synthetic vitamin A for long periods of
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| needed for normal haematopoiesis;
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| | time, pregnant women require large
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| deficiency causes abnormalities in iron
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| | amounts of vitamin A from preferably
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| metabolism.
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| | natural animal sources, such as liver,
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| Growth Vitamin A affects the production
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| | raw (non-pasteurized) butter, and cod
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| of human growth hormone.
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| | liver oil. However excess in retinoid
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| Units of measurement When referring to
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| | form must at all costs be avoided due to
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| dietary allowances or nutritional
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| | its well known teratogenic effects.
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| science, retinol is usually measured in
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| | Closely related chemicals Isotretinoin
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| international units (IU). IU refers to
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| | (Tradename: Accutane) Retinyl palmitate
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| biological activity and therefore is
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| | ('vitamin A' aka.
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| unique to each individual compound,
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| | "pro-vitamin A") All-trans retinoic acid
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| however 1 IU of retinol is equivalent to
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| | Genetically engineered vitamin A enriched
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| approximately 0.3 µg (300 nanograms).
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| | rice Due to the high prevalence of
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| Dietary intake During the absorption
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| | vitamin A deficiency in developing
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| process in the intestines, retinol is
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| | countries, there are efforts to produce
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| incorporated into chylomicrons as the
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| | genetically modified rice rich in beta
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| ester form, and it is these particles
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| | carotene. The idea is that this would
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| that mediate transport to the liver.
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| | help poor people, who can't afford a
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| Liver cells (hepatocytes) store vitamin A
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| | varied diet containing sufficient natural
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| as the ester, and when retinol is needed
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| | sources of vitamin A, meet their dietary
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| in other tissues, it is de-esterifed and
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| | needs. The golden rice project is one
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| released into the blood as the alcohol.
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| | such effort, and is already undergoing
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| Retinol then attaches to a serum carrier,
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| | trials.
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| retinol binding protein, for transport to
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