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Retinol

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

                                          Vitamin A (Retinol)
                                                                  General
                                            Chemical formula   C[20]H[30]O
                                            Molecular weight 286.456 g/mol
                                                       Vitamin properties
                                                  Solubility           Fat
                                            RDA (adult male)    900 µg/day
                                          RDA (adult female)    700 µg/day
                                RDA upper limit (adult male)  3,000 µg/day
                              RDA upper limit (adult female)  3,000 µg/day
                                                      Deficiency symptoms
                                                        * Night blindness
                                                        * Keratomalacia
                                                        * Pale, dry skin

                                                          Excess symptoms
                                    * Liver toxicity
                                    * Dry skin
                                    * Hair loss
                                    * Teratological effects
                                    * Osteoporosis (suspected, long-term)

                                                           Common sources
                                                  * Liver
                                                  * Dairy products
                                                  * Darkly colored fruits
                                                  * Leafy vegetables

   Retinol, the animal form of vitamin A, is a yellow 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 pro-vitamin A
   carotenoids. Hydrolysis of retinyl esters results in retinol while
   pro-vitamin A carotenoids can be cleaved to produce retinal. Retinal,
   also known as retinaldehyde, can be reversibly reduced to produce
   retinol or it can be irreversibly oxidized to produce retinoic acid.
   The best described active retinoid metabolites are 11-cis-retinal and
   the all-trans and 9-cis-isomers of retinoic acid.

Discovery

   In 1913, Elmer McCollum, a biochemist at the University of
   Wisconsin-Madison, and colleague Marguerite Davis identified a
   fat-soluble nutrient in butterfat and cod liver oil. Their work
   confirmed that of Thomas Osborne and Lafayette Mendel, at Yale, which
   suggested a fat-soluble nutrient in butterfat, also in 1913 . Vitamin A
   was first synthesized in 1947.

Chemical structure and function

   Many 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.
   As can be seen from the structure, retinol is derived from isoprene,
   and has an alcohol functional group. The first full synthesis route for
   the compound was found by David Adriaan van Dorp and Jozef Ferdinand
   Arens in 1947.
   Enlarge
   As can be seen from the structure, retinol is derived from isoprene,
   and has an alcohol functional group. The first full synthesis route for
   the compound was found by David Adriaan van Dorp and Jozef Ferdinand
   Arens in 1947.

   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 cells of this subtype of leukemia
   are sensitive to agonists of the retinoic acid receptors (RARs).

Vision

   Vitamin A is required in the production of rhodopsin, the visual
   pigment used in low light levels. This is why eating foods rich in
   vitamin A is said to allow you to see in the dark.

Epithelial Cells

   Vitamin A is essential for the correct functioning of epithelial cells.
   In Vitamin A deficiency, mucus-secreting cells are replaced by keratin
   producing cells, leading to xerosis.

Glycoprotein synthesis

   Glycoprotein synthesis requires adequate Vitamin A status. In severe
   Vitamin A deficiency, lack of glycoproteins may lead to corneal ulcers
   or liquefaction.

Immune System

   Vitamin A is essential to maintain intact epithelial tissues as a
   physical barrier to infection; it is also involved in maintaining a
   number of immune cell types from both the innate and acquired immune
   systems. These include the lymphocytes ( B-cells, T-cells, and natural
   killer cells), as well as many myelocytes ( neutrophils, macrophages,
   and myeloid dendritic cells).

Formation of red blood cells (Haematopoiesis)

   Vitamin A may be needed for normal haematopoiesis; deficiency causes
   abnormalities in iron metabolism.

Growth

   Vitamin A affects the production of human growth hormone.

Units of measurement

   When referring to dietary allowances or nutritional science, retinol is
   usually measured in international units (IU). IU refers to biological
   activity and therefore is unique to each individual compound, however 1
   IU of retinol is equivalent to approximately 0.3 micrograms (300
   nanograms).

Nutrition

   This vitamin plays an essential role in vision, particularly night
   vision, normal bone and tooth development, reproduction, and the health
   of skin and mucous membranes (the mucus-secreting layer that lines body
   regions such as the respiratory tract). Vitamin A also acts in the body
   as an antioxidant, a protective chemical that may reduce the risk of
   certain cancers.

   There are two sources of dietary vitamin A. Active forms, which are
   immediately available to the body are obtained from animal products.
   These are known as retinoids and include retinal and retinol.
   Precursors, also known as provitamins, which must be converted to
   active forms by the body, are obtained from fruits and vegetables
   containing yellow, orange and dark green pigments, known as
   carotenoids, the most well-known being beta-carotene. For this reason,
   amounts of vitamin A are measured in Retinal Equivalents (RE). One RE
   is equivalent to 0.001mg of retinal, or 0.006mg of beta-carotene, or
   3.3 International Units of vitamin A.

   In the intestine, vitamin A is protected from being chemically changed
   by vitamin E. Vitamin A is fat-soluble and can be stored in the body.
   Most of the vitamin A you eat is stored in the liver. When required by
   a particular part of the body, the liver releases some vitamin A, which
   is carried by the blood and delivered to the target cells and tissues.

Dietary intake

   The Dietary Reference Intake (DRI) Recommended Daily Amount (RDA) for
   Vitamin A for a 25-year old male is 900 micrograms/day, or 3,000 IU.

   During 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. Carotenoid bioavailability ranges
   between 1/5 to 1/10 of retinol's. Carotenoids are better absorbed when
   ingested as part of a fatty meal. Also, the carotenoids in vegetables,
   especially those with tough cell walls (e.g. carrots), are better
   absorbed when these cell walls are broken up by cooking or mincing.

Topical use

   All retinoid 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. Isotretinoin is also used orally (under the trade names
   Accutane and Roaccutane), generally for severe or recalcitrant acne.

   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 deficiency

   Prevalence of vitamin A deficiency. Source: WHO
   Enlarge
   Prevalence of vitamin A deficiency. Source: WHO

   Vitamin 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 also diminishes the ability to fight infections.
   In countries where children are not immunized, infectious disease like
   measles have relatively higher fatality rates. As elucidated by Dr.
   Alfred Sommer, even mild, 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.

   In addition to dietary problems, there are other causes of vitamin A
   deficiency. Iron deficiency can affect vitamin A uptake. Excess alcohol
   consumption can deplete vitamin A, and a stressed liver may be more
   susceptible to vitamin A toxicity. People who consume large amounts of
   alcohol should seek medical advice before taking vitamin A supplements.

   Treatment of vitamin A deficiency can be undertaken with both oral and
   injectable forms, generally as vitamin A palmitate.

Vitamin A overdose (Toxicity)

   The Tolerable Upper Intake Level (UL) for vitamin A, for a 25-year old
   male, is 3,000 micrograms/day, or about 10,000 IU.

   Too much vitamin A can be harmful or fatal, resulting in what is known
   as hypervitaminosis A. 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. Thus, vitamin A
   toxicity is typically reported in arctic explorers and people taking
   large doses of synthetic vitamin A. 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. Just 0.3 grams of the liver of
   the polar bear contains the upper intake level. If eaten in one meal,
   30 to 90 grams is enough to kill a human being, or to make even sled
   dogs very ill.

   Excess vitamin A has also been suspected to be a contributor to
   osteoporosis. This seems to happen at much lower doses than those
   required to induce acute intoxication. Only preformed vitamin A can
   cause these problems, because the conversion of carotenoids into
   vitamin A is downregulated when physiological requirements are met. An
   excessive uptake of carotenoids can, however, cause carotenosis.

   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. In non-smokers, the opposite effect has been
   noted.

   Excess preformed vitamin A during early pregnancy has also been
   associated with a significant increase in birth defects. These defects
   may be severe, even life-threatening. Even twice the daily recommended
   amount can cause severe birth defects. The FDA currently recommends
   that pregnant women get their Vitamin A from foods containing beta
   carotene and that they should ensure that they consume no more than
   5,000 IU of preformed Vitamin A (if any) per day. Although Vitamin A is
   necessary for fetal development, most women carry stores of Vitamin A
   in their fat cells, so oversupplementation should be strictly avoided.

Night vision

   Night blindness - the inability to see well in dim light - is
   associated with a deficiency of vitamin A. This vitamin is needed for
   the formation of rhodopsin. This is a pigment located in the eye's
   retina, which is the light-sensitive tissue lining in the back of the
   eye.

   When stimulated by light, rhodopsin splits into two proteins: opsin and
   retinal (a form of vitamin A); when it is dark the reverse reaction
   occurs - the retinal and opsin combine to re-form rhodopsin, a reaction
   that requires extra retinal.

   Without adequate amounts of retinal, regeneration of rhodopsin is
   incomplete and night blindness occurs. Since carrots are a good source
   of beta-carotene, there is truth in the old belief that carrots help
   you see better in the dark.

Closely related chemicals

     * Isotretinoin (Tradename: Accutane(US), Roaccutane)
     * Retinyl palmitate ('vitamin A' aka. "pro-vitamin A")
     * All-trans retinoic acid

Genetically engineered vitamin A enriched rice

   Due to the high prevalence of vitamin A deficiency in developing
   countries, there are efforts to produce genetically modified rice rich
   in beta carotene. The idea is that this would help poor people, who
   can't afford a varied diet containing sufficient natural sources of
   vitamin A, meet their dietary needs. The golden rice project is one
   such effort, and is already undergoing trials.
   Retrieved from " http://en.wikipedia.org/wiki/Retinol"
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