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The structure of retinol, the most common dietary form of vitamin A
The structure of retinol, the most common dietary form of vitamin A

Vitamin A is an essential human nutrient. Vitamin A actually refers to a family of similarly shaped molecules: the retinoids. The retinoids are a class of Chemical compounds that are related chemically to Vitamin A. Its important part is the retinyl group, which can be found in several forms. In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to an alcohol (retinol) in the small intestine. Esters are a class of Chemical compounds and Functional groups Esters consist of an inorganic or organic Acid in which at least Retinyl palmitate, or vitamin A palmitate, is a common Vitamin supplement, with formula C36H60O2 In Chemistry, an alcohol is any Organic compound in which a Hydroxyl group ( - O[[hydrogen H]]) is bound to a Carbon Retinol (Afaxin, the animal form of Vitamin A, is a fat-soluble vitamin important in vision and Bone growth Vitamin A can also exist as an aldehyde (retinal), or as an acid (retinoic acid). An aldehyde is an organic compound containing a terminal Carbonyl group. Retinal, technically called retinene1 or retinaldehyde, is a light-sensitive Retinene molecule found in the Photoreceptor cells of Precursors to the vitamin (provitamins) are present in foods of plant origin as some of the members of the carotenoid family of compounds. A provitamin is a substance that can be converted into a Vitamin by animal tissues. Carotenoids are organic Pigments that are naturally occurring in Chromoplasts of plants and some other photosynthetic Organisms [1]

All forms of vitamin A have a Beta-ionone ring to which an isoprenoid chain is attached. The ionones are a series of closely related Chemical substances that are part of a group of compounds known as rose ketones which also includes Damascones and Isoprene is a common synonym for the chemical compound 2-methylbuta-13-diene This structure is essential for vitamin activity. [1] The orange pigment of carrot - Beta-carotene - can be represented as two connected retinyl groups. The carrot ( Daucus carota subsp sativus, Etymology: Middle French carotte, from Late Latin carōta, from Greek karōton Beta-carotene is an Organic compound - a Terpenoid, a red-orange pigment abundant in plants and fruits The retinyl group, when attached to a specific protein, is the only primary light absorber in visual perception, and the compound name is related to the retina of the eye. In Psychology, visual perception is the ability to interpret information from Visible light reaching the Eyes The resulting Perception is also The vertebrate retina is a light sensitive part inside the inner layer of the Eye.

Vitamin A can be found in various forms:

Contents

Discovery of vitamin A

The discovery of vitamin A stemmed from research dating back to 1906, indicating that factors other than carbohydrates, proteins, and fats were necessary to keep cattle healthy. Year 1906 ( MCMVI) was a Common year starting on Monday (link will display full calendar of the Gregorian calendar (or a Common year starting Carbohydrates (from ' Hydrates of Carbon ' or saccharides ( Greek σάκχαρον meaning " Sugar " are the most Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Fats consist of a wide group of compounds that are generally soluble in organic solvents and largely insoluble in water Cattle, colloquially referred to as cows, are domesticated Ungulates a member of the Subfamily Bovinae of the family [3] By 1917 one of these substances was independently discovered by Elmer McCollum at the University of Wisconsin-Madison, and Lafayette Mendel and Thomas Osborne at Yale University. Year 1917 ( MCMXVII) was a Common year starting on Monday (link will display the full calendar of the Gregorian calendar (or a Common year Elmer McCollum ( March 3, 1879 &ndash November 15, 1967) was an American biochemist Lafayette Benedict Mendel ( February 5, 1872 &ndash December 9, 1935) was an American Biochemist known for his work in Since "water-soluble factor B" (Vitamin B) had recently been discovered, the researchers chose the name "fat-soluble factor A" (vitamin A). Solubility is the characteristic Physical property referring to the ability of a given substance the Solute, to dissolve in a Solvent. The B vitamins are eight water-soluble Vitamins that play important roles in cell Metabolism. [3] Vitamin A was first synthesized in 1947 by two Dutch chemists, David Adriaan van Dorp and Jozef Ferdinand Arens.

Equivalencies of retinoids and carotenoids (IU)

Since some carotenoids can be converted into vitamin A, attempts have been made to determine how much of them in the diet is equivalent to a particular amount of retinol, so that comparisons can be made of the benefit of different foods. Unfortunately the situation is confusing because the accepted equivalences have changed. For many years, a system of equivalencies was used in which an international unit (IU) was equal to 0. In Pharmacology, the International Unit is a unit of measurement for the amount of a substance based on measured Biological activity or effect 3 micrograms of retinol, 0. 6 μg of β-carotene, or 1. In the Metric system, a microgram is 1/1000000 of a Gram (1 × 10-6 or 1/1000 of a milligram is one of the smallest units of weight/mass commonly used 2 μg of other provitamin-A carotenoids. [4] Later, a unit called retinol equivalent (RE) was introduced. 1 RE corresponded to 1 μg retinol, 2 μg β-carotene dissolved in oil (as in supplement pills), 6 μg β-carotene in normal food (because it is not absorbed as well as from supplements), and 12 μg of either α-carotene or β-cryptoxanthin in food.

However, new research showed that the absorption of provitamin-A carotenoids was only half as much as previously thought, so in 2001 the US Institute of Medicine recommended a new unit, the retinol activity equivalent (RAE). The Institute of Medicine (IOM is one of the United States National Academies, and is a not-for-profit non-governmental American organization chartered in 1970 as a part of the United 1 μg RAE corresponds to 1 μg retinol, 2 μg of β-carotene in oil, 12 μg of "dietary" beta-carotene, or 24 μg of other dietary provitamin-A carotenoids. [5]

Substance and its chemical environment Micrograms of retinol equivalent per microgram of the substance
retinol 1
beta-carotene, dissolved in oil 1/2
beta-carotene, common dietary 1/12
alpha-carotene, common dietary 1/24
beta-cryptoxanthin, common dietary 1/24

Because the production of retinol from provitamins by the human body is regulated by the amount of retinol available to the body, the conversions apply strictly only for vitamin A deficient humans. Beta-carotene is an Organic compound - a Terpenoid, a red-orange pigment abundant in plants and fruits The absorption of provitamins also depends greatly on the amount of lipids ingested with the provitamin; lipids increase the uptake of the provitamin. [6]

The conclusion that can be drawn from the newer research is that fruits and vegetables are not as useful for obtaining vitamin A as was thought--in other words, the IU's that they were reported to contain were worth much less than the same number of IU's of fat-dissolved supplements. This is important for vegetarians. Vegetarianism is the practice of a diet that excludes Meat (including game and slaughter by-products Fish (including Shellfish and other sea (Night blindness is prevalent in countries where little meat or vitamin A-fortified foods are available. Nyctalopia (Greek for " night blindness " is a condition making it difficult or impossible to see in relatively low light ) A sample vegan diet for one day that provides sufficient vitamin A has been published by the Food and Nutrition Board (page 120[5]). On the other hand, reference values for retinol or its equivalents, provided by the National Academy of Sciences, have decreased. The RDA (for men) of 1968 was 5000 IU (1500 μg retinol). The Dietary Reference Intake (or DRI is a system of Nutrition recommendations from the Institute of Medicine (IOM of the US National Academy of Sciences In 1974 the RDA was set to 1000 RE (1000 μg retinol), whereas now the Dietary Reference Intake is 900 RAE (900 μg or 3000 IU retinol). The Dietary Reference Intake (or DRI is a system of Nutrition recommendations from the Institute of Medicine (IOM of the US National Academy of Sciences This is equivalent to 1800 μg of β-carotene supplement (3000 IU) or 10800 μg of β-carotene in food (18000 IU).

Recommended daily intake

Vitamin A
Dietary Reference Intake:

Life Stage Group RDA/AI*

ug/day

 UL

ug/day
Infants

0-6 months
7-12 months
400*
500*
600
600
Children

1-3 years
4-8 years
300
400
600
900
Males

9-13 years
14-18 years
19 - >70 years
600
900
900
1700
2800
3000
Females

9-13 years
14-18 years
19 - >70 years
600
700
700
1700
2800
3000
Pregnancy

<19 years
19 - >50 years
750
770
2800
3000
Lactation

<19 years
19 - >50 years
1200
1300
2800
3000

RDA = Recommended Dietary Allowances
AI* = Adequate Intakes
UL = Upper Limit

(Note that the limit refers to retinoid forms of vitamin A. The Dietary Reference Intake (or DRI is a system of Nutrition recommendations from the Institute of Medicine (IOM of the US National Academy of Sciences The retinoids are a class of Chemical compounds that are related chemically to Vitamin A. Carotene forms from dietary sources are not toxic. The term carotene is used for several related substances having the formula C40H56 This article is primarily about the human diet For a discussion of animal diets see List of feeding behaviours. Toxicity is the degree to which a substance is able to damage an exposed organism [7])

According to the Institute of Medicine of the National Academies, "RDAs are set to meet the needs of almost all (97 to 98 percent) individuals in a group. For healthy breastfed infants, the AI is the mean intake. The AI for other life stage and gender groups is believed to cover the needs of all individuals in the group, but lack of data prevent being able to specify with confidence the percentage of individuals covered by this intake. "[8]

[1]

Sources of Vitamin A

Vitamin A is found naturally in many foods:

Note: bracketed values are retinol equivalences and percentage of the adult male RDA per 100g. The sweet potato ( Ipomoea batatas) is a Dicotyledonous plant which belongs to the family Convolvulaceae Kale or Borecole is a form of Cabbage ( Brassica oleracea Acephala Group) green in color in which the central leaves do not Butter is a Dairy product made by churning fresh or fermented Cream or Milk. Spinach ( Spinacia oleracea) is a Flowering plant in the family of Amaranthaceae. Leaf vegetables, also called potherbs, greens, or leafy greens, are plant leaves eaten as a Vegetable, sometimes accompanied Pumpkin is a Gourd -like squash of the genus Cucurbita and the family Cucurbitaceae (which also includes gourds Collards, also called borekale (from the Dutch boerenkool (farmers' kale are various loose-leafed Cultivars of Brassica oleracea ( Acephala Cantaloupe (also cantaloup) refers to two varieties of Muskmelon ( Cucumis melo), which is a Species in the family Cucurbitaceae (a An egg is a round or oval body laid by the female of many animals consisting of an Ovum surrounded by layers of Membranes and an outer casing which acts to nourish The Apricot ( Prunus armeniaca, "Armenian plum" in Latin syn The papaya (from Carib via Spanish) is the Fruit of the Plant Carica papaya, in the genus Carica. Mangoes belong to the genus Mangifera, consisting of numerous species of tropical fruiting Trees in the Flowering plant family Anacardiaceae A pea (inaccurately called a '''sweet pea''' by food distubutors is most commonly the small spherical Seed or the seed-pod of the Legume Pisum Broccoli is a plant of the Cabbage family Brassicaceae (formerly Cruciferae Winter squash (Australia pumpkin) is a warm-seasoned vegetable representing several species within the genus Cucurbita The Dietary Reference Intake (or DRI is a system of Nutrition recommendations from the Institute of Medicine (IOM of the US National Academy of Sciences

Conversion of carotene to retinol varies from person to person and bioavailability of carotene in food varies. [10][11]

Metabolic Functions of Vitamin A

Vitamin A plays a role in a variety of functions throughout the body, such as:

Vision

The role of vitamin A in the vision cycle is specifically related to the retinal form. Within the eye, 11-cis-retinal is bound to rhodopsin (rods) and iodopsin (cones) at sites called opsins. As light enters the eye the 11-cis-retinal is isomerized to the all-"trans" form. The all-"trans" retinal dissociates from the opsin in a series of steps called bleaching. This dissociation induces a nervous signal along the optic nerve to the visual center of the brain. Upon completion of this cycle, the all-"trans"-retinal can be recycled and converted back to the 11-"cis"-retinal form via a series of enzymatic reactions. Additionally, some of the all-"trans" retinal may be converted to all-"trans" retinol form and then transported with an interphotoreceptor retinol-binding protein (IRBP) to the pigment epithelial cells. Further esterification into all-"trans" retinyl esters allow this final form to be stored within the pigment epithelial cells to be reused when needed. [12] The final conversion of 11-cis-retinal will rebind to opsin to reform rhodopsin in the retina. Rhodopsin is needed to see black and white as well as see at night. It is for this reason that a deficiency in vitamin A will inhibit the reformation of rhodopsin and lead to night blindness. [13]

Gene Transcription

Vitamin A, in the retinoic acid form, plays an important role in gene transcription. Once retinol has been taken up by a cell, it can be oxidized to retinal (by retinol dehydrogenases) and then retinal can be oxidized to retinoic acid (by retinal oxidase). The conversion of retinal to retinoic acid is an irreversible step, meaning that the production of retinoic acid is tightly regulated, due to its activity as a ligand for nuclear receptors. [12] Retinoic acid can bind to two different nuclear receptors to initiate (or inhibit) gene transcription: the retinoic acid receptors (RARs) or the retinoid "X" receptors (RXRs). RAR and RXR must dimerize before they can bind to the DNA. RAR will form a heterodimer with RXR (RAR-RXR), but it does not readily form a homodimer (RAR-RAR). RXR, on the other hand, readily forms a homodimer (RXR-RXR) and will form heterodimers with many other nuclear receptors as well, including the thyroid hormone receptor (RXR-TR), the Vitamin D3 receptor (RXR-VDR), the peroxisome proliferator-activated receptor (RXR-PPAR) and the liver "X" receptor (RXR-LXR). [14] The RAR-RXR heterodimer recognizes retinoid acid response elements (RAREs) on the DNA whereas the RXR-RXR homodimer recognizes retinoid "X" response elements (RXREs) on the DNA. The other RXR heterodimers will bind to various other response elements on the DNA. [12] Once the retinoic acid binds to the receptors and dimerization has occurred, the receptors undergo a conformational change that causes co-repressors to dissociate from the receptors. Coactivators can then bind to the receptor complex, which may help to loosen the chromatin structure from the histones or may interact with the transcriptional machinery. [14] The receptors can then bind to the response elements on the DNA and upregulate (or downregulate) the expression of target genes, such as cellular retinol-binding protein (CRBP) as well as the genes that encode for the receptors themselves. [12]

Dermatology

Vitamin A appears to function in maintaining normal skin health. The mechanisms behind retinoid's therapeutic agents in the treatment of dermatological diseases are being researched. For the treatment of acne, the most effective drug is 13-cis retinoic acid (isotretinoin). Although its mechanism of action remains unknown, it is the only retinoid that dramatically reduces the size and secretion of the sebaceous glands. isotretinoin reduces bacterial numbers in both the ducts and skin surface. This is thought to be a result of the reduction in sebum, a nutrient source for the bacteria. Isotretinoin reduces inflammation via inhibition of chemotatic responses of monocytes and neutrophils. [12] Isotretinoin also has been shown to initiate remodeling of the sebaceous glands; triggering changes in gene expression that selectively induces apoptosis. [15] Isotretinoin is a teratogen and its use is confined to medical supervision.

Deficiency

Vitamin A deficiency is estimated to affect millions of children around the world. Approximately 250,000-500,000 children in developing countries become blind each year owing to vitamin A deficiency, with the highest prevalence in Southeast Asia and Africa. [16] According to the World Health Organization (WHO), vitamin A deficiency is under control in the United States, but in developing countries vitamin A deficiency is a significant concern. With the high prevalence of vitamin A deficiency, the WHO has implemented several initiatives for supplementation of vitamin A in developing countries. Some of these strategies include intake of vitamin A through a combination of breast feeding, dietary intake, food fortification, and supplementation. Through the efforts of WHO and its partners, an estimated 1. 25 million deaths since 1998 in 40 countries due to vitamin A deficiency have been averted. [17]

Vitamin A deficiency can occur as either a primary or secondary deficiency. A primary vitamin A deficiency occurs among children and adults who do not consume an adequate intake of yellow and green vegetables, fruits and liver. Early weaning can also increase the risk of vitamin A deficiency. Secondary vitamin A deficiency is associated with chronic malabsorption of lipids, impaired bile production and release, low fat diets, and chronic exposure to oxidants, such as cigarette smoke. Vitamin A is a fat soluble vitamin and depends on micellar solubilization for dispersion into the small intestine, which results in poor utilization of vitamin A from low-fat diets. Zinc deficiency can also impair absorption, transport, and metabolism of vitamin A because it is essential for the synthesis of the vitamin A transport proteins and the oxidation of retinol to retinal. In malnourished populations, common low intakes of vitamin A and zinc increase the risk of vitamin A deficiency and lead to several physiological events. [12] A study in Burkina Faso showed major reduction of malaria morbidity with combined vitamin A and zinc supplementation in young children. Burkina Faso (bɚˌkiːnəˈfɑːsoʊ burr-KEE-na FAH-soh) also known by its short-form name Burkina, is a Landlocked nation in West Africa Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 [18]

Since the unique function of retinyl group is the light absorption in Retinylidene protein, one of the earliest and specific manifestations of vitamin A deficiency is impaired vision, particularly in reduced light - Night blindness. Retinylidene proteins are a family of proteins that use Retinal as Chromophore for Light reception Vitamin A deficiency is common in developing countries but rarely seen in developed countries Retinol (Afaxin, the animal form of Vitamin A, is a fat-soluble vitamin important in vision and Bone growth Persistent deficiency gives rise to a series of changes, the most devastating of which occur in the eyes. Some other ocular changes are referred to as xerophthalmia. Xerophthalmia ( Greek for dry eyes) is a medical condition in which the Eye fails to produce Tears. First there is dryness of the conjunctiva (xerosis) as the normal lacrimal and mucus secreting epithelium is replaced by a keratinized epithelium. Xerosis is the Medical term for dry Skin. It can have many different causes including general Dehydration, Vitamin A deficiency and Diabetes This is followed by the build-up of keratin debris in small opaque plaques (Bitot's spots) and, eventually, erosion of the roughened corneal surface with softening and destruction of the cornea (keratomalacia) and total blindness. Bitot's spots are the buildup of Keratin debris located superficially in the Conjunctiva, which are oval triangular or irregular in shape Keratomalacia is an Eye disorder that leads to a dry Cornea. One of its major causes is a deficiency of Vitamin A. [19] Other changes include impaired immunity, hypokeratosis (white lumps at hair follicles), keratosis pilaris and squamous metaplasia of the epithelium lining the upper respiratory passages and urinary bladder to a keratinized epithelium. Keratosis pilaris (KP also follicular keratosis is a very common genetic follicular condition that is manifested by the appearance of rough bumps on the skin Squamous metaplasia refers to Benign ( non-cancerous) changes in the epithelial linings of certain organs within the body With relations to dentistry, a deficiency in Vitamin A leads to enamel hypoplasia.

Adequate supply of Vitamin A is especially important for pregnant and breastfeeding women, since deficiencies cannot be compensated by postnatal supplementation. Postnatal ( Latin for 'after birth' from post meaning "after" and natalis meaning "of birth" is the period beginning immediately after [20][21].

Toxicity

Main article: Hypervitaminosis A

As vitamin A is fat-soluble, disposing of any excesses taken in through diet is much harder than with water-soluble vitamins B and C. Hypervitaminosis A refers to the effects of excessive Vitamin A (specifically retinoid intake As such, vitamin A toxicity can result. This can lead to nausea, jaundice, irritability, anorexia (not to be confused with anorexia nervosa, the eating disorder), vomiting, blurry vision, headaches, muscle and abdominal pain and weakness, drowsiness and altered mental status. Anorexia (deriving from the Greek "α(ν-" ( a(n-, a prefix that denotes absence + "όρεξη (orexe = appetite is the decreased sensation Anorexia Nervosa is a psychiatric Diagnosis that describes an Eating disorder characterized by low Body weight and Body image distortion

Acute toxicity generally occurs at doses of 25,000 IU/kg, with chronic toxicity occurring at 4,000 IU/kg daily for 6-15 months. [22] However, liver toxicities can occur at levels as low as 15,000 IU per day to 1. 4 million IU per day, with an average daily toxic dose of 120,000 IU per day. In people with renal failure 4000 IU can cause substantial damage. Additionally excessive alcohol intake can increase toxicity. Children can reach toxic levels at 1500IU/kg of body weight. [23]

In chronic cases, hair loss, drying of the mucous membranes, fever, insomnia, fatigue, weight loss, bone fractures, anemia, and diarrhea can all be evident on top of the symptoms associated with less serious toxicity. Insomnia is a symptom of a sleeping disorder characterized by persistent difficulty falling asleep or staying asleep despite the opportunity [24]

It has been estimated that 75% of people may be ingesting more than the RDA for vitamin A on a regular basis in developed nations. Intake of twice the RDA of preformed vitamin A chronically may be associated with osteoporosis and hip fractures. High vitamin A intake has been associated with spontaneous bone fractures in animals. Cell culture studies have linked increased bone resorption and decreased bone formation with high vitamin A intakes. This interaction may occur because vitamins A and D may compete for the same receptor and then interact with parathyoid hormone which regulates calcium. [23]

Toxic effects of vitamin A have been shown to significantly affect developing fetuses. Therapeutic doses used for acne treatment have been shown to disrupt cephalic neural cell activity. The fetus is particularly sensitive to vitamin A toxicity during the period of organogenesis. [12]

These toxicities only occur with preformed (retinoid) vitamin A (such as from liver). The carotenoid forms (such as beta-carotene as found in carrots), give no such symptoms, but excessive dietary intake of beta-carotene can lead to carotenodermia, which causes orange-yellow discoloration of the skin. Carotenoderma (also carotenodermia is the name given to the yellow to yellow-orange discoloration of the skin that is caused by increased serum carotenoids ( carotenemia) and [25][26][27]

A study by Siri Forsmo et al. shows a correlation between low bone mineral density and too high intake of vitamin A. [28]

Researchers have succeeded in creating water-soluble forms of vitamin A, which they believed could reduce the potential for toxicity. [29] However, a 2003 study found that water-soluble vitamin A was approximately 10 times as toxic as fat-soluble vitamin. [30] A 2006 study found that children given water-soluble vitamin A and D, which are typically fat-soluble, suffer from asthma twice as much as a control group supplemented with the fat-soluble vitamins. [31]

See also

References

  1. ^ a b Carolyn Berdanier. Beta-carotene is an Organic compound - a Terpenoid, a red-orange pigment abundant in plants and fruits The retinoids are a class of Chemical compounds that are related chemically to Vitamin A. Hypervitaminosis A refers to the effects of excessive Vitamin A (specifically retinoid intake 1997. Advanced Nutrition Micronutrients. pp 22-39
  2. ^ American Cancer Society: Retinoid Therapy
  3. ^ a b Wolf, George (2001-04-19). "Discovery of Vitamin A". Encyclopedia of Life Sciences. doi:10.1038/npg.els.0003419. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  4. ^ Composition of Foods Raw, Processed, Prepared USDA National Nutrient Database for Standard Reference, Release 20 USDA, Feb. 2008
  5. ^ a b Chapter 4, Vitamin A of Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, hickel, Silicon, Vanadium, and Zinc, Food and Nutrition Board of the Institute of Medicine, 2001
  6. ^ NW Solomons, M Orozco. The Institute of Medicine (IOM is one of the United States National Academies, and is a not-for-profit non-governmental American organization chartered in 1970 as a part of the United Alleviation of Vitamin A deficiency with palm fruit and its products. Asia Pac J Clin Nutr, 2003
  7. ^ Sources of vitamin A. Retrieved on 2007-08-27. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 479 BC - Greco-Persian Wars: Persian forces led by Mardonius are routed by Pausanias, the Spartan
  8. ^ Food and Nutrition Board. Institute of Medicine. National Academies. (2001) "Dietary Reference Intakes"
  9. ^ The RAE value in the USDA data for broccoli leaves is similar to the IU value for broccoli florets, which implies rather improbably that the leaves have about 20 times as much beta-carotene. It is possible that there was an error in the table.
  10. ^ Borel P, Drai J, Faure H, et al (2005). "[Recent knowledge about intestinal absorption and cleavage of carotenoids]" (in French). Ann. Biol. Clin. (Paris) 63 (2): 165–77. PMID 15771974.  
  11. ^ Tang G, Qin J, Dolnikowski GG, Russell RM, Grusak MA (2005). "Spinach or carrots can supply significant amounts of vitamin A as assessed by feeding with intrinsically deuterated vegetables". Am. J. Clin. Nutr. 82 (4): 821–8. PMID 16210712.  
  12. ^ a b c d e f g Combs, Gerald F. (2008). The Vitamins: Fundamental Aspects in Nutrition and Health, 3rd edition, Burlington: Elsevier Academic Press. ISBN 9780121834937.  
  13. ^ McGuire, Michelle; Beerman, Kathy A. (2007). Nutritional sciences: from fundamentals to food. Belmont, CA: Thomson/Wadsworth. ISBN 0534537170.  
  14. ^ a b Stipanuk, Martha H. (2006). Biochemical, Physiological and Molecular Aspects of Human Nutrition, 2nd edition, Philadelphia: Saunders. ISBN 978116002093.  
  15. ^ Nelson, A. M. ; et al. (2008). "Neutrophil gelatinase-associated lipocalin mediates 13-cis retinoic acid-induced apoptosis of human sebaceous gland cells". Journal of Clinical Investigation 118 (4): 1468–1478. doi:10.1172/JCI33869. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  16. ^ Office of Dietary Supplements. Vitamin A. National Institute of Health. Retrieved on 2008-04-08. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 217 - Roman Emperor Caracalla is Assassinated (and succeeded by his Praetorian
  17. ^ Micronutrient Deficiencies-Vitamin A. World Health Organization. Retrieved on 2008-04-09. 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 193 - Septimius Severus is proclaimed Roman Emperor by the army in Illyricum (in the Balkans)
  18. ^ Zeba AN, Sorgho H, Rouamba N, et al (2008). "Major reduction of malaria morbidity with combined vitamin A and zinc supplementation in young children in Burkina Faso: a randomized double blind trial". Nutr J 7: 7. doi:10.1186/1475-2891-7-7. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 18237394.  
  19. ^ Roncone DP (2006). "Xerophthalmia secondary to alcohol-induced malnutrition". Optometry (St. Louis, Mo. ) 77 (3): 124-33. doi:10.1016/j.optm.2006.01.005. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16513513.  
  20. ^ Strobel M, Tinz J, Biesalski HK (2007). "The importance of beta-carotene as a source of vitamin A with special regard to pregnant and breastfeeding women". Eur J Nutr 46 Suppl 1: I1–20. doi:10.1007/s00394-007-1001-z. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 17665093.  
  21. ^ Schulz C, Engel U, Kreienberg R, Biesalski HK (2007). "Vitamin A and beta-carotene supply of women with gemini or short birth intervals: a pilot study". Eur J Nutr 46 (1): 12–20. doi:10.1007/s00394-006-0624-9. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 17103079.  
  22. ^ Rosenbloom, Mark. Toxicity, Vitamin. eMedicine.
  23. ^ a b Penniston, Kristina L. ; Tanumihardjo, Sherry A. (2006). "The acute and chronic toxic effects of vitamin A". Am. J. Clin. Nutr. 83 (2): 191–201. PMID 16469975.  
  24. ^ Eledrisi, Mohsen S. . Vitamin A Toxicity. eMedicine.
  25. ^ Sale TA, Stratman E (2004). "Carotenemia associated with green bean ingestion". Pediatr Dermatol 21 (6): 657–9. doi:10.1111/j.0736-8046.2004.21609.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 15575851.  
  26. ^ Nishimura Y, Ishii N, Sugita Y, Nakajima H (1998). "A case of carotenodermia caused by a diet of the dried seaweed called Nori". J. Dermatol. 25 (10): 685–7. PMID 9830271.  
  27. ^ Takita Y, Ichimiya M, Hamamoto Y, Muto M (2006). "A case of carotenemia associated with ingestion of nutrient supplements". J. Dermatol. 33 (2): 132–4. doi:10.1111/j.1346-8138.2006.00028.x. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16556283.  
  28. ^ Forsmo, Siri; Fjeldbo,Sigurd Kjørstad; Langhammer, Arnulf (2008). "Childhood Cod Liver Oil Consumption and Bone Mineral Density in a Population-based Cohort of Peri- and Postmenopausal Women: The Nord-Trøndelag Health Study". Am. J. Epidemiol. 167 (4): 406–411. doi:10.1093/aje/kwm320. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
  29. ^ Science News. Water-soluble vitamin A shows promise.
  30. ^ Water-miscible, emulsified, and solid forms of retinol supplements are more toxic than oil-based preparations. PMID: 14668278
  31. ^ Early-life supplementation of vitamins A and D, in water-soluble form or in peanut oil, and allergic diseases during childhood. doi:10. 1016/j. jaci. 2006. 08. 022

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External links

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Dictionary

vitamin A

-noun

  1. (biochemistry) Any of several closely related fat-soluble vitamins that are essential for normal vision; retinol.
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