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Vitamin A
Overview

Vitamin A is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is absolutely necessary for both low-light and color vision. Vitamin A also functions in a very different role, as an irreversibly oxidized form of retinol known as retinoic acid, which is an important hormone-like growth factor for epithelial and other cells.

In foods of animal origin, the major form of vitamin A is an ester, primarily retinyl palmitate, which is converted to the retinol (chemically an alcohol) in the small intestine. The retinol form functions as a storage form of the vitamin, and can be converted to and from its visually active aldehyde form, retinal. The associated acid (retinoic acid), a metabolite that can be irreversibly synthesized from vitamin A, has only partial vitamin A activity, and does not function in the retina for the visual cycle.

All forms of vitamin A have a beta-ionone ring to which an isoprenoid chain is attached, called a retinyl group. Both structural features are essential for vitamin activity. The orange pigment of carrots – beta-carotene – can be represented as two connected retinyl groups, which are used in the body to contribute to vitamin A levels. Alpha-carotene and gamma-carotene also have a single retinyl group, which give them some vitamin activity. None of the other carotenes have vitamin activity. The Carotenoid beta-cryptoxanthin possesses an ionone group and has vitamin activity in humans.

Vitamin A can be found in two principal forms in foods:

arw Retinol, the form of vitamin A absorbed when eating animal food sources, is a yellow, fat-soluble substance. Since the pure alcohol form is unstable, the vitamin is found in tissues in a form of retinyl ester. It is also commercially produced and administered as esters such as retinyl acetate or palmitate.
   
arw The carotenes alpha-carotene, beta-carotene, gamma-carotene; and the xanthophyll beta-cryptoxanthin (all of which contain beta-ionone rings), but no other carotenoids, function as vitamin A in herbivores and omnivore animals, which possess the enzyme required to convert these compounds to retinal. In general, carnivores are poor converters of ionine-containing carotenoids, and pure carnivores such as cats and ferrets lack beta-carotene 15,15'-monooxygenase and cannot convert any carotenoids to retinal
 

What is Vitamin A ?
A is a group of compounds that play an important role in vision, bone growth, reproduction, cell division, and cell differentiation (in which a cell becomes part of the brain, muscle, lungs, blood, or other specialized tissue.) Vitamin A helps regulate the immune system, which helps prevent or fight off infections by making white blood cells that destroy harmful bacteria and viruses. Vitamin A also may help lymphocytes (a type of white blood cell) fight infections more effectively.

Vitamin A promotes healthy surface linings of the eyes and the respiratory, urinary, and intestinal tracts .When those linings break down, it becomes easier for bacteria to enter the body and cause infection. Vitamin A also helps the skin and mucous membranes function as a barrier to bacteria and viruses.

In general, there are two categories of vitamin A, depending on whether the food source is an animal or a plant. Vitamin A found in foods that come from animals is called preformed vitamin A. It is absorbed in the form of retinol, one of the most usable (active) forms of vitamin A. Sources include liver, whole milk, and some fortified food products. Retinol can be made into retinal and retinoic acid (other active forms of vitamin A) in the body.

Vitamin A that is found in colorful fruits and vegetables is called provitamin a carotenoid. They can be made into retinol in the body. In the United States, approximately 26% of vitamin A consumed by men and 34% of vitamin A consumed by women is in the form of provitamin A carotenoids .Common provitamin A carotenoids found in foods that come from plants are beta-carotene, alpha-carotene, and beta-cryptoxanthin. Among these, beta-carotene is most efficiently made into retinol. Alpha-carotene and beta-cryptoxanthin are also converted to vitamin A, but only half as efficiently as beta-carotene.
Of the 563 identified carotenoids, fewer than 10% can be made into vitamin A in the body. Lycopene, lutein, and zeaxanthin are carotenoids that do not have vitamin A activity but have other health promoting properties. The Institute of Medicine (IOM) encourages consumption of all Carotenoid-rich fruits and vegetables for their health-promoting benefits.

Some provitamin A carotenoids have been shown to function as antioxidants in laboratory studies; however, this role has not been consistently demonstrated in humans. Antioxidants protect cells from free radicals, which are potentially damaging by-products of oxygen metabolism that may contribute to the development of some chronic diseases.

Where it is found
Vitamin A comes from animal sources, such as eggs, meat, milk, cheese, cream, liver, kidney, cod, and halibut fish oil. However, all of these sources - except for skim milk that has been fortified with Vitamin A - are high in saturated fat and cholesterol.

Sources of beta-carotene are carrots, pumpkin, sweet potatoes, winter squashes, cantaloupe, pink grapefruit, apricots, broccoli, spinach, and most dark green, leafy vegetables. The more intense the color of a fruit or vegetable, the higher the beta-carotene content. These vegetable sources of beta-carotene are free of fat and cholesterol.

 
See Vitamin A related videos:
video icon Vitamin A (video module – 1.13 minutes)
Product related PDF file
The A Vitamin
THE VITAMIN A DEFICIENCY
 

Benefits / uses
Vitamin A helps form and maintain healthy teeth, skeletal and soft tissue, mucous membranes, and skin. It is also known as retinol because it produces the pigments in the retina of the eye.

Vitamin A promotes good vision, especially in low light. It may also be needed for reproduction and breast-feeding.

Retinol is an active form of vitamin A. It is found in animal liver, whole milk, and some fortified foods.

Carotenoids are dark colored dyes found in plant foods that can turn into a form of vitamin A. One such Carotenoid is beta-carotene. Beta-carotene is an antioxidant. Antioxidants protect cells from damage caused by unstable substances called free radicals. Free radicals are believed to contribute to certain chronic diseases and play a role in the degenerative processes seen in aging.

When to take/Type to take
Vitamin A supplement are best to be taken with meal. Vitamin A is available as fish liver oil or as a palmitate (from palmitic acid) However, beta-carotene is a preferential method for supplementing with vitamin A. Unless the user is diabetic, the body can convert beta-carotene into vitamin A as needed. As such, risk for vitamin A toxicity is virtually nonexistent.

Doses
Infants
0 - 6 months: 400 micrograms per day (mcg/day)
7 - 12 months: 500 mcg/day

Children
1 - 3 years: 300 mcg/day
4 - 8 years: 400 mcg/day
9 - 13 years: 600 mcg/day

Adolescents and Adults
Males age 14 and older: 900 mcg/day
Females age 14 and older: 700 mcg/day

Possible Side effects / Precautions / Possible Interactions:
If you don't get enough vitamin A, you are more susceptible to infectious diseases and vision problems.If you get too much vitamin A, you can become sick. Large doses of vitamin A can also cause birth defects. Acute vitamin A poisoning usually occurs when an adult takes several hundred thousand IU. Symptoms of chronic vitamin A poisoning may occur in adults who regularly take more than 25,000 IU a day. Babies and children are more sensitive and can become sick after taking smaller doses of vitamin A or vitamin A-containing products such as retinol (found in skin creams).
Increased amounts of beta-carotene can turn the color of skin to yellow or orange. The skin color returns to normal once the increased intake of beta-carotene is reduced.

Research studies / References

arw Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. National Academy Press, Washington, DC, 2001.
   
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arw Ross AC, Gardner EM. The function of vitamin A in cellular growth and differentiation, and its roles during pregnancy and lactation. Adv Exp Med Biol 1994;352:187-200. [PubMed abstract]
   
arw .Ross AC. Vitamin A and retinoids. In: Modern Nutrition in Health and Disease. 9th Edition (edited by Shils ME, Olson J, Shike M, Ross AC). Lippincott Williams and Wilkins, New York, 1999, pp. 305-27.
   
arw Ross AC, Stephensen CB. Vitamin A and retinoids in antiviral responses. FASEB J 1996;10:979-85. [PubMed abstract]
   
arw Semba RD. The role of vitamin A and related retinoids in immune function. Nutr Rev 1998;56:S38-48. [PubMed abstract]
   
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arw U.S. Department of Health and Human Services. Advance Data from Vital and Health Statistics. Dietary Intake of Selected Vitamins for the United States Population: 1999-2000. Centers for Disease Control and Prevention. National Center for Health Statistics. Number 339, 2004.
   
arw Rodrigues MI, Dohlman CH. Blindness in an American boy caused by unrecognized vitamin A deficiency. Arch Ophthalmol 2004;122:1228-9.
   
arw .Sommer A. Nutritional Blindness: Xeropthalmia and Keratomalacia. Oxford University Press, London and New York, 1982.
   
arw Ross AC. Vitamin A status: Relationship to immunity and the antibody. Proc Soc Exp Biol Med 1992;200:303-20. [PubMed abstract]
   
arw Stephens D, Jackson PL, Gutierrez Y. Subclinical vitamin A deficiency: A potentially unrecognized problem in the United States. Pediatr Nurs 1996;22:377-89. [PubMed abstract]
   
arw Butler JC, Havens PL, Sowell AL, Huff DL, Peterson DE, Day SE, Chusid MJ, Benning RA, Circo R, Davis JP. Measles severity and serum retinol (vitamin A) concentration among children in the United States. Pediatrics 1993;91:1176-81. [PubMed abstract]
   
arw Leo MA, Lieber CS. Alcohol, vitamin A, and beta-carotene: Adverse interactions, including hepatotoxicity and carcinogenicity. Am J Clin Nutr 1999;69:1071-85. [PubMed abstract]
   
arw Committee on Infectious Diseases. Vitamin A treatment of measles. Pediatrics 1993;91:1014-5. [PubMed abstract]
   
arw Tursi A. Gastrointestinal motility disturbances in celiac disease. J Clin Gastroenterol 2004;38:642-5.
   
arw Krok KL, Lichtenstein GR. Nutrition in Crohns disease. Curr Opin Gastroenterol 2003;19:148-53.
   
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arw Koo LC. Diet and lung cancer 20+ years later: more questions than answers? Int J Cancer 1997;Suppl10:22-9. [PubMed abstract]
   
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arw Albanes D, Heinonen OP, Taylor PR, Virtamo J, Edwards BK, Rautalahti M, Hartman AM, Palmgren J, Freedman LS, Haapakoski J, Barrett MJ, Pietinen P, Malila N, Tala E, Lippo K, Salomaa ER, Tangrea JA, Teppo L, Askin FB, Taskinen E, Erozan Y, Greenwald P, Huttunen JK. Alpha-tocopherol and beta-carotene supplement and lung cancer incidence in the alpha-tocopherol, beta-carotene cancer prevention study: Effects of base-line characteristics and study compliance. J Natl Cancer Inst 1996;88:1560-70. [PubMed abstract]
   
arw Redlich CA, Blaner WS, Van Bennekum AM, Chung JS, Clever SL, Holm CT, Cullen MR. Effect of supplementation with beta-carotene and vitamin A on lung nutrient levels. Cancer Epidemiol Biomarkers Prev 1998;7:211-14. [PubMed abstract]
   
arw Pryor WA, Stahl W, Rock CL. Beta carotene: from biochemistry to clinical trials. Nutr Rev 2000;58:39-53.
   
arw National Institutes of Health. Osteoporosis prevention, diagnosis, and therapy. NIH Consensus Statement Online, 2000 March 27-29, 2000:1-36.
   
arw National Osteoporosis Foundation. NOF osteoporosis prevention-risk factors for osteoporosis. 2003. http://www.nof.org/prevention/risk.htm.
   
arw Binkley N, Krueger D. Hypervitaminosis A and bone. Nutr Rev 2000;58:138-44. [PubMed abstract]
   
arw Forsyth KS, Watson RR, Gensler HL. Osteotoxicity after chronic dietary administration of 13-cis-retinoic acid, retinyl palmitate or selenium in mice exposed to tumor initiation and promotion. Life Sci 1989;45:2149-56. [PubMed abstract]
   
arw Whiting SJ, Lemke B. Excess retinol intake may explain the high incidence of osteoporosis in northern Europe. Nutr Rev 1999;57:249-50. [PubMed abstract]
   
arw Johansson S, Melhus H. Vitamin A antagonizes calcium response to vitamin D in man. J Bone Miner Res 2001;16:1899-1905. [PubMed abstract]
   
arw Melhus H, Michaelsson K, Kindmark A, Bergstrom R, Holmberg L, Mallmin H, Wolk A, Ljunghall S. Excessive dietary intake of vitamin A is associated with reduced bone mineral density and increased risk of hip fracture. Ann Intern Med 1998;129:770-8. [PubMed abstract]
   
arw Feskanich D, Singh F, Willett WC, Colditz GA. Vitamin A intake and hip fractures among postmenopausal women. J Am Med Assoc 2002;287:47-54. [PubMed abstract]
   
arw .Michaelsson K, Lithell H, Vessby B, Mehus H. Serum retinol levels and the risk of fracture. N Engl J Med 2003;348:287-94.
   
arw Ballew C, Galuska D, Gillespie C. High serum retinyl esters are not associated with reduced bone mineral density in the third National Health and Nutrition Examination Survey, 1988-94. J Bone Miner Res 2001;16:2306-12. [PubMed abstract]
   
arw Bendich A, Langseth L. Safety of vitamin A. Am J Clin Nutr 1989;49:358-71. [PubMed abstract]
   
arw Udall JN, Greene HL. Vitamin update. Pediatr Rev 1992;13:185-94. [PubMed abstract]
   
arw Soprano DR, Soprano KJ. Retinoids as teratogens. Annu Rev Nutr 1995;15:111-32. [PubMed abstract]
   
arw Orafanos CE, Zouboulis CC, Almond-Roesler B, Geilen CC. Current use and future potential role of retinoids in dermatology. Drugs 1997;53:358-88. [PubMed abstract]
   
arw Meigel WN. How safe is oral isotretinoin? Dermatology 1997;195:22-28, 38-40. [PubMed abstract]
   
arw Hathcock JN. Vitamin and Mineral Safety. Council for Responsible Nutrition, Washington, DC, 1997, pp. 26-27.
   
arw Hennekens CH, Buring JE, Manson JE, Stampfer M, Rosner B, Cook NR, Belanger C, LaMotte F, Gaziano JM, Ridker PM, Willett W, Peto R. Lack of effect of long-term supplementation with beta-carotene on the incidence of malignant neoplasm and cardiovascular disease. N Eng J Med 1996;334:1145-9. [PubMed abstract]
   
arw Blot WJ, Li J-Y, Taylor PR, Guo W, Dawsey S, Wang G-Q, Yang CS, Zheng S-F, Gail M, Li G-Y, Yu Y, Liu B-Q, Tangrea J, Sun Y-H, Liu F, Fraumeni JF, Zhang Y-H, Li B. Nutrition intervention trials in Linxian, China: supplementation with specific vitamin/mineral combinations, cancer incidence, and disease-specific mortality in the general population. J Natl Cancer Inst 1993;85:1483-92. [PubMed abstract]
   
arw U.S. Department of Health and Human Services, U.S. Department of Agriculture. Dietary Guidelines for Americans 2005. Washington, DC: U.S. Government Printing Office, 2005. http://www.health.gov/dietaryguidelines/dga2005/document/pdf/DGA2005.pdf.
   
arw U.S. Department of Agriculture. MyPyramid.gov. 2005. http://www.mypyramid.gov/.