Skip to main content
Select Source:

Vitamin A

Vitamin A

Description

Vitamin A is one of four fat-soluble vitamins necessary for good health. It serves an important role as an antioxidant by helping to prevent free radicals from causing cellular damage. Adequate levels are important for good eyesight, and poor night vision may be one of the first symptoms of a deficiency. It is also necessary for proper function of the immune, skeletal, respiratory, reproductive, and integumentary (skin) systems.

General Use

An adequate level of vitamin A unquestionably contributes to good health. It is essential for the proper function of the retina, where it can act to prevent night blindness , as well as lower the odds of getting age-related macular degeneration (AMD), which is the most common cause of blindness in the elderly. There is also evidence that good levels of vitamin A in the form of carotenoids may decrease the risk of certain cancers, heart attacks, and strokes. The immune system is also strengthened. It is unclear, however, if supplemental forms of vitamin A have the same benefit as consuming the nutrient in natural foods in the case of a person without deficiency. Taking high levels of vitamin A in any supplemental form is not advisable without the counsel of a healthcare professional.

Preparations

Natural sources

There are two basic forms of vitamin A. Retinoids, the active types, are contained in animal sources, including meat, whole milk, and eggs. Liver is particularly rich in vitamin A, since it is one of the storage sites for excess. Precursor forms of the vitamin (carotenoids) are found in orange and leafy green produce such as sweet potatoes, carrots, collard greens, spinach, winter squash, kale, and turnip greens. Very fresh foods have the highest levels, followed by frozen foods. Typically, canned produce has little vitamin A. Preparing vegetables by steaming, baking, or grilling helps them to release the carotenes they contain. Alpha and beta carotene , as well as some of the other lesser-known carotenoids, can be converted to vitamin A in the small intestine. This is done by the body on an as-needed basis, so there is no risk of overdose as there is with the active form.

Supplemental sources

Supplements may contain either the active or precursor forms of vitamin A. The active form may be more desirable for those who may have some difficulty in converting the carotenoids into the active vitamin. This is more often true in those over age 55 or who have a condition that impairs the absorption of fat. There is a water-soluble form of the vitamin, retinyl palmitate, which may be better utilized in the latter case. Carotenes are also available either as oil-based or natural water-based formulas. Be sure to store both away from light and heat, which will destroy them.

Units

There are several units that can express the amount of vitamin A activity in a product. Many supplements are still labeled with the old International Unit (IU), although the more current and most accurate unit is the Retinol Equivalent (RE). The new measurement distinguishes between the differences in absorption of retinol and beta carotene. One RE is equal to one microgram (MICROg) of retinol, or six MICROg of beta carotene.

Dose limits

Adults should take no more than 25,000 IU (5,000 RE) per day of vitamin A in its active form, except in the case of women who are pregnant or may become pregnant. The latter group should not exceed 10,000 IU (2,000 RE) per day in order to avoid potential toxic effects to the fetus. The best way to get vitamins is in the natural food form, as the complexities are not always either known or reproducible in a supplement. A diet rich in foods containing carotenoids is optimal, but in the event of nutritional deficiencies, supplements may be needed. Mixed carotenoids are preferable to either large doses of vitamin A or pure beta carotene supplements to avoid toxicity and maximize healthful benefits. Some of the minor carotenoids appear to have beneficial effects that are still being explored. A good mixture will contain alpha and beta carotene, as well as lycopene and xanthophylls. Eating foods high in many carotenoids may confer some benefitssuch as a lower risk of cancer , heart attacks, and strokeswhich a supplement may not.

Deficiency

Levels of vitamin A low enough to cause symptomatic deficiency are uncommon in people of normal health in industrialized nations. Symptoms of deficiency may include, but are not limited to, loss of appetite, poor immune function causing frequent infections (especially respiratory), hair loss, rashes , dry skin and eyes, visual difficulties including night blindness, poor growth, and fatigue . Generally symptoms are not manifested unless the deficiency has existed for a period of months. Deficiencies are more likely in people who are malnourished, including alcoholics, the chronically ill, and those with impaired fat absorption. Another group at increased risk of vitamin A deficiency are persons with type 1 diabetes whose disorder is poorly controlled. People with normal health and nutritional status have a considerable vitamin A reserve.

In countries where nutritional status tends to be poor and deficiency is more common, vitamin A has been found to reduce the mortality rate of children suffering from a number of different viral infections.

Experts in plant genetics have been working on a strain of rice that contains beta carotene, hoping to help people in developing countries avoid the risk of vitamin A deficiency. Known as Golden Rice, the new strain is being sent to research institutes in developing countries for further study.

Risk factors for deficiency

Taking the RDA level of a nutrient will prevent a deficiency in most people, but under certain circumstances, an individual may require higher doses of vitamin A. Those who consume alcoholic beverages may be more prone to vitamin A deficiency. People taking some medications, including birth control pills, methotrexate, cholestyramine, colestipol, and drugs that act to sequester bile will also need larger amounts. Those who are malnourished, chronically ill, or recovering from surgery or other injuries may also benefit from a higher than average dose. Patients undergoing treatments for cancer, including radiation and chemotherapy, typically have compromised immune systems that may be boosted by judicious supplementation with vitamin A. Other conditions that may impair vitamin A balance include chronic diarrhea , cystic fibrosis, and kidney or liver disease. Diabetics are often deficient in vitamin A, but may also be more susceptible to toxicity. Any supplementation for these conditions should be discussed with a healthcare provider. Supplements are best taken in the form of carotenoids to avoid any potential for toxicity. There is not an established RDA for beta carotene. Recommendations for how much to take vary between 6 and 30 mg a day, but the middle rangearound 15 mgis a reasonable average.

Precautions

Overdose can occur when taking megadoses of the active form of this vitamin. Amounts above what is being utilized by the body accumulate in the liver and fatty tissues. Symptoms may include dry lips and skin, bone and joint pain , liver and spleen enlargement, diarrhea, vomiting , headaches, blurry or double vision, confusion, irritability, fatigue, and bulging fontanel (soft spot on the head) in infants; these are most often reversible, but a doctor should be contacted if a known overdose occurs. Very high levels of vitamin A may also create deficiencies of vitamins C, E, and K. Symptoms will generally appear within six hours following an acute overdose, and take a few weeks to resolve after ceasing the supplement. Children are more sensitive to high levels of vitamin A than adults are, so instructions on products designed for children should be followed with particular care. Vitamin supplements should always be kept out of reach of children.

It is especially important to avoid overdoses in pregnancy , as it may cause miscarriage or fetal malformations. Using supplements that provide carotenoids will avoid the potential of overdose. Those with kidney disease are also at higher risk for toxicity due to either vitamin A or beta carotene, and should not take these supplements without professional healthcare advice.

There is some evidence that taking beta carotene supplements puts smokers at a higher risk of lung cancers. The CARET (Beta Carotene and Retinol Efficacy Trial) study is one that demonstrated this effect. Clarification through more study is needed, as evidence also exists showing that beta carotene, along with other antioxidants , can be a factor in cancer prevention. For example, a team of American researchers has recently reviewed evidence that vitamin A protects against bladder cancer , and a group in Germany is testing an aerosol form of vitamin A to prevent lung cancer . Some of the lesser-known carotenoids may be key factors in the relationship between vitamin intake and cancer. Whole sources of vitamin A are better obtained from foods than from supplements. Smokers should consult with a healthcare provider before taking supplemental beta carotene.

Side effects

Very high levels of carotenoids (carotenemia) may cause an orange discoloration of the skin, which is harmless and transient.

Interactions

Vitamin A supplements should not be taken in conjunction with any retinoid medications, including isotretinoin (Accutane), a drug used to treat acne . There is a higher risk of toxicity.

A very low fat diet or use of fat substitutes impairs absorption of all the fat-soluble vitamins, including A. Mineral oil and aluminum-containing antacids may also inhibit absorption, as do the cholesterol-lowering drugs cholestyramine and colestipol. Vitamin A reserves of the body are depleted by a number of substances, including alcohol, barbiturates, caffeine , cortisone, tobacco, and very high levels of vitamin E . Overuse of alcohol and vitamin A together may increase the possibility of liver damage.

Taking appropriate doses of vitamin C , vitamin E, zinc , and selenium optimizes absorption and use of vitamin A and carotenoids. As vitamin A is fat-soluble, a small amount of dietary fat is also helpful.

Studies of both children and pregnant women with iron deficiency anemia show that this condition is better treated with a combination of iron supplements and vitamin A than with iron alone.

Resources

BOOKS

Bratman, Steven, and David Kroll. Natural Health Bible. Prima Publishing, 1999.

Feinstein, Alice. Prevention's Healing with Vitamins. Rodale Press, 1996.

Griffith, H. Winter. Vitamins, Herbs, Minerals & Supplements: The Complete Guide. Fisher Books, 1998.

Jellin, Jeff, Forrest Batz, and Kathy Hitchens. Pharmacist's letter/Prescriber's Letter Natural Medicines Comprehensive Database. Therapeutic Research Faculty, 1999.

PERIODICALS

Baena, R. M., C. Campoy, R. Bayes, et al. "Vitamin A, Retinol-Binding Protein and Lipids in Type 1 Diabetes Mellitus." European Journal of Clinical Nutrition 56 (January 2002): 4450.

Beyer, P., S. Al-Babili, X. Ye, et al. "Golden Rice: Introducing the Beta-Carotene Biosynthesis Pathway Into Rice Endosperm by Genetic Engineering to Defeat Vitamin A Deficiency." Journal of Nutrition 132 (March 2002): 506S510S.

Kamat, A. M., and D. L. Lamm. "Chemoprevention of Bladder Cancer." Urology Clinics of North America 29 (February 2002): 157168.

Kohlhaufl, M., K. Haussinger, F. Stanzel, et al. "Inhalation of Aerosolized Vitamin A: Reversibility of Metaplasia and Dysplasia of Human Respiratory EpitheliaA Prospective Pilot Study." European Journal of Medical Research 7 (February 21, 2002): 7278.

Miksad, R., V. de Ledinghen, C. McDougall, et al. "Hepatic Hydrothorax Associated with Vitamin A Toxicity." Journal of Clinical Gastroenterology 34 (March 2002): 275279.

van den Berg, H., M. van der Gaag, and H. Hendriks. "Influence of Lifestyle on Vitamin Bioavailability." International Journal of Vitamin and Nutrition Research 72 (January 2002): 5359.

Yeum, K. J., and R. M. Russell. "Carotenoid Bioavailability and Bioconversion." Annual Review of Nutrition 22 (2002): 483504.

Judith Turner

Rebecca J. Frey, PhD

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Vitamin A." Gale Encyclopedia of Alternative Medicine. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"Vitamin A." Gale Encyclopedia of Alternative Medicine. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/vitamin-1

"Vitamin A." Gale Encyclopedia of Alternative Medicine. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/vitamin-1

Vitamin A Deficiency

Vitamin A Deficiency

Definition

Vitamin A deficiency exists when the chronic failure to eat sufficient amounts of vitamin A or beta-carotene results in levels of blood-serum vitamin A that are below a defined range. Beta-carotene is a form of pre-vitamin A, which is readily converted to vitamin A in the body. Night blindness is the first symptom of vitamin A deficiency. Prolonged and severe vitamin A deficiency can produce total and irreversible blindness.

Description

Vitamin A (called retinol in mammals) is a fat-soluble vitamin. The recommended dietary allowance (RDA) for vitamin A is 1.0 mg/day for the adult man and 0.8 mg/day for the adult woman. Since beta-carotene is converted to vitamin A in the body, the body's requirement for vitamin A can be supplied entirely by beta-carotene. Six mg of beta-carotene are considered to be the equivalent of 1 mg of vitamin A. The best sources of vitamin A are eggs, milk, butter, liver, and fish, such as herring, sardines, and tuna. Beef is a poor source of vitamin A. Plants do not contain vitamin A, but they do contain beta-carotene and other carotenoids. The best sources of beta-carotene are dark-green, orange, and yellow vegetables; spinach, carrots, oranges, and sweet potatoes are excellent examples. Cereals are poor sources of beta-carotene.

Vitamin A is used for two functions in the body. Used in the eye, it is a component of the eye's light-sensitive parts, containing rods and cones, that allow for night-vision or for seeing in dim-light circumstances. Vitamin A (retinol) occurs in the rods. Another form of Vitamin A, retinoic acid, is used in the body for regulating the development of various tissues, such as the cells of the skin, and the lining of the lungs and intestines. Vitamin A is important during embryological development, since, without vitamin A, the fertilized egg cannot develop into a fetus.

Causes and symptoms

Vitamin A deficiency occurs with the chronic consumption of diets that are deficient in both vitamin A and beta-carotene. When vitamin A deficiency exists in the developed world, it tends to happen in alcoholics or in people with diseases that affect the intestine's ability to absorb fat. Examples of such diseases are celiac disease (chronic nutritional disorder), cystic fibrosis, and cholestasis (bile-flow failure or interference). Vitamin A deficiency occurred in infants during the early 1900s in Denmark. The deficiency resulted when milk fat was made into butter for export, leaving the by-product (skimmed milk) for infant feeding. Vitamin A deficiency has taken place in infants in impoverished populations in India, where the only foods fed to the infants were low in beta-carotene. Vitamin A deficiency is also common in areas like Southeast Asia, where polished rice, which lacks the vitamin, is a major part of the diet.

KEY TERMS

Bitot's spots Bitot's spots are superficial, foamy gray, triangular spots on the white of the eyeball.

Carotenoids Carotenoids are yellow to deep-red pigments.

Conjunctiva The conjunctiva is a clear layer of cells that covers the eye and directly contacts the atmosphere. The conjunctiva is about five-cells thick.

Cornea The cornea is a clear layer of cells that covers the eye, just under the conjunctiva. The cornea is about 50-cells thick.

Fat-soluble vitamin Fat-soluble vitamins can be dissolved in oil or in melted fat. Water-soluble vitamins can be dissolved in water or juice.

Keratomalacia Keratomalacia is ulceration of the cornea.

Recommended Dietary Allowance (RDA) The Recommended Dietary Allowances are quantities of nutrients in the diet that are required to maintain good health in people. RDAs are established by the Food and Nutrition Board of the National Academy of Sciences, and may be revised every few years. A separate RDA value exists for each nutrient. The RDA values refer to the amount of nutrient expected to maintain good health in people. The actual amounts of each nutrient required to maintain good health in specific individuals differ from person to person.

Xerophthalmia Xerophthalmia is a dry, thickened, lusterless condition of the eyeball resulting from vitamin A deficiency.

The earliest symptom of vitamin A deficiency is night blindness. Prolonged deficiency results in drying of the conjunctiva (the mucous membrane that lines the inner surface of the eyelids and extends over the forepart of the eyeball). With continued vitamin A deficiency, the drying extends to the cornea (xerophthalamia). The cornea eventually shrivels up and becomes ulcerated (keratinomalacia). Superficial, foamy gray triangular spots may appear in the white of the eye (Bitot's spots). Finally, inflammation and infection occur in the interior of the eye, resulting in total and irreversible blindness.

Diagnosis

Vitamin A status is measured by tests for retinol. Blood-serum retinol concentrations of 30-60 mg/dl are considered in the normal range. Levels that fall below this range indicate vitamin A deficiency. Night blindness is measured by a technique called electroretinography. Xerophthalamia, keratinomalacia, and Bitot's spots are diagnosed visually by trained medical personnel.

Treatment

Vitamin A deficiency can be prevented or treated by taking vitamin supplements or by getting injections of the vitamin. The specific doses given are oral retinyl palmitate (110 mg), retinyl acetate (66 mg), or injected retinyl palmitate (55 mg) administered on each of two successive days, and once a few weeks later if symptoms are not relieved.

Prognosis

The prognosis for correcting night blindness is excellent. Xerophthalamia can be corrected with vitamin A therapy. Ulcerations, tissue death, and total blindness, caused by severe vitamin A deficiency, cannot be treated with vitamin A.

Prevention

Vitamin A deficiency can be prevented by including foods rich in vitamin A or beta-carotene as a regular component of the diet; liver, meat, eggs, milk, and dairy products are examples. Foods rich in beta-carotene include red peppers, carrots, pumpkins, as well as those just mentioned. Margarine is rich in beta-carotene, because this chemical is used as a coloring agent in margarine production. In Africa, Indonesia, and the Philippines, vitamin A deficiency is prevented by public health programs that supply children with injections of the vitamin.

Resources

BOOKS

Brody, T. Nutritional Biochemistry. San Diego: Academic Press, Inc., 1998.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Vitamin A Deficiency." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"Vitamin A Deficiency." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/vitamin-deficiency

"Vitamin A Deficiency." Gale Encyclopedia of Medicine, 3rd ed.. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/vitamin-deficiency

Vitamin A

Vitamin A

Since its discovery in the early part of the 20th century, researchers have learned a great deal about vitamin A. They now know that this once-mysterious compound is essential for normal growth and development, maintaining the body's skin cells, and forming part of the two pigments needed by the retina to help the eye adjust to varying degrees of light.

McCollum's Research

By 1906 English biochemist Frederick Gowland Hopkins (1861-1947) was looking into what he termed "accessory food factors." But the scientific community generally believed that all dietary needs could be met by proper amounts of carbohydrates, proteins, fats, minerals, and water. The concept of trace nutrients was largely unknown.

In 1907 Elmer McCollum joined the University of Wisconsin as an instructor. Born and raised in Kansas, McCollum had recently received his doctorate from Yale University. McCollum originally studied inorganic chemistry, but had switched to biochemistry simply because no position was immediately available in his chosen field. Biochemistry was known as agricultural chemistry at the turn of the century.

The first research project McCollum was assigned to was to study the effects of different single-grain diets on dairy cattle. It was assumed that the three chemically similar grains would have similar dietary effects. To the research team's surprise, however, only the corn-fed cows remained healthy. Those animals that were fed wheat or oats did not thrive. McCollum was immediately intrigued. He reasoned that there must be some undiscovered difference in the structures of the similar foods.

Before long, McCollum was designing his own nutritional studies. Because he felt that dairy cattle were too cumbersome to work with, he set up a colony of albino rats. This was the country's first such colony devoted to experimental research. McCollum was assisted in his work by Marguerite Davis, a recent University of Wisconsin graduate. It was in his own laboratory that the studies began that would make him famous.

By 1913 McCollum reported that when laboratory rats were put on diets in which lard or olive oil was the only source of fat they eventually stopped growing. These same rats, however, quickly resumed normal growth when fed extracts of eggs or butter. McCollum concluded that butterfat and egg yolks must contain some growth-promoting factor missing in other fats.

The Missing Ingredient

Within two years, McCollum had isolated the growth-promoting factor. He named it "fat-soluble A" to distinguish it from a water-soluble factor. The water-soluble factor was previously discovered in rice polishings by Christiaan Eijkman (1858-1930). McCollum termed Eijkman's discovery "water-soluble B." Fat-soluble A was soon known as vitamin A, the first vitamin actually to be named and described.

[See also Eijkman, Christiaan ; Hopkins, Frederick Gowland ; Vitamin ]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Vitamin A." Medical Discoveries. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"Vitamin A." Medical Discoveries. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/medicine/medical-journals/vitamin

"Vitamin A." Medical Discoveries. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/medical-journals/vitamin

vitamin A

vitamin A Essential in the diet either as the preformed vitamin (retinol) found in animal foods or as a precursor, carotene, found in plant foods (usually both are present in the diet). Required for control of growth, cell turnover and fetal development, maintenance of fertility, and maintenance of the epithelial tissues lining the mouth and respiratory and urinary tracts; essential in vision.

Deficiency leads to slow adaptation to see in dim light (poor dark adaptation), an early sign of deficiency, and later to night blindness; then drying of the tear ducts (xerophthalmia) and ulceration of the cornea (keratomalacia) resulting in blindness.  Retinol occurs in animal products, especially liver, kidney, fish liver oils, milk, and butter. Carotene is found in green‐ and orange‐coloured vegetables and fruits; especially rich in red palm oil and carrots.

The vitamin A content of foods is expressed as retinol equivalents, i.e. retinol plus carotene; 1 μg retinol = 6 μg β‐carotene = 12 μg other active carotenoids = 3.33 international units.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A." A Dictionary of Food and Nutrition. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A." A Dictionary of Food and Nutrition. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0

"vitamin A." A Dictionary of Food and Nutrition. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0

vitamin A

vitamin A (retinol) A fat-soluble vitamin that cannot be synthesized by mammals and other vertebrates and must be provided in the diet. Green plants contain precursors of the vitamin, notably carotenes, that are converted to vitamin A in the intestinal wall and liver. The aldehyde derivative of vitamin A, retinal, is a constituent of the visual pigment rhodopsin. Deficiency affects the eyes, causing night blindness, xerophthalmia (dryness and thickening of the cornea), and eventually total blindness. The role of vitamin A in other aspects of metabolism is less clear; it may be involved in controlling ATP production and the growth of epithelial cells.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A." A Dictionary of Biology. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A." A Dictionary of Biology. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/vitamin-2

"vitamin A." A Dictionary of Biology. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/vitamin-2

vitamin A toxicity

vitamin A toxicity Retinol in excess of requirements is stored in the liver, bound to proteins, and is a cumulative poison. When the storage capacity is exceeded, free retinol causes damage to cell membranes. Carotene is not toxic in excess, since there is only a limited capacity to form retinol from carotene. The recommended upper limits of habitual daily intake of retinol are about 12.5 × reference intake for adults, but only 2.5 × reference intake for infants. Retinol is also teratogenic in excess, and for pregnant women the recommended upper limit of daily intake is 3000–3300 μg.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A toxicity." A Dictionary of Food and Nutrition. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A toxicity." A Dictionary of Food and Nutrition. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/vitamin-toxicity

"vitamin A toxicity." A Dictionary of Food and Nutrition. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/vitamin-toxicity

vitamin A

vitamin A (retinol) n. a fat-soluble vitamin that occurs preformed in foods of animal origin (especially milk products, egg yolk, and liver) and is formed in the body from the pigment β-carotene, present in some vegetable foods (for example cabbage, lettuce, and carrots). Retinol is essential for growth, vision in dim light, and the maintenance of soft mucous tissue. A deficiency causes stunted growth, night blindness, xerophthalmia, keratomalacia, and eventual blindness. Recommended daily intake: 750 μg retinol equivalents.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A." A Dictionary of Nursing. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A." A Dictionary of Nursing. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0

"vitamin A." A Dictionary of Nursing. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0

Bitot's spots

Bitot's spots (bee-tohz) pl. n. cheesy foamy greyish spots that form on the surface of dry patches of conjunctiva at the sides of the eyes. They consist of fragments of keratinized epithelium. A common cause is vitamin A deficiency. [ P. A. Bitot (1822–88), French physician]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Bitot's spots." A Dictionary of Nursing. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"Bitot's spots." A Dictionary of Nursing. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/bitots-spots

"Bitot's spots." A Dictionary of Nursing. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/bitots-spots

Bitots spots

Bitot's spots Irregularly shaped foam‐like plaques on the conjunctiva of the eye, characteristically seen in vitamin A deficiency, but not considered to be a diagnostic sign without other evidence of deficiency.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Bitots spots." A Dictionary of Food and Nutrition. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"Bitots spots." A Dictionary of Food and Nutrition. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/bitots-spots

"Bitots spots." A Dictionary of Food and Nutrition. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/bitots-spots

vitamin A

vi·ta·min A • n. another term for retinol.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/vitamin-1

"vitamin A." The Oxford Pocket Dictionary of Current English. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/vitamin-1

vitamin A

vitamin A See RETINOL.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"vitamin A." A Dictionary of Zoology. . Encyclopedia.com. 19 Aug. 2017 <http://www.encyclopedia.com>.

"vitamin A." A Dictionary of Zoology. . Encyclopedia.com. (August 19, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0

"vitamin A." A Dictionary of Zoology. . Retrieved August 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/vitamin-0