Vitamin A
Vitamin A
Definition
Vitamin A is a fat-soluble organic compound that the body needs to remain healthy. Humans cannot make vitamin A, so they must get it from foods in their diet. Vitamin A is sometimes called retinol.
Purpose
Vitamin A affects many different systems of the body. It is especially important to maintaining good vision, a healthy immune system, and strong bones. Vitamin A also helps turn on and off certain genes (gene expression) during cell division and differentiation. Getting the correct amount—not too little and not too much—of vitamin A is essential for health. People who get too little vitamin A have vision defects, are more likely to have damaged cells in the lining of
Vitamin A
| Age | Recommended Dietary Allowance | Tolerable Upper Intake Level |
| Children 0–6 mos. | 1,330 IU 400 RAE | 2,000 IU 600 RAE |
| Children 7–12 mos. | 1,670 IU 500 RAE | 2,000 IU 600 RAE |
| Children 1–3 yrs. | 1,000 IU 300 RAE | 2,000 IU 600 RAE |
| Children 4–8 yrs. | 1,330 IU 400 RAE | 3,000 IU 900 RAE |
| Children 9–13 yrs. | 2,000 IU 600 RAE | 5,610 IU 1,700 RAE |
| Boys 14–18 yrs. | 3,000 IU 900 RAE | 9,240 IU 2,800 RAE |
| Girls 14–18 yrs. | 2,310 IU 700 RAE | 9,240 IU 2,800 RAE |
| Men 19≥yrs. | 3,000 IU 900 RAE | 10,000 IU 3,000 RAE |
| Women 19≥yrs. | 2,310 IU 700 RAE | 10,000 IU 3,000 RAE |
| Pregnant women 19≥yrs. | 2,500 IU 750 RAE | 10,000 IU 3,000 RAE |
| Breastfeeding women 19≥yrs. | 4,300 IU 1,300 RAE | 10,000 IU 3,000 RAE |
| Food | Vitamin A (retinol) |
| Beef liver, cooked, 3 oz. | 27,185 IU |
| Chicken liver, cooked, 3 oz. | 12,325 IU |
| Skim milk, vitamin A fortified, 1 cup | 500 IU |
| Butter, 1 tbsp. | 325 IU |
| Egg, 1 whole | 300 IU |
| Whole milk cheddar cheese, 1 oz. | 280 IU |
| Whole milk, 1 cup | 250 IU |
| Food | Vitamin A (provitamin A carotenoid) |
| Spinach, cooked, ½ cup | 11,460 IU |
| Kale, cooked, ½ cup | 9,560 IU |
| Carrot, raw, unpeeled, 1 whole (7.5") | 8,670 IU |
| Cantaloupe, 1 cup | 5,410 IU |
| Spinach, raw, 1 cup | 2,800 IU |
| Papaya, 1 cup | 1,530 IU |
| Carrot, raw, peeled, sliced, ½ cup | 1,285 IU |
| Mango, 1 cup | 1,260 IU |
| Tomato juice, 6 oz. | 820 IU |
| Cereal, vitamin A fortified, 1 serving | 500–770 IU |
IU = International Unit
RAE = retinol activity equivalents
(Illustration by GGS Information Services/Thomson Gale.)
body cavities, and are more susceptible to infection. People who get too much vitamin A have weaken bones that tend to break easily and have a chronic feeling of illness, including headache, nausea, irritability, fatigue, and muscle and joint pain. Women who get too much vitamin A may have disrupted menstrual cycles. Excess vitamin A can also cause birth defects in a developing fetus.
Description
Vitamin A was the first fat-soluble vitamin to be discovered. In 1913, two groups of American scientists experimenting with animal feed almost simultaneously discovered a substance essential to health that was present in whole milk but absent in fat-free milk. They called this “fat-soluble factor A,” later renamed vitamin A. Today scientists know that vitamin A is found in food that comes from both animal and plants, is used by many systems in the body besides vision, and comes in several different forms.
Vitamin A from animal sources
Vitamin A found in food that comes from animals is in the form of a compound called retinol or preformed vitamin A. Sometimes retinol is called “true” vitamin A because it can be used by the body without any chemical changes. It can also be converted into retinoic acid, a compound involved in the control of gene expression. About 80% of the retinol in an individual's diet is absorbed by the body.
Good sources of retinol include beef or chicken liver, whole eggs, whole milk, and cheese made with whole milk. Some manufactured foods such as breakfast cereals and fat-free milk are fortified with vitamin A in the form of retinol. Dietary supplements of vitamin A and multivitamin tablets or capsules also contain this form of vitamin A. Americans who eat meat get about 70% of the vitamin A in their diet from animal sources.
Vitamin A from plant sources
Vitamin A found in plants is called provitamin A carotenoid. Provitamins cannot be directly used by the body but can be chemically convert into usable vitamins. Carotenoids are a family of more than 560 compounds, some of which can be converted into retinol. The carotenoids that can be converted into retinol by humans are mainly bets-carotene, alpha-carotene, and beta-cryptoxanthin. Of these, beta-carotene is converted twice as efficiently as alpha-carotene or beta-cryptoxanthin. However it takes 12 micrograms (mcg) of beta-carotene to equal the activity of 1 mcg of retinol. Carotenoids are found in yellow and orange vegetables and in some deep green vegetables where their orange color is not noticeable. Good sources of provitamin A carotenoid include carrots, cantaloupe, apricots, mango, papaya, spinach, and kale. Vegans (people who do not eat any animal products) must be especially careful to get enough of these vegetables.
Vitamin A's role in health
Almost everyone living in the developed world gets enough vitamin A to maintain health from their normal diet. The same is not true in the developing world where famine and limited food choices often prevent individuals, especially children, from getting enough vitamin A and other nutrients. When too little
KEY TERMS
Cell differentiation —The process by which stem cells develop into different types of specialized cells such as skin, heart, muscle, and blood cells.
Fat-soluble vitamin—A vitamin that dissolves in and can be stored in body fat or the liver.
Provitamin —A substance that the body can convert into a vitamin.
Vitamin —A nutrient that the body needs in small amounts to remain healthy but that the body cannot manufacture for itself and must acquire through diet.
vitamin A is in the diet, the effects can be seen in many different systems.
vision. The first function of vitamin A to be well understood was its role in maintaining good vision. Much of the research that explained how vitamin A was critical to vision was done by Harvard scientist George Wald (1906–1997), who won the Nobel Prize in 1967 for his work. When light enters the eye, it is absorbed by cells lining the retina at the back of the eye. This activates a chain of events that results in vision. Vitamin A (in the form of retinol) is part of a pigment in the retina called rhodopsin that absorbs the light. Without enough vitamin A, the eye does not detect low levels of light. People with this deficiency develop night blindness. They can see well in bright light, but cannot see in dim light. Night blindness was known in early Egyptian, Chinese, and Greek cultures, all of whom discovered independently that eating liver (an excellent source of retinol) would cure the disorder. Night blindness disappears almost immediately when vitamin A is added to the diet. If left untreated, however, dry eye (xeropthalmia) and permanent blindness can occur because of damage to the cornea, the clear covering of the eye.
skin. Vitamin A helps skin (epithelial) cells to remain healthy. Skin disorders such as acne can be treated by prescription drugs such as tretinoin (Avita, Renova, Retina-A) and isotretinoin (Accu-tane) that contain synthetic Vitamin A. Vitamin A supplements are also often given to burn victims to help them grow large amounts of new skin.
resistancetoinfection. Vitamin A is necessary for proper functioning of the immune system. The cells that line the throat, lungs, intestine, bladder, and other internal cavities are the first line of defense against bacteria and viruses entering the body. These cells need vitamin A to grow normally and form a continuous barrier against invading microorganisms. When these cells br eak down, it is easier for bacteria and viruses to infect the body. In addition, vitamin A is needed for the proper development white blood cells that fight infection. However vitamin A taken in excess of recommended amounts does not appears to benefit the immune system.
cancer prevention. There are mixed results from research on whether Vitamin A can help prevent cancer. The prescription drug All-Trans-Retinoic Acid (ATRA, Vesanoid) has been proved successful in increasing survival time for individuals with acute pro-myelocytic leukemia. This drug contains retinoic acid, a derivative of retinol. Research results on whether vitamin A is helpful in preventing or treating skin cancer and breast cancer are unclear. Clinical trials are underway to determine safety and effectiveness of vitamin A in a variety of situations. Individuals interested in participating in a clinical trial at no charge can find a list of open trials at <http://www.clinicaltrials.gov>.
Normal vitamin A requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs consist of three sets of values. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97–98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
RDAs for vitamin A are measured in both weight (micrograms) and international units (IU). The IU measurement is what is used on dietary supplement labels. Vitamin A comes in two different forms, preformed retinol from animal sources and provitamin A carotenoid from plant sources. These forms have different activity levels. To adjust for this, dietitians have developed an equivalency measure called the Retinol Activity Equivalent. This allows a direct comparison between the two sources of vitamin A.
For vitamin A from food:
- RAE = 1 mcg retinol
- RAE = 12 mcg beta-carotene
- RAE = 24 mcg any other provitamin A carotenoid
- RAE = about 3 IU
The following are the RDAs and ULs for vitamin A for healthy individuals:
- children birth-6 months: RDA 1,330 IU or 400 RAE; UL 2,000 IU or 600 RAE
- children 7–12 months: RDA 1,670 IU or 500 RAE; UL 2,000 IU or 600 RAE
- children 1–3 years: RDA 1,000 IU of 300 RAE; UL 2,000 IU or 600 RAE
- children 4–8 years: RDA 1,330 IU of 400 RAE; UL 3,000 IU or 900 RAE
- children 9–13 years: RDA 2,000 IU or 600 RAE; UL 5,610 IU or 1,700 RAE
- boys 14–18 years: RDA 3,000 IU or 900 RAE; UL 9,240 IU or 2,800 RAE
- girls 14–18 years: RDA 2,310 IU or 700 RAE; UL 9,240 IU or 2,800 RAE
- men age 19 and older: RDA 3,000 IU or 900 RAE; UL 10,000 IU or 3,000 RAE
- women age 19 and older: RDA 2,310 IU or 700 RAE; UL 10,000 IU or 3,000 RAE
- pregnant women age 19 and older: RDA 2,500 IU or 750 RAE; UL 10,000 IU or 3,000 RAE
- breastfeeding women age 19 and older: RDA 4,300 IU or 1,300 RAE; UL 10,000 IU or 3,000 RAE
The following list gives the approximate vitamin A (retinol) content for some common animal foods:
- beef liver, 3 ounces cooked: 27,185 IU
- chicken liver, 3 ounces cooked: 12,325 IU
- whole milk, 1 cup: 250 IU
- skim milk fortified with vitamin A, 1 cup: 500 IU
- whole milk cheddar cheese, 1 ounce: 280 IU
- egg, 1 whole: 300 IU
- butter, 1 tablespoon: 325 IU
The following list gives the approximate vitamin A (provitamin A carotenoid) content for some common plant foods:
- carrot, 1 whole raw: 8,670 IU
- carrot, 1/2 cup raw: 1,285 IU
- cantaloupe, 1 cup: 5,410 IU
- kale, 1/2 cup cooked: 9,560 IU
- spinach, 1/2 cup cooked: 11,460 IU
- spinach, raw, 1 cup: 2,800
- papaya, 1 cup: 1,530 IU
- mango, 1 cup: 1,260 IU
- tomato juice, 6 ounces: 820 IU
- breakfast cereal fortified with vitamin A, 1 serving: 500–770 IU
- adult multivitamin, 1 tablet or capsule: usually 5,000 IU (The UL of vitamin A has recently been reduced— see vitamin A excess below—so manufacturers may begin reducing this amount.)
Precautions
Vitamin A excess
Vitamin A is definitely a vitamin where more is not better, and excesses can be seriously harmful to health. It is a fat-soluble vitamin that is stored in the liver. Over time vitamin A can build up to dangerous levels and cause liver damage. Vitamin A excess can also cause birth defects. For this reason, certain prescription acne medications that contain synthetic vitamin A (e.g. tretinoin Avita, Renova, Retina-A, isotretinoin, Accutane) should not be taken by pregnant women or women who have the chance of becoming pregnant. Pregnant women should discuss their vitamin needs with their healthcare provider.
Acute vitamin A excess usually occurs when a person takes vitamin A in large quantities as a dietary supplement. Acute excess can cause nausea, vomiting, blurred vision, headache, drowsiness, and altered mental states. Chronic vitamin A excess occurs when vitamin A builds up in the body gradually. Symptoms include loss of appetite, dry skin, hair loss, insomnia, fatigue, irritability, diarrhea, menstrual irregularities, bone pain, and reduced growth rate in children.
Too much vitamin A activates the cells that break down bone (osteoclasts) and interferes with the activities of vitamin D, a vitamin involved in building and preserving bone. Studies have linked high levels of retinol in the blood with increased risk of hip fracture among people over age 50. Most multivitamins contain 5,000 IU of vitamin A. This amount is based on 1968 RDAs, which have now been revised downward. Since the risk of osteoporosis (bone weakening) is greatest in the elderly, some experts question the value of a daily multivitamin for people over age 55.
Vitamin A deficiency
Vitamin A deficiency is not a problem for healthy people in most industrial countries. However, the following groups in these countries have a greater likelihood of developing vitamin A deficiency:
- strict vegans, especially vegan children, who eat no animal products
- people with gastrointestinal diseases such as Crohn's disease, celiac disease, or inflammatory bowel disease that interfere with the absorption of nutrients from the intestine
- people with disorders of the pancreas that interfere with the absorption of nutrients
- people with anorexia nervosa (self-starvation)
- people with alcoholism
In the developing world, especially parts of Africa and Southeast Asia, vitamin A deficiency is common. The World Health Organization (WHO) estimates between 100 and 140 million children are at high risk of developing vitamin A deficiency and that each year 250,000–500,000 children become blind because of inadequate vitamin A in their diet. These children also have up to a 50% higher risk of dying from measles, diarrhea, malaria, and similar infections. These risks are lowered when vitamin A is added to the diet. WHO recommends that malnourished and at risk children under age five to receive a high-dosage capsule of vitamin A every six months as a safe and cost-effective way to prevent blindness and other problems associated with vitamin A deficiency in children. The excess vitamin A from the supplement is stored in the liver and released gradually as it is needed by the body.
Interactions
Vitamin A may interact with the following medications:
- antacids, which may be more effective in when used in combination with vitamin A
- birth control pills, which increase the level of vitamin A in a woman's blood
- blood thinning medicine such as warfarin (Coumadin), whose effect may be enhanced by long-term use of vitamin A
- cholesterol-lowering drugs, which may reduce the body's ability to absorb vitamin A
- orlistat, a weight-loss drug marketed as Xenical or Alli that prevents fat from being absorbed and oles-tra, substance used to replace fat in some foods. These may decrease the amount of vitamin A absorbed from the intestine.
- Alcohol, which increases the likelihood of vitamin A excess possibly because regular use of alcohol damages the liver and interferes with vitamin A storage
Complications
Vitamin A is safe when taken in amounts listed above as recommended by the Institute of Medicine. Too much or too little vitamin A results in side effects listed above in the Precautions section.
Parental concerns
Parents should be aware that the RDA and UL for vitamins and minerals are much lower for children than for adults. Accidental overdose may occur if children are give adult vitamins or dietary supplements.
Resources
BOOKS
Gaby, Alan R., ed. A-Z Guide to Drug-Herb-Vitamin Interactions Revised and Expanded 2nd Edition: Improve Your Health and Avoid Side Effects When Using Common Medications and Natural Supplements Together. New York: Three Rivers Press, 2006.
Lieberman, Shari and Nancy Bruning. The Real Vitamin and Mineral Book: The Definitive Guide to Designing Your Personal Supplement Program, 4th ed. New York: Avery, 2007.
Pressman, Alan H. and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals, 3rd ed. Indianapolis, IN: Alpha Books, 2007.
Rucker, Robert B., ed. Handbook of Vitamins. Boca Raton, FL: Taylor & Francis, 2007.
PERIODICALS
Wolf, George. “A History of Vitamin A.” The FASEB Journal, 10, no. 9 (1996): 1102–8.
ORGANIZATIONS
American Dietetic Association. 120 South Riverside Plaza, Suite 2000, Chicago, Illinois 60606–6995. Telephone: (800) 877–1600. Website: <http://www.eatright.org>
Linus Pauling Institute. Oregon State University, 571 Weniger Hall, Corvallis, OR 97331–6512. Telephone: (541) 717–5075. Fax: (541) 737–5077. Website: <http://lpi.oregonstate.edu>
Office of Dietary Supplements, National Institutes of Health. 6100 Executive Blvd., Room 3B01, MSC 7517, Bethesda, MD 20892–7517 Telephone: (301)435–2920. Fax: (301)480–1845. Website: <http://dietary-supplements.info.nih.gov>
OTHER
Eledrisi, Mosheen S. “Vitamin A Toxicity.” emedicine.com, July 20, 2005. <http://www.emedicine.com/med/topic2382>
Harvard School of Public Health. “Vitamins.” Harvard University, November 10, 2006. <http://www.hsph.harvard.edu/nutritionsource/vitamins.html>
Higdon, Jane. “Vitamin A.”Linus Pauling Institute-Oregon State University, December 12, 2003. <http://lpi.oregonstate.edu/infocenter/vitamins/VitaminA>
Maryland Medical Center Programs Center for Integrative Medicine. “Vitamin A (Retinol).” University of Maryland Medical Center, April 2002. <http://www.umm.edu/altmed/ConsSupplements/VitaminARetinolcs.html>
Medline Plus. “Vitamin A (Retinol).” U. S. National Library of Medicine, August 1, 2006. <http://www.nlm.nih/gov/medlineplus/druginfo/natural/patient-vitamina.html>
Office of Dietary Supplements. “Dietary Supplements Fact Sheet: Vitamin A and Carotenoids.” National Institutes of Health, April 23, 2006. <http://dietary-supplements.info.nih.gov/factsheets/Vitamin_A.asp>
Stenson, Jacqueline. “A Vitamin A Day May Do More Harm Than Good.” http://MSNBC.com January 19, 2007. <http://www.msnbc.com/id/16655168>
Thakore, Jigna. “Vitamin A Deficiency.” http://emedicine.com May 12, 2006. <http://www.emedicine.com/med/topic2381.htm>
UNICEF. “Vitamin A Deficiency.” United Nations, May 2006. <http://www.childinfo.org/areas/vitamina>
Tish Davidson, A.M.
Vitamin B5 see Pantothenic acid
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 benefits—such as a lower risk of cancer , heart attacks, and strokes—which 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 range—around 15 mg—is 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): 44–50.
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): 506S–510S.
Kamat, A. M., and D. L. Lamm. "Chemoprevention of Bladder Cancer." Urology Clinics of North America 29 (February 2002): 157–168.
Kohlhaufl, M., K. Haussinger, F. Stanzel, et al. "Inhalation of Aerosolized Vitamin A: Reversibility of Metaplasia and Dysplasia of Human Respiratory Epithelia—A Prospective Pilot Study." European Journal of Medical Research 7 (February 21, 2002): 72–78.
Miksad, R., V. de Ledinghen, C. McDougall, et al. "Hepatic Hydrothorax Associated with Vitamin A Toxicity." Journal of Clinical Gastroenterology 34 (March 2002): 275–279.
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): 53–59.
Yeum, K. J., and R. M. Russell. "Carotenoid Bioavailability and Bioconversion." Annual Review of Nutrition 22 (2002): 483–504.
Judith Turner
Rebecca J. Frey, PhD
Vitamin A
Vitamin A
Description
Vitamin A is one of the 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, that supplemental forms have the same benefit as consuming them 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 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 (μg) of retinol, or six μg of beta carotene.
Dose limits
Adults should take no more than 25,000 IU (5000 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 (2000 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 benefits—such as lower risk of cancer , heart attacks, and strokes—which a supplement may not.
KEY TERMS
Antioxidant —Substance, such as vitamin A, which blocks the destructive action of free radicals.
Carotenoids —Any of a group of over 600 orange or red substances which are found primarily in vegetables, many of which are vitamin A precursors.
Free radical —Highly reactive atoms which are very reactive as a result of having one or more unpaired electrons. They form through exposure to smoke and other environmental pollutants, as well as radiation and other sources. They have great potential to cause cellular damage, and may even be a factor in aging.
Retinoids —Any of the group of substances which comprise active vitamin A, including retinaldehyde, retinol, and retinoic acid.
Deficiency
Low enough levels of vitamin A 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 the chronically ill and those with impaired fat absorption. Those 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.
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 dose than average. 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 six and 30 milligrams a day, but the middle range—around 15mg—is 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 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. Some of the lesser-known carotenoids may be key factors. Whole sources 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.
Pressman, Alan H. and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals. Alpha Books, 1997.
Judith Turner
Vitamin A
Vitamin A
Description
Vitamin A is one of the 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, that supplemental forms have the same benefit as consuming them 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 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 (μg) of retinol, or six μg 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 benefits—such as lower risk of cancer, heart attacks, and strokes—which a supplement may not.
Deficiency
Levels of vitamin A that are 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 the chronically ill and those with impaired fat absorption. Those 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.
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 dose than average. 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 six and 30 milligrams a day, but the middle range—around 15mg—is 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 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. Some of the lesser-known carotenoids may be key factors. Whole sources 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.
KEY TERMS
Antioxidant— Substance, such as vitamin A, which blocks the destructive action of free radicals.
Carotenoids— Any of a group of over 600 orange or red substances which are found primarily in vegetables, many of which are vitamin A precursors.
Free radical— Highly reactive atoms which are very reactive as a result of having one or more unpaired electrons. They form through exposure to smoke and other environmental pollutants, as well as radiation and other sources. They have great potential to cause cellular damage, and may even be a factor in aging.
Retinoids— Any of the group of substances which comprise active vitamin A, including retinaldehyde, retinol, and retinoic acid.
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.
Pressman, Alan H., and Sheila Buff. The Complete Idiot's Guide to Vitamins and Minerals. Alpha Books, 1997.
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.
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 ]
vitamin A
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.