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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.
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
(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.
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
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:
The following are the RDAs and ULs for vitamin A for healthy individuals:
The following list gives the approximate vitamin A (retinol) content for some common animal foods:
The following list gives the approximate vitamin A (provitamin A carotenoid) content for some common plant foods:
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:
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.
Vitamin A may interact with the following medications:
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.
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.
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