Vitamin A Overview
In the case of vitamin A, the eyes have it. The essential nutrient vitamin A, or retinol, plays a vital role in vision, reproduction and it helps healthy cells create accurate copies of themselves during cell division. In this article, we'll discuss the importance of vitamin A in your diet and sources for this vital nutrient. Here's a preview:- What Is Vitamin A?
Vitamin A is is important to vision, especially the ability to see in the dark. A deficiency of vitamin A leads to xerophthalmia, which causes irreversible damage to the eyes and blindness. Vitamin A deficiency is a major cause of blindness in the world. Vitamin A is also important to maintaining healthy skin and it helps the body to resist infection. - Benefits of Vitamin A
Vitamin A is important for the immune system, and it keeps skin and mucous membrane cells healthy. Vitamin A helps to fight cancer by inhibiting the production of DNA in cancerous cells. It slows down tumor growth in established cancers and may keep leukemia cells from dividing. - Foods That Contain Vitamin A
Vitamin A can be found in both animal and plant foods. It is found as retinol in animal foods and as carotenoids in plant foods. These are compounds the body can convert into vitamin A. The single best source of vitamin A is liver. - Vitamin A Deficiency
Vitamin A deficiency causes an inability to see well in the dark, rough skin, and a susceptibility to infectious diseases. In the United States, vitamin A deficiency is most common among low-income groups. - Vitamin A Supplements
Vitamin A supplementation should be limited to 5,000 IU (1,000 RE) for men and 2,500 IU (500 RE) for women. Use caution when taking vitamin A supplements -- large doses of the vitamin can be toxic. - Carotenoids
Carotenoids, also called provitamin A, are precursors to vitamin A. These colorful plant pigments can be found in yellow and orange fruits and vegetables. Carotenoids are powerful antioxidants that can help prevent some forms of cancer and heart disease.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
What Is Vitamin A?
As indicated by its position at the head of the vitamin alphabet, vitamin A was the first vitamin discovered. In the early 1900s, researchers recognized that a certain substance in animal fats and fish oils was necessary for the growth of young animals. Scientists originally called the substance fat-soluble A to signify its presence in animal fats. Later, they renamed it vitamin A.
 ©2007 Publications International, Ltd. Vitamin A is essential for healthy eyesight, especially our ability to see in the dark. |
Moreover, a deficiency of vitamin A dries out the transparent coating of the eye (the cornea) and the membrane over the whites of the eye (the conjunctiva). If not treated, this condition, called xerophthalmia, causes irreversible damage and blindness. Vitamin A deficiency is a major cause of blindness in the world.
Vitamin A is also important for normal growth and reproduction -- especially proper development of bones and teeth. Animal studies show that vitamin A is essential for normal sperm formation, for growth of a healthy fetus, and perhaps for the synthesis of steroid hormones.
Another important, but misunderstood, role of vitamin A involves preserving healthy skin -- inside and out. Taking extra vitamin A won't make your sagging skin suddenly beautiful, but a deficiency of it will cause major skin problems. Furthermore, an adequate vitamin A intake ensures healthy mucous membranes of the gastrointestinal and respiratory tracts. In this way, vitamin A helps the body resist infection.
Vitamin A is also being studied for its function in creating normal copies of each of our cells as they reproduce. Cells that do not differentiate into specific cell types have a greater chance of undergoing pre-cancerous changes than normally differentiated cells. Current studies suggest that adequate retinol levels can help prevent cancers of the liver, prostate and possibly the colon.
As we've just learned, vitamin A is essential for healthy eyesight. Keep reading to learn more about vitamin A's therapeutic benefits.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
Benefits of Vitamin A
In addition to treating deficiency syndromes, vitamin A has several potential preventive and therapeutic uses. Vitamin A is important "medicine" for the immune system. It keeps skin and mucous membrane cells healthy. When membranes are healthy they stay moist and resistant to cell damage. The moistness inhibits bacteria and viruses from "putting down stakes" and starting infectious diseases.
 ©2007 Publications International, Ltd. Vitamin A helps to keep skin and mucous membrane cells healthy. |
This vitamin is particularly helpful in diseases caused by viruses. Measles, respiratory viruses, and even human immunodeficiency virus (HIV), the virus that causes AIDS, may retreat in the presence of vitamin A. Blood levels of vitamin A are often low in people with viral illnesses. After receiving additional amounts of this vitamin, the body is able to mount its defenses, often resulting in a quicker recovery. However, vitamin A can also increase the risk of infections, according to the findings of a recent study. In HIV infection, for instance, preformed vitamin A may also encourage HIV to replicate, so limiting the intake of vitamin A is the best bet for these patients.
Stroke victims who have high levels of vitamin A in their systems are less likely to die or suffer disabilities from the stroke. Eating plenty of fruits and vegetables is a good defense against stroke and its complications.
Topical application of vitamin A helps relieve dry-eye disorder. When tear production and lubrication stop, the resulting dry eyes can be extremely uncomfortable. Many treatment avenues are often disappointing, with the exception of vitamin A eyedrops. Used clinically, these drops improve cell function and moistness returns to the eyes. Vision Pharmaceuticals (1-800-325-6789) sells vitamin A drops called Viva-Drops in many chain drugstores throughout the U.S.
Vitamin A taken orally and applied topically looks promising in preventing and possibly even treating skin cancers. It may be helpful, too, in lightening liver spots, those dark spots that often appear on aging skin. Topical application used in one study significantly lightened liver spots within one month.
Vitamin A derivatives are used to treat skin disorders. Isotretinoin acne medicine is an oral medication used for severe cystic acne. Because of the possibility of such serious side effects as liver damage and elevated blood triglycerides, a doctor must closely monitor treatment with this medication. Any woman capable of becoming pregnant needs to use reliable birth control when taking this medicine because it can cause spontaneous abortion or serious birth defects. Pregnant women must avoid it and other sources of high-dose vitamin A.
Tretinoin is a topical medication primarily used for acne, with less potential for serious side effects than oral isotretinoin. It treats baldness when prescribed along with minoxidil. It also may reduce the appearance of wrinkles and reverse the effects of sun damage on the skin. Another vitamin A derivative, etretinate, may treat psoriasis.
Experts agree that the best way to get essential nutrients is through diet. Keep reading to learn about foods that contain vitamin A.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
Foods That Contain Vitamin A
Both animal and plant foods have vitamin A activity. Retinol, also called preformed vitamin A, is the natural form found in animals. Carotenoids, found in plants, are compounds that the body can convert to vitamin A.Liver is the single best food source of vitamin A. However, many experts recommend eating liver only once or twice a month because of the toxic substances it can contain. Environmental pollutants tend to congregate in an animal's liver. Egg yolk, cheese, whole milk, butter, fortified skim milk, and margarine are also good sources of vitamin A. Be careful, though, as all these foods -- except fortified skim milk -- are also high in total fat and saturated fat, and all except margarine are high in cholesterol. Red palm oil, used for cooking in many tropical countries, and fish liver oils taken as supplements are also rich in vitamin A. One tablespoon of cod liver oil contains more than 12,000 international units (IU), more than twice the daily recommended intake for adults.
 ©2007 Publications International, Ltd. Egg yolks are a good source of vitamin A. |
This chart will help you find foods that are a good source of vitamin A.
| Food | Quantity | Vitamin A Content International Units (IU) | Retinol Equivalents (RE) |
| Baked sweet potatoes, peeled | 1 medium | 28,805 | 2,881 |
| Pumpkin, canned | 1/2 cup | 27,018 | 2,702 |
| Sweet potatoes, candied | 1 medium | 25,188 | 2,519 |
| Beef liver, cooked | 2 ounces | 20,230 | 6,130 |
| Spinach, canned, drained | 1 cup | 18,781 | 1,878 |
| Sweet potatoes, canned | 1 cup | 15,966 | 1,597 |
| Spinach, cooked, fresh or frozen | 1 cup | 14,790 | 1,479 |
| Carrot, raw | 1 medium | 12,767 | 1,277 |
| Cantaloupe | 1/2 medium | 12,688 | 1,269 |
| Peas and carrots, frozen (boiled, drained) | 1 cup | 12,418 | 1,242 |
Vitamin A deficiency can cause a variety of health problems, including increased susceptibility to infectious diseases. Go to the next page to learn more about vitamin A deficiency and recommended daily intake.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
Vitamin A Deficiency
Vitamin A deficiency is common in the United States among low-income groups. Children are especially vulnerable because they are still growing rapidly. People who eat very-low-fat diets and who limit their consumption of liver, dairy foods and dark green vegetables, and those who experience fat malabsorption from conditions like celiac disease or infectious hepatitis can also become deficient in vitamin A. A zinc deficiency can also trigger a vitamin A deficiency by making it difficult to use the body's own stores of the vitamin.
 ©2007 Publications International, Ltd. One of the first signs of a vitamin A deficiency is the inability to see well in the dark, or night blindness. |
Vitamin A deficiency also causes dry and rough skin, making it take on a kind of "goose flesh" appearance. In addition, one can become more susceptible to infectious diseases. That's because a lack of vitamin A damages the lining of the gastrointestinal and respiratory tracts, so they can't act as effective barriers against bacteria. Infections of the vagina and the urinary tract are also more likely.
Treatment for children with xerophthalmia starts with large doses of vitamin A, decreasing to smaller amounts after a few days. Blindness can be averted if treatment is started before too much eye damage has occurred.
Diseases such as obstructive jaundice or cystic fibrosis cause poor absorption of dietary fat and the fat-soluble vitamins. So even if people with these diseases consume adequate vitamin A, they may still develop a deficiency because of poor absorption. To overcome this obstacle, patients can take large amounts of a water-soluble form of vitamin A.
A disease accompanied by prolonged fever, such as infectious hepatitis or rheumatic fever, can rapidly deplete the liver's reserves of vitamin A. As part of the treatment, a doctor may prescribe this vitamin in amounts greater than the RDA to prevent deficiency. Zinc is needed to transport vitamin A, so zinc may also be recommended at low levels.
Dietary Requirements
The Recommended Dietary Allowance (RDA) for vitamin A is 900 retinol equivalents (RE) for men and 700 RE for women. Retinol equivalents are the preferred measure for vitamin A, because this method takes into account both forms of the vitamin -- retinol and carotenoids. One RE is equal to 3.33 international units (IU) of retinol or 10 IU of beta-carotene or 12 IU of mixed carotenes. Assuming you get the vitamin from both sources, the RDAs are equivalent to about 5,000 IU for men and 4,000 IU for women.
It's not necessary to obtain the RDA amount for vitamin A each day. Because vitamin A is not soluble in water, you do not excrete excess amounts of the vitamin. The liver stores vitamin A, and the body can tap into the reserves whenever dietary intake is too low. For most adults it takes months to deplete stored amounts. As long as you have a well-balanced diet that includes milk and yellow-orange and green vegetables, your overall intake should be sufficient to provide the vitamin A your body needs. Strict vegetarians, such as vegans, can obtain sufficient vitamin A if they eat a lot of pigmented vegetables.
If you choose to take vitamin A supplements to get your daily requirements, make sure you don't overdo it -- too much vitamin A can be toxic. Keep reading to learn more about vitamin A supplements.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
Vitamin A Supplements
How much is enough? For general health, limit vitamin A supplementation to 5,000 IU (1,000 RE) for men and 2,500 IU (500 RE) for women. A high dose of up to 50,000 IU (10,000 RE) for one or two days only to treat a viral infection is acceptable. Pregnant women should use carotenes instead.When shopping for carotene supplements, look for "mixed carotenoids" so that you get some of the other helpful carotenes besides just beta-carotene. The best source of these supplements is palm oil. You don't need to worry about fat intake, because the carotenes are extracted from the palm oil. Carotenes from the algae D. salina are second best. Avoid synthetic preparations. A daily amount of 25,000 IU is recommended for general health purposes.
Don't overdo it. Large amounts of vitamin A are clearly toxic. One massive dose or large doses taken over an extended period of time can cause hair loss, joint pain, nausea, bone and muscle soreness, headaches, dry and flaky skin, diarrhea, rashes, enlarged liver and spleen, cessation of menstruation, and stunted growth.
 ©2007 Publications International, Ltd. Large doses of vitamin A can be toxic, so don't overdo it with your supplements. |
Excessive intake of retinol can be detrimental to your health, specifically your heart and bone health. One clinical trial showed that people who took 25,000 IU of vitamin A per day for 3-4 years saw their triglycerides and total cholesterol increase while their good cholesterol (HDL) decreased. Both animal and human studies show that excessive vitamin A intake can accelerate bone loss and inhibit formation of new bone, increasing the risk of osteoporosis. In humans, small studies have found these effects at about 85,000–125,000 IU per day. Therefore you should limit retinol supplementation to 500-600 RE per day if you are at risk for either cardiovascular disease or osteoporosis. In other words, limit intake if you plan to live past age 50!
In a few reported instances, vitamin A toxicity has occurred after eating large amounts of liver. (Polar bear liver is especially high in vitamin A; it contains as much as 560,000 IU (169,697 RE) per ounce!) Because the liver stores vitamin A, eating large amounts daily is not wise.
Carotenoids can also be a good source of vitamin A. Keep reading to learn more about these colorful plant pigments.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.
Carotenoids
Carotenoids, the colorful plant pigments some of which the body can turn into vitamin A, are powerful antioxidants that can help prevent some forms of cancer and heart disease, and act to enhance your immune response to infections.These precursors to vitamin A are sometimes called provitamin A. Bright-orange beta-carotene is the most important carotenoid for adequate vitamin A intake because it yields more vitamin A than alpha- or gamma-carotene.
Some carotenoids, such as lycopene, do not convert to vitamin A at all. Lycopene, the orange-red pigment found in tomatoes and watermelon, is still of value, however, because it's an antioxidant even more potent than beta-carotene. The other carotenoids are also valuable antioxidants. Antioxidants help the body reduce the inflammatory action of singlet or free-radical oxygen. Oxygen atoms like to combine into pairs. Singlet oxygen atoms are unstable and interact with the lipids found in cell walls causing inflammation and damage. Sometimes, your own body uses these free radicals to fight infections and abnormal cells. Most of the time, these free radicals cause inflammation and damage to cells, such as those that line your arteries.
 ©2007 Publications International, Ltd. Carotenoids, precursors to vitamin A, can be found in orange and yellow fruits and vegetables. |
Most other carotenoids, such as alpha- and gamma-carotene, plus cryptoxanthin and beta-zeacarotene have less vitamin A activity than beta-carotene, but offer ample cancer prevention. Some carotenoids, such as lycopene, zeaxanthin, lutein, capsanthin, and canthaxanthin are not converted into vitamin A in the body. But again, they are powerful cancer fighters, prevalent in fruits and vegetables. There is abundant evidence that lycopene in particular helps reduce the risk for prostate cancer.
Carotenes are valuable preventive medicines, too. Research shows that people who eat a lot of foods rich in beta-carotene -- the carotenoid with the greatest vitamin A value -- are less likely to develop lung cancer. Even among smokers, lung cancer is less likely to occur in those people who eat a diet that includes lots of vegetables and fruits containing beta-carotene. Taking a beta-carotene supplement in pill form does not always have the same effect, however. Perhaps this is because in these foods there may be other substances that offer protection as well. In three studies involving 69,000 participants, many of them smokers, beta-carotene supplements increased the rate of lung cancer. Lutein/zeaxanthin, lycopene and alpha-carotene show evidence of being significantly more protective against lung and some other cancers.
Many experts now believe that the protective effect of carotenoids depends on the timing of when you take them. If you take betacarotene before your cells have undergone any pre-cancerous changes, the antioxidant action of the carotenoids can help reduce the likelihood that any mutations will take place. The carotenoids at this point can prevent free radicals from damaging cells and the DNA inside of cells, both of which can start cancerous growth. But if you take supplemental betacarotene AFTER cells have already mutated, the beta-carotene may protect the mutated cells from being destroyed by your own immune system.
Some of the most potent cancer fighting cells in your body utilize free radicals to fight infections and to destroy precancerous cells. So eating foods high in carotenoids or taking supplements helps with what is called primary protection from cancer – cancer never gets started. But after you have a growing colony of cancerous cells in your system, betacarotene supplements may prevent your own system from fighting the cancer, making carotenoid supplementation significantly less safe for what is called secondary prevention – stopping a recurrence of cancer.
In addition to their role in cancer prevention, the carotenes offer us protection from heart disease, too. Again, it's their antioxidant behavior that protects the lining of the arteries and the fats in the blood from free radicals' oxidative damage. And age-related macular degeneration of the eye, which leads to vision loss, may be counteracted by carotenes' antioxidant power.
Beta-carotene is used to treat skin problems caused by sun exposure. Some people have conditions in which swelling, redness, itching, and pain occur after being in the sun. Typically this is the result of excessive free radical damage due to a cellular problem. Beta-carotene supplementation helps alleviate these symptoms by protecting cells from damage.
Carotenes, like vitamin A, support immune function, but in a different way. They stimulate the production of special white blood cells that help determine overall immune status. They improve the communication between cells, too, which results in fewer cell mutations. White blood cells attack bacteria, viruses, cancer cells, and yeast. Women with high levels of carotenes in their blood tend to have fewer incidences of vaginal yeast infections.
While the liver stores retinol, excess carotenoids accumulate in the fat just beneath the skin. If you eat a lot of carotene-rich foods, you may notice a yellowing of your skin, especially on the palms of your hands and soles of your feet. This is generally considered to be harmless, though carotene-containing tanning pills used in Europe reportedly cause infertility in women.
Deficiency
There is no known carotenoid deficiency state. Deficiency symptoms are linked to vitamin A deficiency instead. While carotenoids can help prevent vitamin A deficiency, people who have impaired thyroid function are less able to convert betacarotene into vitamin A (retinol).
Contraindications
Taking more than 100,000 IU betacarotene per day sometimes gives the skin a yellow-orange hue, which may look like jaundice but is not harmful. People taking beta-carotene for long periods of time should also supplement with vitamin E, as beta-carotene may reduce vitamin E levels. Supplementing with doses of over 50,000 IU of beta carotene may also decrease blood levels of lutein, lycopene and other carotenoids.
Vitamin A and carotenoids are vital to your overall health, especially for good eyesight. Just be sure you're getting appropriate amounts of this nutrient and safeguard against overdose to avoid toxic levels, and vitamin A will work to keep your body running smoothly.
Vitamin A is just one of many essential nutrients that need to be a part of your diet. For more information, visit these links.
ABOUT THE AUTHOR:
Jennifer Brett, N.D. is director of the Acupuncture Institute for the University of Bridgeport, where she also serves on the faculty for the College of Naturopathic Medicine. A recognized leader in her field with an extensive background in treating a wide variety of disorders utilizing nutritional and botanical remedies, Dr. Brett has appeared on WABC TV (NYC) and on Good Morning America to discuss utilizing herbs for health.
This information is solely for informational purposes. IT IS NOT INTENDED TO PROVIDE MEDICAL ADVICE. Neither the Editors of Consumer Guide (R), Publications International, Ltd., the author nor publisher take responsibility for any possible consequences from any treatment, procedure, exercise, dietary modification, action or application of medication which results from reading or following the information contained in this information. The publication of this information does not constitute the practice of medicine, and this information does not replace the advice of your physician or other health care provider. Before undertaking any course of treatment, the reader must seek the advice of their physician or other health care provider.