|Wednesday, September 1
What is it?Vitamin A is a fat-soluble vitamin which comes in two forms. The first is retinol, which is found already pre-formed in animal foods. The other is pro-vitamin A, which is found in plant foods in the forms of compounds called carotenoids.
The best known and most prominent carotenoid is beta-carotene, which provides about two-thirds of vitamin A in our diets. Beta-carotene is not as well absorbed as retinol, and it is only about half as active in vitamin activity. Other carotenoids are even less active than beta-carotene.
What does it do?Vitamin A is needed to:
Where do you get it?
The main sources of pre-formed vitamin A are animal foods, including:
Vitamin A is also found in:
How much do we need?Vitamin A is often measured in retinol equivalents -- RE, for short. The Recommended Daily Allowance, or RDA, is: 1,000 RE for boys and men 800 RE for girls over 12 and adult women (1,300 RE if breast-feeding).
Vitamin A also can be measured in international units, which is abbreviated as IU. This is how it is normally measured for food labels and supplements. To calculate IU, multiply the RE number by 5. For example, a male needs 5,000 IU.
If you are considering vitamin A supplements, keep in mind that deficiency is extremely rare in the U.S. and that there are health risks.
Is it safe?
In excessive amounts, vitamin A can be toxic. The use of high-potency vitamin supplements, fad diets, and the tendency to over- and self-medicate have led to many cases of vitamin A intoxication. Acute toxicity results from extremely high doses (500,000 IU) of vitamin A consumed over a short period of time. The symptoms include nausea, headache, bone pain, blurred vision and flaking skin. The symptoms resolve if the high doses are discontinued. Chronic toxicity may occur in adults with long-term intakes of 25,000 IU per day. Symptoms include headache, bone thickening, anemia, enlarged liver and spleen, menstrual abnormalities, stiffness and joint pain. These are some specific conditions under which vitamin A may be toxic, even in relatively low doses.
Recently, concerns have arisen that the level of vitamin A in multi-vitamin supplements (5000 IU) may be associated with abnormal liver function tests in the elderly. Vitamin A in excess is also capable of causing birth defects, including deformities of the head, heart, brain or spinal cord. The vitamin A analogue used to treat skin disorders like acne may be problematic in this regard, and those who use it should be careful to avoid pregnancy.
Pregnant women should never exceed 5,000 IU a day and should get most of their vitamin A from food sources -- liver, fortified cereals and carrot juice -- to avoid excessive intakes. Amounts around 10,000 IU could cause birth defects.
As far as supplements are concerned, women who are pregnant -- or planning to be should avoid supplements with more than 5,000 IU of vitamin A. In fact, supplements should be avoided altogether during the first trimester of pregnancy unless they are needed to correct a deficiency. Women of child-bearing age should choose fortified foods that contain vitamin A in the form of beta-carotene, rather than pre-formed vitamin A, whenever possible. There is no risk in eating lots of fruits and vegetables that are rich in beta-carotene and other carotenoids, and it should be encouraged.
What about beta-carotene?
Beta-carotene is the most abundant of the carotenoids that the body converts to vitamin A when we eat foods rich in carotenes. The liver makes only the amount of vitamin A needed by the body.
Beta-carotene is an anti-oxidant that neutralizes the "free radicals" in your blood. Left unchecked, these highly reactive compounds can damage tissues and cells and possibly lead to diseases such as cancer and heart disease.
More than 200 studies suggest beta-carotene plays a role in cancer prevention. One study in China showed beta-carotene decreases risk of stomach cancer if it is combined with other anti-oxidants such as vitamin E and selenium.
Conflict about beta-carotene arose when recent major studies provided conflicting and problematic results. In one study, men who smoked or were heavy drinkers took 30-50 milligrams of beta-carotene and had an increase in the risk of lung cancer.
Another study tracked a large group of doctors who took 50 milligrams of beta-carotene. Results showed neither had an increased or decreased risk of cancer. In another clinical trial people were given beta-carotene to see if it would prevent skin or lung cancer or precancerous lesions in the colon, and it did not.
Beta-carotene and other carotenoids may reduce the transmission of AIDS from mother to infant.
Where do you get beta-carotene?
Major food sources of beta-carotene are the brightly colored orange vegetables and fruits as well as dark green leafy vegetables. Carrots, sweet potatoes, apricots, pumpkin, cantaloupe,mango, broccoli, spinach and collard greens are excellent sources.
What about the other carotenoids?
A-carotene: This anti-oxidant may inhibit cell proliferation and halt the spread of cancer. It can be found in carrots, pumpkin and carrot juice.
Lutein/zeaxanthin: They are the carotenoids that form the yellow pigment of the macula, a tiny portion in the center of the retina of the eye. These pigments filter out blue light which could damage the eye. People who eat a lot of spinach and leafy green vegetables have less risk for macular degeneration, a common cause of irreversible blindness in people over 65. Studies show that a high intake of leafy green vegetables decrease the risk of cataracts. Good sources for these two carotenoids are collard greens, swiss chard, mustard greens, red pepper, okra and romaine lettuce.
Lycopene: Found in watermelon, guava, pink grapefruit, tomatoes, tomato sauces and juice. It has been found to reduce the risk of prostate cancer if consumed frequently. An Italian study also indicated frequent consumption of tomato products decreased the incidence of several forms of cancer.
b-Cryptoxanthin: Found in papayas, oranges and tangerines.
Is beta-carotene safe?
Carotenoids, even in very large amounts taken for years, are not known to be toxic. However, people who smoke or drink more than two drinks a day should avoid b-carotene supplements. Most people can take between 10-20 milligrams of beta-carotene daily without much risk. A milligram of beta-carotene is the same amount as 1,666 IU. Most diets provide about 2 milligrams of beta-carotene.
If you take the supplement or eat foods high in carotenes they might turn your skin yellow or orange. Supplements with beta-carotene may keep the body from absorbing other carotenoids that work together to promote lung health. Eating fruits and vegetables (3-5 servings) containing carotenes is a much better nutritional choice than taking the supplements.