Vitamin A

Daily Value: 5,000 international units

Good Food Sources: Carrot juice, pumpkin, sweet potatoes, carrots, spinach, butternut squash, tuna, dandelion greens, cantaloupe, mangoes, turnip greens, beet greens

Give 200,000 international units of vitamin A to a malnourished child in Indonesia, Nepal, India or Ghana, and it could save his life. Give 25,000 international units of beta-carotene, a vitamin A precursor, to an adult every day, and it could help prevent macular degeneration, which, after cataracts, is the leading cause of blindness in people age 50 and older. One study at Johns Hopkins School of Hygiene and Public Health in Baltimore showed that between 9,000 and 20,000 international units of vitamin A, given daily to men infected with HIV, may help slow the disease's progression to AIDS by about 40 percent.

What is this powerful substance? Actually, vitamin A is the generic name given to a group of naturally occurring molecules called retinoids. Study after study shows that your body uses these retinoids, which are powerful compounds drawn from plant and animal sources, to build or maintain an effective immune system.

Without an adequate amount of vitamin A, your body is vulnerable to a whole host of infectious creatures that can cause anything from measles to AIDS. Those who lack vitamin A on their defense teams also face increased risk of cancer and blindness.

Symptoms of a vitamin A deficiency include night blindness, difficulty recovering vision (after looking into the headlights of an oncoming car, for example), distorted color vision, dry eyes, loss of appetite, poor taste and smell and difficulty keeping your balance.

Fortunately, most people in the United States get enough vitamin A from their diets. Those most at risk are people who have cancer, tuberculosis, pneumonia, chronic nephritis, urinary tract infections or prostate disease, all of which may increase the body's demands for vitamin A. People who have digestive conditions that impair fat absorption--celiac disease and cystic fibrosis are two examples--are also at risk.

Vitamin A is a fat-soluble substance, which means that you should eat a food that contains a small amount of fat when you take it. You can get vitamin A directly from a supplement. Or you can get it indirectly by eating fruits and vegetables that are loaded with beta-carotene. Your body will turn beta-carotene into the vitamin A it needs.

Many doctors prefer the second route. There are two reasons: One, foods contain hundreds of other substances that may have healthful benefits; and two, although excessive amounts of vitamin A are usually toxic, large amounts of beta-carotene usually are not. There is one exception: Both vitamin A and beta-carotene can damage the liver in someone who drinks heavily.

Using Vitamin A Safely

For centuries, Arctic adventurers, Eskimos and even sled dogs have known that eating polar bear liver can make them sick.

The reason? It's loaded with enough vitamin A to poison a full-grown adult. A single meal consisting of a half-pound to one pound of polar bear liver contains a whopping 3 million to 13 million international units of vitamin A, which is 6 to 26 times the amount needed to cause acute vitamin A poisoning.

Only 500,000 international units of vitamin A, taken over a short period of time, can cause irritability, headaches, vomiting, bone pain, weakness and blurred vision. Regular use of even 50,000 international units a day can cause hair loss, weakness, headaches, enlarged liver and spleen, anemia, stiffness and joint pain. And at least one death has been reported from the regular use of 25,000 international units every day.

Women of childbearing age need to be particularly careful when supplementing vitamin A. Daily doses of 10,000 international units, an amount found in some The bottom line? In different amounts and situations, vitamin A can be either a miraculous healing agent or a malicious, toxic compound. It's a good idea to consult your doctor before supplementing.