Vitamins: The Good, the Bad, and the Useless

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Vitamins and minerals are essential to health, but that doesn't mean that megadoses will keep you out of the hospital or make you live longer. Some may be harmful. In most cases, it's preferable to get these nutrients from a balanced diet. Here's the lowdown on eight common supplements — most of which you probably don't need.

High doses of certain vitamins and minerals may be appropriate for certain people, though. Talk to your doctor about supplements if you are a woman of childbearing age, are a vegetarian or vegan, have limited exposure to the sun, are an athlete in training, or suspect for any reason you may be malnourished.

1. Beta-Carotene

Best food sources
Carrots, spinach, kale, cantaloupe RDA*
3,000 IU (in the form of vitamin A) for males, 2,130 IU for females Why you may be taking it
As an antioxidant, especially to prevent cancer. What you should know
Beta-carotene supplements increase the risk of lung cancer in smokers. There is no evidence that they prevent any other form of cancer. Bottom line
Don't take it *Recommended dietary allowances for healthy adults are listed; they may be lower or higher for children, women who are pregnant or nursing, the elderly, and other groups.

2. Folic Acid

Best food sources
Fortified bread and breakfast cereal, legumes, asparagus RDA
400 micrograms Why you may be taking it
To prevent birth defects. What you should know
Supplements reduce the risk of neural tube defects in newborns. However, some doctors say supplementation of food with folic acid could be fueling rising rates of colon cancer. Bottom line
Only women who are pregnant or may become pregnant are advised to take it.

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3. Selenium

Best food sources
Brazil nuts, tuna, beef RDA
55 micrograms Why you may be taking it
To prevent cancer, especially prostate cancer. What you should know
One 2007 study found a 50% increased risk of type 2 diabetes in people who took 200 micrograms a day. A major study will determine whether selenium supplements prevent prostate cancer, but it won't be completed until 2013. Bottom line
Don't take it.

4. Vitamin B6

Best food sources
Baked potatoes, bananas, chickpeas RDA
Males/females 19–50: 1.3 milligrams; males over 50: 1.7 milligrams; females over 50: 1.5 milligrams Why you may be taking it
To prevent mental decline and lower homocysteine levels. What you should know
Two studies failed to show cognitive benefits. B6 does reduce homocysteine, but it's not clear whether this prevents heart attacks. Bottom line
Take it only if your doctor recommends it.

5. Vitamin B12

Best food sources
Fish and shellfish, lean beef, fortified breakfast cereal RDA
2.4 micrograms Why you may be taking it
To prevent age-related mental decline and boost energy. What you should know
Vitamin B12 deficiency, which can cause anemia and dementia, is a problem for some seniors, so supplements can help. However, high doses of B12 have not been proven to prevent cognitive loss, nor do they boost energy. Bottom line
Take it only if your doctor recommends it.

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6. Vitamin C

Best food sources
Citrus fruits, melons, tomatoes RDA
90 milligrams for adult males, 75 milligrams for females Why you may be taking it
To prevent the common cold. Also as an antioxidant to help fight cancer or heart disease. What you should know
A review of 30 clinical trials found no evidence that vitamin C prevents colds. Exceptions: It may reduce the risk in people who live in cold climates or experience extreme physical stress, such as running marathons. Smokers may need extra vitamin C. High doses of vitamin C do not seem to prevent cancer or heart disease. Bottom line
Most people don't need C supplements.

7. Vitamin E

Best food sources
Vegetable oil, nuts, leafy green vegetables RDA
15 milligrams Why you may be taking it
To help prevent heart disease, cancer, and Alzheimer's disease. What you should know
Not only have studies failed to show that vitamin E supplements prevent heart attacks or cancer, but high doses may increase the risk of strokes. One study found that vitamin E from food — but not supplements — helps prevent Alzheimer's disease. Bottom line
Don't take it.

8. Zinc

Best food sources
Oysters, lean beef, breakfast cereal RDA
11 milligrams for males; 8 milligrams for females Why you may be taking it
To prevent and treat symptoms of the common cold. What you should know
A few studies suggest that cold symptoms are less severe and resolve sooner in zinc users, but others show no benefit. High doses can weaken the immune system. Bottom line
Don't take it except for occasional use of zinc lozenges or sprays for colds.

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