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Beta-carotene

Found in carrots, sweet potatoes, and green peppers, among other foods, this antioxidant is converted in the body to vitamin A and is important for healthy vision, a functioning immune system, and good skin. But the evidence isn't really there to recommend it for staving off cancer. In fact, a 2004 study found that supplements may actually raise the risk of lung cancer in smokers.

Bottom line: Skip the supplements if you're a smoker, and try to get your beta-carotene from fruits and veggies, whether you smoke or not.

Next: Calcium