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Selenium

The body needs only small amounts of this antioxidant, found in meats, seafood, eggs, and bread. A study found that taking 200 micrograms daily reduced the risk of prostate, lung, and colorectal cancers, but other studies have been "complete flops," Anderson says. That includes a 2011 analysis.

Bottom line: Don't count on selenium to lower your chances of getting or dying from cancer. It's likely you're getting enough from food sources, anyway.

Next: Vitamin C