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Red peppers

Like citrus fruits, red peppers are high in vitamin C. In fact, one red pepper has 150 milligrams of the nutrient—that's twice the recommended daily allowance for women. (A large orange, by comparison, only has about 100 milligrams.)

Even that may not be enough, however, as studies suggest you need much more than that to harness the nutrient's cold-fighting benefits. "If you're sick, you should be eating a lot of vitamin C throughout the day—400 to 500 milligrams," Ansel says.

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