Soak up the sun.
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Exposing yourself to the sun for 30 minutes will give you up to 15,000 international units of vitamin D, an essential nutrient for immunity. With shortened winter days, that's often easier said than done. But supplements are also effective, and people who take vitamin D daily come down with far fewer colds and flus. Getting enough D is also associated with a lowered risk of cancer, heart disease, diabetes, and other serious illnesses. Maintain your levels by taking 2,000 IU daily.