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Leafy greens

The darker the greens, the higher the nutrient content. So when you're shoring up your defenses for cold and flu season, choose arugula and kale over iceberg lettuce.

Bitter greens like arugula may even help relieve chest congestion, sniffles, and coughs. How? It's not entirely clear, although a 2011 British study found that mice that were fed green vegetables had more infection-fighting white blood cells in their intestines than those who were not.

Next: Blueberries