Sweet potatoes
Often confused with yams, this tuber is one of the healthiest foods on the planet. In addition to countering the effects of secondhand smoke and preventing diabetes, sweet potatoes contain glutathione, an antioxidant that can enhance nutrient metabolism and immune-system health, as well as protect against Alzheimer's, Parkinson's, liver disease, cystic fibrosis, HIV, cancer, heart attack and stroke. "One sweet potato a day is a great alternative to the traditional variety," says Clark.