Blockage of the arteries that supply blood to the heart (coronary heart disease) still causes more deaths in the United States than any other condition. Treating high cholesterol, high blood pressure, and diabetes; not smoking; and exercising regularly are all known to help prevent coronary heart disease. Whether particular diets are effective in preventing coronary heart disease is controversial, however. Several studies have suggested that low-fat diets may be particularly helpful; others have suggested that diets high in fruits and vegetables are key. Many fruits and vegetables contain large amounts of fiber; potassium; flavonoids; and antioxidant vitamins, such as vitamin C and vitamin E. Large studies that follow participants for a long time are needed to see whether diets rich in such ingredients help protect against coronary heart disease.