When you exercise, cancer causing free radicals are created. Consuming foods rich in antioxidants, such as sweet potatoes, can help counter the negative effects of free radicals. Antioxidants also help repair and rebuild tissues. Sweet potatoes are a rich source of potassium. Eating acid producing foods, like processed foods, meat, and dairy products, speed up the loss of muscle mass, but potassium can counteract the affects of acidic foods. If you're not a sweet potato fan, other sources of potassium include kiwi, peppers, lettuce, melon, and oranges. Eat three foods a day that contain potassium.