The glycemic index of sweet potatoes is a lot lower, which is better for diabetes control, according to a 2002 article in the American Journal of Clinical Nutrition.. Eating sweet potatoes in moderate amounts will help you keep your blood sugar levels in the healthy range even if you have diabetes. A medium sweet potato contains 26 grams of carbohydrates, of which 3.8 grams are dietary fiber, while a cup of mashed sweet potatoes has 58 grams of carbohydrates and 8.2 grams of fiber.