Glycemic Index. White potatoes, whether you have them mashed, baked, as french fries or potato chips, have a high glycemic index, which means that their carbohydrates are quickly turned into sugar, which elevates your blood sugar levels after your meal.ating 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.