Bananas are eaten raw as a snack, sliced onto cereal or blended into smoothies. A medium banana of about 120 g gives about 105 kcal and 27 g of carbohydrate, about half of it sugar, and it is a good source of vitamin B6. Its 430 mg of potassium is useful but less than a portion of potatoes or beans provides. As bananas ripen, starch turns into sugar, so a greener banana is less sweet and has a lower glycemic index.