Carole Conn, PhD, RD

Pick up any packaged food, and you will see the number of calories listed on the label. Of course, nonpackaged foods, such as fresh produce, have calories, too; they just don’t carry labels telling you how many. Most people know that the body uses the calories contained in foods for energy and that, if they consume more calories than they expend, they will gain weight. (They also know that, if they do the opposite, they will lose weight.) Nonetheless, what exactly is a calorie, why do foods have calories and how does the body use them?