As long as this popular crunchy treat isn't doused in movie-theater butter, it makes an excellent weight-loss snack. Not only is popcorn high in fiber, it even delivers some protein. A 1-ounce serving of air-popped corn (about 3½ cups) has 4 grams of fiber, almost 4 grams of protein and clocks in at 110 calories. This combination makes it a snack with staying power. Popcorn is filled with air, so you get a pretty large portion without a lot of calories. You can eat 3 whole cups of popcorn for only 100 calories.