That's not surprising since more than 80 percent of U.S. consumers (73 million people) have a microwave at home. Popcorn also substitutes as a meal for many consumers in a pinch, and some munch on it when they are in diet mode - it's low in both calories and fat. In fact, one gallon of unbuttered popcorn has a mere 390 calories. A whole-grain food that is high in fiber, protein and other nutrients, popcorn is one of the healthiest snacks around.