Maybe it's the satisfying crunch of each kernel that makes popcorn so popular. Or the sheer enjoyment of eating a snack food without worrying about the waistline -- provided it's not popped in oil, slathered with butter and doused with salt.

Americans munch on 56 quarts per person of the fluffy stuff annually, according to the Chicago-based Popcorn Institute, a trade association. Popcorn sales topped $1 billion last year.

High in fiber and low in calories, popcorn is one of the few "junk foods" that earns qualified praise from nutritionists. "People can eat as much as three cups and still have a low-calorie snack," said Jayne Hurley, associate nutritionist for the Center for Science in the Public Interest, a Washington-based health advocacy group.

Three cups of plain air-popped corn contain 90 calories, 3.9 grams of fiber and less than half a gram of fat. Add butter, however, and the calories skyrocket -- one tablespoon adds 11 grams of fat and 100 calories.

Admittedly, it's hard for some people to eat popcorn in its most healthful form -- air-popped and unadorned -- when tastier alternatives abound on the shelves of grocery stores. About 23 million pounds of caramel-coated popcorn -- a traditional favorite among ready-to-eat versions -- were consumed last year, according to the Snack Food Association, an industry group.

Popcorn tossed with white cheddar cheese has been making a splash since Smartfood Inc., a subsidiary of Frito-Lay, introduced the product in New England in 1985. The flavor -- a mixture of corn oil, aged cheddar, buttermilk and whey -- has been one of the fastest-growing segments of the market, up 33 percent in 1989 over 1988. One half-ounce serving, however, has 80 calories and 5 grams of fat.

When it comes to nutrition, popping the corn at home is still the best alternative, said Hurley. For flavor, sprinkle on a favorite herb or spice -- oregano, garlic powder, dried dill, cinnamon or ginger. Although one ounce of parmesan cheese has 111 calories and 7.3 grams of fat, grating a small amount on popcorn also can boost the flavor. Hurley suggests that people first spray a fine mist of water over the popcorn before adding spice. "This helps the flavor adhere to the popcorn without sinking to the bottom of the bowl," she said.

For people who can't take the time to pop their own corn, Hurley recommends two microwave versions -- Orville Redenbacher's Light Natural and Weight Watchers. "They are not as pure as the no frills air-popped corn, but they are good alternatives." Hurley also suggests that people be wary of brands that say they are air popped. "The corn is often sprayed with oil which adds a lot more calories and fat."