Energy bars have gone mainstream. No longer under the domain of health food stores, energy bars can now be found in most grocery, drug, mass merchandise, and convenience stores today, according to a study by ACNielsen. In September 2000, sales through all of these retail outlets combined were up 53 percent from September 1999. As a whole, the $700 million category has seen double-digit sales growth for each of the last four years. That's because nearly 10 percent of U.S. households purchase these power snacks, compared with 5.9 percent in 1997. Who's munching on all of these vitamin-packed vittles? Energy bars are most popular among single men, upper-income professionals, and households in large cities.