听力题 Exercise is one of the few factors with a positive role in long-term maintenance of body weight. Unfortunately, that message has not gotten through to the average American, who would rather try switching to "light" beer and low-calorie bread than increase physical exertion. The Center for Disease Control, for example, found that fewer than one-fourth of overweight adults who were trying to lose pounds said they were combining exercise with their diet. In rejecting exercise, some people may be too discouraged by calorie-expenditure charts; for example, one would have to quickly walk three miles just to work off the 275 calories in one delicious Danish pastry. Even exercise professionals concede Americans have a point here. "Exercise by itself is a very tough way to lose weight," says York Onnen, program director of the President''s Council on Physical Fitness and Sports. Still, exercise''s supporting role in weight reduction is vital. A study at the Boston University Medical Center of overweight police officers and other public employees confirmed that those who dieted without exercise regained almost all their old weight, while those who worked exercise into their daily routine maintained their new weight. If you have been taking little exercise and decide to start walking one mile a day, the added exercise could burn an extra 100 calories daily. In a year''s time, assuming no increase in food intake, you could lose ten pounds. By increasing the distance of your walks gradually and making other dietary adjustments, you may lose even more weight.