Exercise is one of the few factors with a positive role in longterm maintenance of body weight. Unfortunately, that message has not gotten through to the average American, who would rather try switching to 'light' beer and lowcalorie bread than increase physical exertion. The Centers for Disease Control, for example, found that fewer than onefourth of overweight s who were trying to shed pounds said they were combining exercise with their diet. In rejecting exercise, some people may be discouraged too much by calorieexpenditure charts, for example, one would have to briskly walk three miles just to work off the 275 calories in one delicious Danish pastry. Even exercise professionals concede half a point here. 'Exercise by itself is a very tough way to lose weight.' said 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 sedentary (极少活动的) 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 other dietary adjustments, you may lose even more weight. What is said about the average American in the passage?