According to Gina Kolata's latest book -- Rethinking Thin: The New Science of Weight Loss--and the Myths and Realities of Dieting -- Not even exercise and reduced calories will help in the long run. The body will force you to readjust to the new weight by slowing down your metabolism until you gain back the weight. Essentially her argument is that practically _all_ scientific studies show that people have a rate range of about 20 pounds and that range cannot be changed by dieting and exercise. She is not saying that this range is genetic -- though that is part of it --- but apparently factors like if your mother smoked while she was pregnant and whether your mother was restricted in her calorie intake while you were pregnant are huge factors in obesity.
Let me repeat, her claim is that practically all of the scientific evidence shows that there is nothing, absolutely nothing, you can do by diet and exercise to effect your weight-range in the long run. But that exercise makes everyone healthy. But if you want to be at the lower end of your weight range, a lot of exercise and a change in the kinds of foods you eat will help.
Jerry