GD9 CTW fileÒ  Medical005, Ø After they become adults, most people don't realize that they gain slightly less than one pound of weight per year. Now, researchers say that they have found out why most dieters regain all of the weight they lose on low-calorie diets: Nature works against them. According to one new study, everyone's metabolism, fat or thin, adjusts to maintain one's prediet weight. In this study, men and women were put on weight-gaining and weight-losing diets. Their metabolisms, that is, the rate at which they burn calories, slowed down when they lost weight and remained that way for the next several months. After the obese people lost weight, they burned fewer calories than other obese people who had not slimmed down. The numbers meant that they would slowly regain their lost weight even if they continued to eat quite reasonably. It worked in reverse for those who were on weight-gaining diets, who gained up to one-tenth of their initial weight. But when their diets stopped, their bodies burned enough additional calories to push their weights back to their original levels. The results were just the same for people regardless of gender, ethnic background, or age. The findings suggest that it is difficult, but not impossible, for people to maintain healthier weight levels. Decreasing one's food intake or increasing energy output for a short period is not going to control weight. Good nutrition and increased physical activity over the long term are necessary to keep weight off.