Obesity is highly prevalent in developed countries and it is casually associated with cardiovascular disease, diabetes and other serious nutritionally related disorders. Although it is clear that obesity is a disease of energy imbalance, it is not clear if obesity is due to high dietary intake or low energy expenditure. A large portion of this uncertainty is due to the absence of a method for measuring total energy expenditure in free-living subjects. This problem has recently been solved through our development of the doubly labeled water method for measuring energy expenditure. Using this method, we and others have obtained data that implicate physical activity as a parameter that inversely correlates with body fat and suggest an important role for low physical activity in the development of obesity. Based on these data, we hypothesize that expending energy in physical activity on weight gain. In Phase I, total energy expenditure and energy expended in physical activity will be measured to test the hypothesis that low physical activity is a predictor of subsequent weight gain. In Phase II, we will stratify the subjects into high and low weight gainers and they will be enrolled in an aerobics class to increase physical activity to test the hypothesis that added physical activity can alter the rate of weight in those who have begun to gain weight. These studies will be the first prospective studies of the influence of energy expended in physical activity on weight change in nonobese adults. More importantly, these studies will quantitate the level of physical activity needed to prevent weight gain and be valuable for the formulation of clinical recommendations for the prevention of obesity.