The current epidemic of obesity has occurred despite the existence of a body weight regulatory system which, for most of mankind's history, has matched energy intake with energy expenditure sufficiently to avoid obesity. This suggests that the primary cause of the current obesity epidemic is not genetic, but may be due to an environment in which the energy balance regulatory system cannot function with sufficient precision to keep the population lean. In this application, we propose research aimed at understanding how factors in the environment, namely high fat/energy dense diets and physical inactivity, can promote obesity by affecting the precision of regulation of energy and fat balance. It is our intent to identify dietary and physical activity patterns that are associated with increased precision of energy balance regulation and which can prevent development of obesity. Laboratory data suggest that high fat diets promote obesity by increasing the probability of overconsumption of total energy. Our first aim is to systematically examine the relationship between dietary fat and energy intake across a range of diet compositions in sedentary subjects. While this has been done for diets with extreme variation in dietary fat (i.e., less than or equal to 20 percent vs greater than or equal to 40-60 percent) it has not been done for dietary fat content within the range of usual consumption of U.S. adults (i.e. 20-40 percent fat diets). We hypothesize that this relationship will not be linear and that there will be a threshold level or a range of dietary fat associated with a low probability of increased energy intake and positive energy balance. This information will be useful in developing dietary guidelines for obesity prevention. Our second aim is to determine how level of physical activity interacts with dietary fat content to affect the likelihood of developing positive energy balance. We hypothesize that the optimum level of dietary fat to minimize the probability of positive energy balance will depend on level of physical activity and the optimum level of physical activity to minimize the likelihood of positive energy balance will vary with dietary fat content. This work will be among the first to study the interaction of dietary and physical activity patterns in promotion and prevention of obesity. The results will help identify the changes required in current dietary and physical activity patterns if we are going to be successful in preventing the development of obesity.