Regular physical exercise and endurance training are associated with low body weight and low body fat mass. The relationship between exercise and body-weight control is complex and incompletely understood. Regular exercise may decrease energy balance through an increase in energy expenditure or an increase in fat oxidation. It may also contribute to weight loss by modulating nutrient intake. An intriguing question that remains unresolved is whether changes in nutrient intake or body composition secondarily affect spontaneous physical activity. If this were the case, physical activity would represent a major adaptative mechanism for body-weight control.