Integration of the overall response to exercise.

Physical activity ellicits major perturbations in metabolism during the period of exertion and continues to influence metabolism afterwards as well. The overall effect of these intermittent and extended effects on the balance between energy expenditure and food intake regulation can best be judged by the impact of exercise on the fat mass. Expansion of the adipose tissue mass promotes fat oxidation more than glucose oxidation, but excessive fat accumulation is often necessary to reach the state where fat oxidation becomes commensurate with fat intake. Since the steady-state of weight maintenance is achieved with less body fat in physically active individuals, exercise is a substitute for an enlarged fat mass in bringing about rates of fat oxidation commensurate with fat intake. The increases in energy expenditure induced by expansion of the adipose tissue mass or by physical activity are quantitatively very different. The fact that they are substitutes for each other in relation to body weight maintenance indicates that the critical issue, under conditions where food availability is not a limiting factor, is not overall energy turnover. It appears therefore that it is the effect of exercise on the 24-h RQ that allows the organism to operate with an average RQ as low as the diet's FQ in the presence of less body fat. This also explains why the diet's carbohydrate-to-fat ratio can be an important parameter in shaping the interactions between physical activity and body weight maintenance.