Diet composition can play a role in obesity treatment because it can influence energy intake and nutrient balance. The greatest potential effect of diet composition occurs not during weight reduction but during maintenance of a reduced body weight. During weight reduction, the extent of negative energy balance is the greatest determinant of the amount and rate of weight loss, and any effects of diet composition are likely to be very small. Conversely, during the maintenance period after weight reduction, maintenance energy requirements are reduced and the rate of fat oxidation may be low. Prevention of fat storage and regain of body weight can be achieved either by increasing fat oxidation (for example, by increasing exercise) or by reducing the fat content of the habitual diet. The treatment with the greatest chance for success combines a reduced-fat diet with aerobic exercise.