Differences in physical activity represent the largest source of variability in energy requirements, both within and between individuals. Chronic changes in physical activity can produce chronic changes in energy requirements that, if not compensated for, can lead to changes in the level at which body weight and body composition are maintained over time. We present a model that can serve as a framework for understanding how energy and macronutrient balance are maintained in steady state conditions, and for illustrating the potential effect of a change in physical activity on these steady state conditions and on energy requirements. According to the model, a chronic change in physical activity forces changes in other aspects of energy and substrate utilization so that a steady state condition can be reestablished. The net effects of the change in physical activity on energy requirements will depend on how and over what period of time these new steady states are reached. Although we cannot at present predict, for any individual, the precise effect of changes in physical activity on energy requirements and on body weight and composition, our model provides a framework for further study of the factors that can influence energy requirements.