Perspective: Is the response of human energy expenditure to increased physical activity additive or constrained?

The idea that increasing physical activity directly adds to total energy expenditure in humans (additive model) has been challenged by the energy constrained hypothesis (constrained model). This model proposes that increased physical activity decreases other components of metabolism to constrain total energy expenditure. There is a logical evolutionary argument for trade-offs in metabolism but, to date, evidence supporting constraint is subject to several limitations including cross-sectional and correlational studies with potential methodological issues from extreme differences in body size/composition and lifestyle, potential statistical issues such as regression dilution and spurious correlations, and conclusions drawn from deductive inference rather than direct observation of compensation. Addressing these limitations in future studies, ideally randomized controlled trials, should improve the accuracy of models of human energy expenditure. The available evidence indicates that in many scenarios, the effect of increasing physical activity on total energy expenditure will be mostly additive – although some energy appears to ‘go missing’ and is currently unaccounted for. The degree of energy balance could moderate this effect even further.