["The Journal of Physiology, EarlyView. ", "\nAbstract figure legend Two models of human energy expenditure were tested. The additive model predicts independent resting and activity costs, while the constrained model predicts a tradeoff between resting and active expenditure. Both longitudinal and cross‐sectional experiments support the additive model, with no tradeoffs between resting and active energy expenditures.\n\n\n\n\n\n\n\n\n\nAbstract\nTotal energy expenditure (TEE) is commonly modelled as the sum of resting energy expenditure (REE), activity energy expenditure (AEE), and diet‐induced thermogenesis (DIT). This additive model has recently been challenged by the constrained energy balance model, which proposes that reductions in mass‐adjusted REE compensate to some extent for increases in AEE, rendering mass‐adjusted TEE effectively invariant. We tested these competing models using two complementary approaches. First, we conducted a short‐term longitudinal, repeated‐measures experiment in 12 adults who completed 10‐day periods of high and low physical activity separated by a washout period. TEE was measured with doubly labelled water, AEE with wearable sensors, and REE with indirect calorimetry as well as anthropometric equations. Second, we analysed a cross‐sectional sample of 268 adults from the Pennington Centre Longitudinal Study that measured TEE and REE independently. In both studies, metabolic rates were analysed using size‐adjusted regression models. In the longitudinal experiment, a ∼28% increase in AEE (∼250 kcal/day) increased TEE by 10% (272 kcal/day) without reducing mass‐adjusted REE, contradicting predictions of metabolic compensation. In the cross‐sectional study, mass‐adjusted REE and AEE were statistically independent, with no evidence for metabolic compensation. These results support the additive model of energy expenditure and indicate that increased physical activity does not suppress REE, contradicting the hypothesis that metabolic compensation thwarts the effects of physical activity on weight management.\n\n\n\n\n\n\n\n\n\nKey points\n\nThere is a debate over whether active activity expenditure (AEE) is independent of resting energy expenditure (REE) corrected for body mass and composition or causes decreases in REE through metabolic compensation.\nWe tested AEE's effects on REE using both a repeated measures longitudinal study and a cross‐sectional study. In both studies, AEE and REE were measured independently, and metabolic rates were analysed using regression models.\nIn the longitudinal study, a 28% increase in AEE had no effect on mass‐adjusted REE, contradicting predictions of metabolic compensation. In the cross‐sectional study, mass‐adjusted REE and AEE were statistically independent with no evidence for metabolic compensation.\nThese results indicate that increased physical activity does not suppress mass‐adjusted REE, supporting the additive model of energy expenditure.\n\n\n"]