Physical activity and metabolic rates in humans

All animals perform physical activity, but humans engage in a special kind of physical activity - exercise, defined as discretionary physical activity for health and fitness. However, the effects of physical activity on whole-organism metabolism and health are unresolved, partly because it is difficult to measure the three major components of metabolism: active energy expenditure (AEE), resting energy expenditure (REE) and dietary induced thermogenesis (DIT), which together equal total energy expenditure (TEE). Three competing models make different predictions about the effects of AEE on REE and TEE. Whereas the traditional 'additive' model of energy balance predicts that AEE is independent of REE, the 'stress' model hypothesizes that AEE temporarily increases REE partly because of transient effects of excess post-exercise oxygen consumption (EPOC). In contrast, the 'constrained energy' model predicts that increases in AEE cause compensatory decreases in REE to maintain a constant TEE. Here, we discuss how different analytical models, measurements, experimental designs and statistical methods affect tests of these three models' hypotheses. After accounting for spurious correlations, we find that longitudinal and cross-sectional data provide most support for the additive model. However, more and better data are needed to test these hypotheses rigorously. To conclude, we also review the evidence, mostly from humans, that increased levels of physical activity slow aging and reduce vulnerability to disease by diverting energy away from processes that improve reproductive success at the expense of long-term health and by increasing energy allocation to repair, maintenance and capacity building.