Quantifying physical activity energy expenditure based on doubly labelled water and basal metabolism calorimetry: what are we actually measuring?

Purpose of review Physical activity impacts energy balance because of its contribution to total energy expenditure. Measuring physical activity energy expenditure (PAEE) is often performed by subtracting the estimated 24 h expenditure on basal metabolism (called basal energy expenditure or BEE) from the total energy expenditure (TEE) measured by doubly labelled water minus an estimate of the thermic effect of food (TEF). Alternatively it can be measured as the ratio of TEE/BEE, which is commonly called the physical activity level (PAL). Recent findings PAEE and PAL are widely used in the literature but their shortcomings are seldom addressed. In this review, we outline some of the issues with their use. Summary TEE and BEE are both measured with error. The estimate of PAEE by difference magnifies these errors and consequently the precision of estimated PAEE is about 3× worse than TEE and 25–35× worse than BEE. A second problem is that the component called PAEE is actually any component of TEE that is not BEE. We highlight how the diurnal variation of BEE, thermoregulatory expenditure and elevations of RMR because of stress will all be part of what is called PAEE and will contribute to a disconnect between what is measured and what energy expenditure is a consequence of physical activity. We emphasize caution should be exerted when interpreting these measurements of PAEE and PAL.