Resting Energy Expenditure (REE)

Resting energy expenditure is the rate at which a body burns energy at rest, awake, fasted and thermally comfortable, and it makes up roughly sixty to seventy-five percent of total daily expenditure. It is measured by indirect calorimetry, which derives heat production from oxygen consumption and carbon dioxide production. Basal metabolic rate is the same quantity under stricter conditions, measured after overnight sleep in the laboratory, and runs slightly lower. The rest of total expenditure is the thermic effect of food, roughly a tenth of intake, plus physical activity.

Fat-free mass explains most of the between-person variance, though not because muscle is expensive at rest. Liver, brain, kidney and heart are a few percent of body mass and account for a large share of resting expenditure, while resting muscle is comparatively quiet per kilogram. Predictive equations such as Mifflin-St Jeor estimate the group mean well and an individual poorly, with errors commonly of ten to fifteen percent, and free-living total expenditure requires doubly labelled water rather than any equation.

That error band is the practical point. An equation output carries a couple of hundred kilocalories of uncertainty, so treating a calculated number as a personal metabolic rate builds on noise. Real change in resting expenditure over months is usually a modest absolute figure that nonetheless compounds.

Consumer scales and wearables feed an impedance estimate of fat-free mass into one of these same equations, inheriting both errors and reporting the result to the kilocalorie. And the slow metabolism invoked to explain a stalled weight loss is rarely present when it is measured: people with obesity have higher absolute resting expenditure than lean people, because there is more tissue to support.