Resting Energy Expenditure

Resting energy expenditure, or REE, is the amount of energy the body expends while a person is awake and resting, under conditions designed to minimize the effects of physical activity, recent food intake, and environmental temperature. It represents the energy required to maintain normal physiological functions at rest, including cellular metabolism, circulation, breathing, and maintenance of body temperature .^1,2

REE is generally the largest component of total daily energy expenditure and often accounts for about 60% to 70% of the total in people who are not highly active. The remainder comes mainly from physical activity and the thermic effect of food, which is the increase in energy expenditure associated with eating and processing nutrients. REE varies among individuals, with body size and body composition being major determinants. Fat-free mass, which includes metabolically active organs and muscle, is particularly important in explaining differences in REE .^1,2

REE is commonly measured using indirect calorimetry. This method measures oxygen consumption and carbon dioxide production while the person rests, then uses these gas-exchange measurements to estimate energy expenditure. REE can also be estimated using predictive equations based on characteristics such as body weight, height, age, and sex, although estimated values can differ from measured values .^1,2

Resting energy expenditure is closely related to basal metabolic rate, or BMR, but the terms are not strictly identical. BMR is measured under more restrictive conditions intended to represent a minimal resting metabolic state, typically after an overnight fast and following sleep. REE is measured under somewhat less stringent resting conditions and is usually slightly higher than BMR. Resting metabolic rate, or RMR, is commonly used as an alternative term for REE .¹