Resting Energy Expenditure: What It Is and Why It Matters

Your body is constantly working, even when you are not. The energy required for these fundamental processes—such as breathing, circulating blood, and producing new cells—is known as Resting Energy Expenditure (REE). It is the baseline number of calories your body burns in a state of complete rest. This figure represents the minimum energy your body needs to sustain its most basic, life-sustaining functions.

Defining Resting Energy Expenditure

Resting Energy Expenditure is measured under less rigid conditions than its counterpart, Basal Metabolic Rate (BMR). REE is assessed when a person is awake, calm, and in a thermally neutral environment, usually a few hours after a meal. BMR, on the other hand, requires stricter measurement conditions, like being tested immediately upon waking after an overnight fast. For practical purposes, REE is more commonly used and the terms are often referenced interchangeably.

This resting energy use forms the largest part of your Total Daily Energy Expenditure (TDEE), the total number of calories you burn in a 24-hour period. For most sedentary or lightly active people, REE accounts for 60-75% of their TDEE. The remaining portion of TDEE is made up of the thermic effect of food and activity energy expenditure.

Factors Influencing Your REE

An individual’s Resting Energy Expenditure is determined by several factors. Body size and composition are primary determinants; larger individuals have a higher REE because they have more tissue to maintain. Lean body mass, which is primarily muscle, is more metabolically active and burns more calories at rest compared to fat mass. For instance, each pound of muscle burns approximately 6 calories per day at rest, whereas a pound of fat burns only about 2 calories.

Age and sex also play a role. REE peaks during infancy and early adulthood and then begins a slow decline, often linked to the natural loss of muscle mass that occurs with aging. On average, males have a higher REE than females, a difference largely attributable to their typically larger body size and greater muscle mass.

Genetic predispositions can influence an individual’s metabolic rate. Hormonal status is another modulator; thyroid hormones are particularly impactful. An overactive thyroid (hyperthyroidism) can increase REE, while an underactive thyroid (hypothyroidism) can lower it. Stress hormones like cortisol and adrenaline can also cause temporary increases in energy expenditure.

Your body’s internal temperature affects its energy use. An elevated body temperature, such as during a fever, forces the body to work harder and leads to a rise in REE. Conversely, prolonged calorie restriction can cause the body to adapt by lowering its REE. This is a protective mechanism to conserve energy during periods of perceived starvation.

Measuring and Estimating REE

Determining your REE can be approached through laboratory measurement or by using predictive equations for an estimation. The most accurate clinical method is indirect calorimetry. This technique involves a person resting comfortably while a device measures their oxygen consumption and carbon dioxide production to calculate energy expenditure. This method requires sophisticated equipment and is generally reserved for clinical or research settings.

Historically, a method called direct calorimetry, which measures the actual heat released by the body, was used but is now rarely employed due to its complexity. For most people, REE is determined through predictive equations. These formulas provide an estimate of REE based on specific personal variables.

The most widely recognized formulas include the Mifflin-St Jeor and the revised Harris-Benedict equations. These equations use inputs such as your age, sex, height, and weight to calculate an approximate REE. Many online calorie calculators use these formulas to give users a baseline for their energy needs. Remember that these are estimations and can have a margin of error, but they offer a valuable starting point.

Why Knowing Your REE Matters

Understanding your Resting Energy Expenditure has several practical applications for weight management. REE helps establish a baseline for setting calorie targets, whether the goal is weight loss, maintenance, or muscle gain. To lose weight, one must consume fewer calories than their Total Daily Energy Expenditure (TDEE), of which REE is the largest component.

Knowledge of your REE can also inform nutritional planning. When you have a clear idea of your body’s baseline energy requirements, you can make more educated decisions about food choices and portion sizes to align with your goals. This awareness provides a personalized context for your metabolism.

Finally, understanding REE provides a clearer context for the role of physical activity. It highlights that while many calories are burned at rest, exercise and daily movement add to this total. This knowledge helps create a more complete picture of energy balance. In clinical settings, REE is also used to develop specialized nutrition support for patients with various medical conditions.