The human body constantly burns calories, even at rest, to power its most basic life-sustaining functions. This continuous energy consumption is known as metabolism, and its largest component is the Resting Energy Expenditure (REE). Understanding REE is foundational because it represents the baseline energy needed just to keep you alive and functioning. This baseline burn is the single most significant factor in your total daily calorie output.
Defining Resting Energy Expenditure
Resting Energy Expenditure (REE) is the number of calories your body uses over a 24-hour period to maintain fundamental physiological processes while you are awake but resting comfortably. These processes include the involuntary work of your organs, such as pumping blood, regulating body temperature, facilitating breathing, and supporting brain function. REE accounts for approximately 60 to 75 percent of the total calories you burn each day.
REE is often confused with Basal Metabolic Rate (BMR), though the two terms have a slight technical difference. BMR is measured under stricter, more controlled conditions, after a 12-hour fast and a full night’s sleep in a thermoneutral environment. REE is measured under less stringent conditions, often requiring only comfortable rest and a few hours of fasting. For practical purposes, REE is more commonly used in clinical and research settings because it is easier to measure. The two values typically differ by less than 10 percent, with REE being slightly higher.
The total number of calories you burn in a day, called Total Daily Energy Expenditure (TDEE), is made up of REE plus two other components. These components are the Thermic Effect of Food (TEF), which is the energy spent digesting and absorbing nutrients, and the energy expended through physical activity. Since REE is the largest component of TDEE, knowing its value provides insight into your body’s overall energy needs.
Key Factors That Influence Your REE
An individual’s REE is not a fixed number but is determined by a complex interplay of biological and physiological factors. The most important determinant is body composition, specifically the amount of fat-free mass a person possesses. Tissues like muscle, organs, and bone are metabolically active and require more calories to sustain than fat tissue. Consequently, individuals with greater muscle mass naturally have a higher REE, even at the same body weight as someone with less muscle.
Body size also plays a significant role in determining REE, as larger individuals generally have a greater surface area and a higher overall mass of metabolically active tissue. Taller people often have a slightly higher REE because increased surface area leads to greater heat loss, requiring more energy to maintain a stable internal temperature. A natural decline in REE is observed with age, primarily attributed to the gradual loss of muscle mass that occurs over the decades.
Genetics and hormonal status introduce further variability into resting energy burn. The function of the thyroid gland is a known regulator of metabolic rate, with imbalances leading to noticeable shifts in REE. Inherited metabolic traits can also account for some differences in REE observed between people who are otherwise similar in age and body composition.
Measuring and Estimating REE
The most accurate method for determining REE is a laboratory technique called Indirect Calorimetry. This procedure measures the volume of oxygen a person consumes and the volume of carbon dioxide they produce while resting. Since energy production requires oxygen, the exchange of these gases provides a precise measure of the calories being burned. The results from Indirect Calorimetry are considered the gold standard for metabolic measurement.
This specialized equipment and expertise are not practical or accessible for the average person. Therefore, predictive equations are widely used to estimate REE based on simple anthropometric data. Formulas like the Mifflin-St Jeor equation or the older Harris-Benedict equation use variables such as age, sex, height, and weight to calculate an approximation of REE.
While these equations are valuable for estimating REE at a population level, they are not always accurate for a single individual. Calculated estimates can have wide limits of agreement compared to a measured REE, meaning they may overestimate or underestimate a person’s true metabolic rate significantly. Despite this limitation, these formulas provide a practical starting point for health and nutrition planning when clinical measurement is unavailable.
Practical Application for Weight Management
Knowing your REE is foundational for effective weight management because it establishes the minimum number of calories your body needs daily. This figure is the basis for calculating your Total Daily Energy Expenditure (TDEE). TDEE is achieved by multiplying the REE by an activity factor that accounts for your level of physical movement and exercise. The resulting TDEE is the estimated total number of calories you burn.
For weight loss, a person must create a caloric deficit, meaning they must consistently consume fewer calories than their calculated TDEE. Using REE to determine TDEE allows individuals to set a safe and effective calorie target below their expenditure. Conversely, consuming too few calories can cause a greater-than-predicted drop in REE, a phenomenon known as metabolic adaptation, which makes continued weight loss more difficult.
Maintaining or optimizing REE is important for long-term weight maintenance. Since muscle tissue is more metabolically active than fat, engaging in strength training can help preserve or increase lean mass, supporting a higher REE. Incorporating physical activity alongside a tailored diet plan is the optimal approach. Exercise helps protect against the loss of fat-free mass and the subsequent decline in REE that often occurs during calorie restriction.