Resting Energy Expenditure (REE) is the minimum energy (calories) your body requires to sustain basic life-supporting functions while you are at rest. This energy is used for involuntary processes like breathing, maintaining body temperature, circulating blood, and neurological function. Knowing your REE is a first step in structured weight management and nutrition planning. It provides the baseline caloric number required to keep your body functioning, offering a more personalized estimate than general calorie recommendations based only on age and sex.
Defining REE and Its Role in Metabolism
Resting Energy Expenditure (REE) is often used interchangeably with Basal Metabolic Rate (BMR). BMR is a stricter measurement taken under highly controlled, fasting conditions immediately upon waking in a thermoneutral environment. REE is a more practical measurement taken under less strict conditions, typically after a short period of rest and fasting, making it the term most commonly referenced in health and fitness. REE tends to be slightly higher than BMR, generally by about 10 to 20%.
REE is the largest component of your Total Daily Energy Expenditure (TDEE), typically accounting for 60% to 75% of the calories you burn each day. The remaining percentage is attributed to the thermic effect of food (digestion) and physical activity. Factors that influence this metabolic rate include age, as metabolism generally slows over time, and sex. Lean body mass, particularly muscle tissue, is a significant determinant of REE because muscle is metabolically active and requires more energy to maintain than fat tissue.
Step-by-Step Calculation Using Predictive Equations
Because direct measurement of REE is not always accessible, scientists use predictive formulas to estimate this value. The most widely accepted and accurate equation for the general healthy population is the Mifflin-St Jeor equation. This formula uses weight, height, age, and sex to provide a practical estimate of your resting calorie burn.
The Mifflin-St Jeor equation requires your weight in kilograms, height in centimeters, and age in years. Using these metric units ensures the constants in the equation work correctly to produce a result in kilocalories per day.
For men, the formula is: REE = (10 x weight) + (6.25 x height) – (5 x age) + 5.
For women, the formula is: REE = (10 x weight) + (6.25 x height) – (5 x age) – 161.
The Revised Harris-Benedict equation is another commonly used formula, often applied in clinical settings, though the Mifflin-St Jeor formula is generally favored for its improved accuracy. These formulas are built on averages derived from large study groups and are estimated to be within 10% of the true measured rate for a majority of healthy individuals.
To illustrate the calculation, consider a 35-year-old woman who weighs 65 kg and is 170 cm tall. The calculation would be: (10 x 65) + (6.25 x 170) – (5 x 35) – 161. This breaks down to: 650 + 1062.5 – 175 – 161, resulting in an estimated REE of 1376.5 calories per day.
The Clinical Standard: Indirect Calorimetry
The gold standard for precisely measuring REE is a technique called Indirect Calorimetry (IC). This non-invasive test works by analyzing the gases a person breathes in and out while resting, measuring oxygen consumption (\(\text{VO}_2\)) and carbon dioxide production (\(\text{VCO}_2\)).
This method relies on the principle that energy production is directly proportional to oxygen use and carbon dioxide release. Measurements are taken using a metabolic cart and a device like a canopy hood or a face mask, and the results are processed using established formulas like the Weir equation to determine calorie expenditure. Indirect Calorimetry is most often performed in a clinical or research setting because it requires specialized, expensive equipment and a trained technician. It is recommended for individuals with significant metabolic abnormalities, such as those with morbid obesity or certain endocrine disorders, where predictive equations may be highly inaccurate.
Translating REE into Daily Calorie Needs
Once your REE is calculated, the next step is to determine your Total Daily Energy Expenditure (TDEE). This is accomplished by multiplying your REE by an Activity Factor, also known as the Physical Activity Level (PAL). The PAL is a multiplier that represents the energy expended through daily activities and structured exercise.
Activity factors are categorized based on lifestyle intensity. For a sedentary lifestyle (little or no exercise), the factor is typically 1.2. A lightly active individual (light exercise 1-3 days per week) might use 1.375, while a moderately active person (moderate exercise 3-5 days per week) would use 1.55. Multiplying the calculated REE by the appropriate factor yields your TDEE, the total calories required to maintain your current weight.
If your goal is weight maintenance, consume calories equal to your TDEE. To achieve weight loss, you must create a caloric deficit by consuming fewer calories than your TDEE or by increasing your activity level. Conversely, a caloric surplus—consuming more than your TDEE—is required for weight gain, often in the form of muscle mass.