When people talk about “metabolism,” they are often referring to how many calories they burn in a day. However, your metabolism is actually a combination of several different processes. The largest piece of that puzzle is Resting Energy Expenditure (REE).
Understanding your REE is the key to managing weight, optimizing athletic performance, and understanding how your body uses fuel.
1. Defining Resting Energy Expenditure (REE)
Resting Energy Expenditure (REE) is the amount of energy (measured in calories) that your body burns while at rest over a 24-hour period. Even when you are sitting on the couch or sleeping, your body is working hard to perform life-sustaining functions, such as:
- Breathing and lung function.
- Circulating blood (heartbeat).
- Maintaining body temperature.
- Brain and nerve function.
- Cell growth and repair.
- Liver and kidney function.
For most sedentary individuals, REE accounts for 60% to 75% of total daily energy expenditure.
2. REE vs. BMR: What’s the Difference?
You may have heard the term Basal Metabolic Rate (BMR) and wondered if it’s the same as REE. While they are similar, there is a slight technical difference:
- BMR (Basal Metabolic Rate): Measured under very strict clinical conditions. The person must be awake but totally relaxed, in a temperature-controlled room, after an overnight fast, and immediately upon waking.
- REE (Resting Energy Expenditure): Measured under less restrictive conditions. It simply requires the person to be at rest. Because it doesn’t require a strict fast or an overnight stay in a lab, REE is the more commonly used metric in clinical and fitness settings. REE is typically about 10% higher than BMR.
3. Factors That Influence Your REE
Not everyone has the same REE. Several variables determine how many calories your body burns at rest:
- Body Composition: This is the most significant factor. Muscle tissue is more metabolically active than fat tissue. This means a person with more muscle mass will have a higher REE than someone of the same weight with a higher body fat percentage.
- Age: Generally, REE decreases as we age. This is partly due to the natural loss of muscle mass (sarcopenia) and changes in hormonal levels.
- Gender: On average, men tend to have a higher REE than women because they typically possess more lean muscle mass and larger internal organs.
- Genetics: Some people naturally have a “faster” or “slower” metabolism due to inherited traits.
- Hormonal Status: Disorders of the thyroid (like hypothyroidism or hyperthyroidism) can significantly increase or decrease REE.
- Environment: Extremely cold or hot temperatures force the body to work harder to maintain its core temperature, raising REE.
4. How is REE Calculated?
There are two main ways to determine REE:
A. Predictive Equations
Scientists have developed formulas to estimate REE based on height, weight, age, and gender. The most common are:
- Mifflin-St Jeor Equation: Currently considered the most accurate for the general population.
- Harris-Benedict Equation: An older formula often used in fitness settings.
B. Indirect Calorimetry
This is the “Gold Standard.” It involves wearing a mask or sitting under a hood that measures the amount of oxygen you breathe in and the carbon dioxide you breathe out. Since oxygen consumption is directly related to energy metabolism, this provides a highly accurate measurement of REE.
5. Why Does REE Matter for You?
Understanding your REE is the foundation of any nutritional plan.
- Weight Loss: If you know your REE, you know your “caloric floor.” To lose weight safely, you generally shouldn’t consume fewer calories than your REE without medical supervision, as your body needs that energy for vital organs.
- Weight Maintenance: By adding your physical activity level to your REE, you can find your Total Daily Energy Expenditure (TDEE)—the exact number of calories you need to stay at your current weight.
- Muscle Building: To gain muscle, you must consume a surplus of calories above your TDEE.
Conclusion
Resting Energy Expenditure is the “hidden” engine of your metabolism. While you can’t change your age or genetics, you can influence your REE by building lean muscle mass through strength training and ensuring your hormonal health is in check. By knowing your REE, you can stop guessing and start fueling your body with precision.