Understanding Resting Energy Expenditure (REE) vs. Basal Metabolic Rate (BMR)
Before determining what a good resting energy expenditure is for you, it's helpful to clarify the difference between REE (or RMR) and BMR. While the terms are often used interchangeably, there are technical distinctions. BMR measures the minimum number of calories your body needs to perform life-sustaining functions, such as breathing, circulation, and cell production, in a completely rested, fasted state. REE, or resting metabolic rate, is a less restrictive measure that also includes the energy needed for low-level daily activities like digestion and simply getting dressed. As a result, RMR is typically about 10% higher than BMR.
There is no one-size-fits-all answer for a 'good' REE. Average figures exist (around 1,600 kcal/day for men and 1,400 kcal/day for women), but the healthy range is extremely wide and varies dramatically between individuals. The right number for you is entirely personal and dependent on a combination of physiological and lifestyle factors. This is why generalized diet plans often fail, as they don't account for individual metabolic needs.
Factors That Influence Your Resting Energy Expenditure
Your individual REE is not a static number. It changes based on several key factors:
- Age: Your metabolism naturally slows with age. After age 20, REE is estimated to decrease by about 2% per decade. This is largely attributed to a loss of lean muscle mass and age-related changes in hormone levels.
- Sex: On average, men have a higher REE than women. This is primarily due to men typically having a greater lean muscle mass and larger body size.
- Body Composition: This is one of the most significant factors. Lean body mass (FFM), which includes muscle and organs, is far more metabolically active than fat mass. Therefore, a person with more muscle mass will have a higher REE, even at the same body weight as someone with more fat mass.
- Body Size: Larger individuals require more energy to maintain their body's functions, leading to a higher REE.
- Genetics: Individual metabolism rates can be influenced by inherited genes. Research on twins suggests that genetics account for a significant portion of the variation in BMR.
- Hormonal Status: Hormones play a major role in regulating metabolism. Thyroid hormone levels, for example, directly impact metabolic speed; too little (hypothyroidism) slows metabolism, while too much (hyperthyroidism) speeds it up. Stress hormones like cortisol can also influence REE.
- Nutritional Intake: Drastic calorie restriction can trigger a protective metabolic slowdown, a process known as adaptive thermogenesis, which lowers REE.
How to Calculate Your Resting Energy Expenditure
While indirect calorimetry in a clinical setting is the most accurate way to measure REE, there are several widely used formulas that provide a good estimate. For most people, these calculators offer a reliable starting point for understanding their metabolic rate.
The Mifflin-St Jeor Equation
Considered one of the more accurate predictive formulas for the general population, the Mifflin-St Jeor equation is a modern standard.
- For Males: $REE = (10 imes ext{weight in kg}) + (6.25 imes ext{height in cm}) - (5 imes ext{age in years}) + 5$
- For Females: $REE = (10 imes ext{weight in kg}) + (6.25 imes ext{height in cm}) - (5 imes ext{age in years}) - 161$
The Harris-Benedict Equation
The original Harris-Benedict equation was developed in 1919 and revised in 1984. It is still widely used but is considered slightly less accurate for certain populations than the Mifflin-St Jeor equation.
- For Males: $BMR = 88.362 + (13.397 imes ext{weight in kg}) + (4.799 imes ext{height in cm}) - (5.677 imes ext{age in years})$
- For Females: $BMR = 447.593 + (9.247 imes ext{weight in kg}) + (3.098 imes ext{height in cm}) - (4.330 imes ext{age in years})$
Comparison of REE Calculation Methods
| Feature | Mifflin-St Jeor Equation | Harris-Benedict Equation | Indirect Calorimetry |
|---|---|---|---|
| Accuracy | High for general population | Less accurate than Mifflin-St Jeor, especially for obese individuals | Considered the gold standard |
| Required Info | Weight, height, age, sex | Weight, height, age, sex | Specialized lab equipment, fasted and rested state |
| Convenience | Highly convenient, calculator-based | Highly convenient, calculator-based | Inconvenient, requires clinical setting |
| Considerations | Does not account for body composition | Does not account for body composition; often overestimates for modern lifestyles | Measures actual metabolic gases to determine energy expenditure |
Can You Improve Your Resting Energy Expenditure?
While you cannot change your age, sex, or genetics, you can influence your REE by altering your body composition. The most effective way to increase your REE is by building and maintaining lean muscle mass through regular strength training. Muscle tissue is more metabolically active than fat tissue, meaning it burns more calories at rest. This makes strength training a valuable strategy for boosting your metabolic rate and supporting weight management goals. Avoiding extreme or prolonged calorie restriction is also important, as this can trigger adaptive thermogenesis and lower your REE.
Conclusion
A 'good' resting energy expenditure is not a single number but a personalized baseline reflecting your body's unique metabolic needs. By using reliable formulas like the Mifflin-St Jeor equation, you can gain a solid estimate of your daily caloric requirements at rest. Knowing this figure is the first step toward making informed decisions about your nutrition and fitness plan. Remember that factors like age, body composition, and genetics play a significant role, and building lean muscle mass is the most effective way to improve your REE over the long term. This personalized approach to metabolism is key to achieving and maintaining your health goals.
For more in-depth scientific literature on the factors affecting resting energy expenditure, including body composition and age, please refer to the NCBI Bookshelf resource.
