What is the Difference Between BMR and RMR?
Basal Metabolic Rate (BMR) and Resting Metabolic Rate (RMR) are terms often used interchangeably but have a technical difference. BMR is the minimum calories needed for basic functions in a completely rested, fasted state, typically measured in a lab. RMR is the calories burned at rest under less strict conditions and is the more practical measure for nutritional planning, which the Harris Benedict equation estimates.
The Original Harris Benedict Equation
Developed in 1919, the Harris Benedict equation calculates basal energy expenditure (BEE) or BMR using formulas that consider weight, height, and age.
For men (metric units):
$BMR = 66.5 + (13.75 \times W) + (5.003 \times H) - (6.755 \times A)$
For women (metric units):
$BMR = 655.1 + (9.563 \times W) + (1.850 \times H) - (4.676 \times A)$
$W$ = weight in kilograms, $H$ = height in centimeters, $A$ = age in years.
For imperial units:
For men (imperial units):
$BMR = 66 + (6.23 \times W) + (12.7 \times H) - (6.8 \times A)$
For women (imperial units):
$BMR = 655 + (4.35 \times W) + (4.7 \times H) - (4.7 \times A)$
$W$ = weight in pounds, $H$ = height in inches, $A$ = age in years.
Example Calculation: Female (Metric Units)
For a 30-year-old woman, 68 kg, 165 cm:
$BMR = 655.1 + (9.563 \times 68) + (1.850 \times 165) - (4.676 \times 30) \approx 1470$ calories per day.
How to Determine Total Daily Energy Expenditure (TDEE)
To estimate total daily caloric needs, multiply your BMR by an activity factor to get your TDEE.
| Activity Level | Activity Factor | TDEE Calculation | Example |
|---|---|---|---|
| Sedentary | $1.2$ | $BMR \times 1.2$ | Little to no exercise |
| Lightly Active | $1.375$ | $BMR \times 1.375$ | Light exercise 1-3 days/week |
| Moderately Active | $1.55$ | $BMR \times 1.55$ | Moderate exercise 3-5 days/week |
| Very Active | $1.725$ | $BMR \times 1.725$ | Hard exercise 6-7 days/week |
| Extra Active | $1.9$ | $BMR \times 1.9$ | Very hard exercise, physical job |
For our example woman, if moderately active: $TDEE = 1470 \times 1.55 \approx 2278.5$ calories per day.
Using the Harris Benedict Results for Weight Management
- Maintain weight: Consume calories equal to your TDEE.
- Lose weight: Eat fewer calories than your TDEE (a 500-calorie deficit aims for about one pound loss per week).
- Gain weight: Consume more calories than your TDEE.
Harris Benedict vs. Mifflin-St. Jeor Equation
The Mifflin-St. Jeor equation (1990) is a more modern alternative often considered more accurate than the 1919 Harris Benedict equation, especially for diverse populations.
| Feature | Harris Benedict Equation | Mifflin-St. Jeor Equation |
|---|---|---|
| Development Year | 1919 | 1990 |
| Accuracy | Can overestimate in obese individuals. | Often considered more accurate. |
| Considerations | Relies on height, weight, and age. | Also relies on height, weight, and age. |
Limitations of Predictive Equations
Predictive equations have limitations:
- Body Composition: They don't account for muscle mass, which burns more calories than fat.
- Individual Variation: Metabolism is influenced by genetics, ethnicity, and hormones.
- Clinical Accuracy: Indirect calorimetry is the standard for precise clinical measurements.
Conclusion
Understanding what is the RMR Harris Benedict equation is a valuable step for personalized nutrition. It provides a baseline caloric estimate to guide dietary and activity choices. Remember it's an estimate, and for precise needs, consult a professional or consider other methods. Use it as a starting point and adjust based on your progress and goals.
