What is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate (BMR) refers to the amount of energy your body needs to maintain basic functions while at complete rest in a neutral environment, when the digestive system is inactive. Think of it like the amount of fuel a parked, idling car consumes. In this state, the body uses energy only to support vital organs such as the heart, brain, kidneys, nervous system, intestines, liver, lungs, sex organs, muscles, and skin.
For most people, around 70% of their daily energy (calorie) expenditure comes from maintaining these essential functions. Physical activity accounts for about 20%, and roughly 10% is used for digesting food, a process known as thermogenesis.
How is BMR Measured?
BMR is measured under very controlled conditions, requiring complete rest with no physical or mental stress. It represents the largest portion of a person’s total daily caloric needs. To estimate your total daily calorie requirement, the BMR is multiplied by an activity factor ranging from 1.2 to 1.9, depending on your lifestyle and activity level.
Formulas Used to Calculate BMR
There are several well-established formulas to estimate BMR:
1. Mifflin-St Jeor Equation (Most Accurate for General Use):
- Men: BMR = 10W + 6.25H – 5A + 5
- Women: BMR = 10W + 6.25H – 5A – 161
2. Revised Harris-Benedict Equation:
- Men: BMR = 13.397W + 4.799H – 5.677A + 88.362
- Women: BMR = 9.247W + 3.098H – 4.330A + 447.593
3. Katch-McArdle Formula (Best for Those Who Know Their Body Fat Percentage):
- Formula: BMR = 370 + 21.6 × (1 – F) × W
Where:
- W = Weight in kilograms
- H = Height in centimeters
- A = Age in years
- F = Body fat percentage
Factors That Influence BMR
- Muscle Mass: Strength training and building muscle increase BMR, since muscle tissue burns more energy at rest than fat tissue.
- Age: BMR naturally decreases with age.
- Genetics: Your inherited traits play a significant role in determining your BMR.
- Weather: Colder or hotter environments can increase BMR as the body works harder to regulate its internal temperature.
- Diet: Eating small, frequent meals can slightly boost BMR, while long-term starvation can significantly lower it.
- Pregnancy: BMR increases during pregnancy to support the growing fetus.
- Supplements: Some supplements, like caffeine, can temporarily raise BMR.
Best Way to Test Your BMR
While online calculators provide a good estimate, for the most accurate results, it’s recommended to consult a health professional or use a medical-grade calorimetry device. These are often available in fitness centers, clinics, and weight-loss facilities.
BMR vs Resting Metabolic Rate (RMR)
Though often used interchangeably, there is a slight difference between BMR and RMR:
- BMR is the measurement of energy expenditure under complete rest and controlled conditions.
- RMR is the energy the body burns while at rest, but with less strict testing conditions.
As a result, RMR tends to be slightly higher than BMR.
How Accurate Are BMR Calculations?
A 2005 meta-analysis found that even after accounting for known factors like age, fat mass, and lean mass, about 26% of BMR variability between individuals remains unexplained. In simple terms, even with perfect calculations, BMR estimates are not 100% accurate.
Thus, while BMR provides a useful starting point for planning health and fitness goals, real-world results can vary. Keeping a daily journal of your exercise, diet, and other habits can help fine-tune your approach over time. Tracking and adjusting based on personal experience usually leads to the best results.