Health⏱ 5 min read
What Is Resting Metabolic Rate and How Is It Measured?
Resting Metabolic Rate (RMR) and Basal Metabolic Rate (BMR) are often used interchangeably but measure different things. Here is the distinction, how to estimate RMR, and what affects it.
RMR is your metabolic "floor" — the calories burned by your organs and body systems when you are at complete rest. It accounts for 60-75% of total daily energy expenditure for sedentary individuals.
RMR vs BMR: The Distinction
BMR (Basal Metabolic Rate):
Measured under strict laboratory conditions:
- Complete rest, supine (lying down)
- Thermoneutral environment (22-24°C)
- Fasted for 12+ hours
- Preceding day without strenuous exercise
- Measured in the morning on waking
RMR (Resting Metabolic Rate):
Less strict conditions — just resting quietly.
Typically 10-20% higher than BMR.
What most online calculators actually estimate.
Practical difference: for calorie target calculations,
RMR-based estimates are more useful because we rarely meet
true BMR conditions in daily life.
How Organs Contribute to RMR
Breakdown of resting energy expenditure by organ:
Liver: ~27% of RMR
Brain: ~19%
Skeletal muscle (at rest): ~18%
Heart: ~7%
Kidneys: ~10%
Other: ~19%
Key insight: Skeletal muscle at rest uses less energy than
the brain and liver combined. This is why building muscle
increases RMR less dramatically than often claimed —
each additional kg of muscle adds approximately 13-15 kcal/day at rest.
10kg of muscle gain: +130-150 kcal/day RMR increase
(over 12-18 months of serious resistance training)
Estimating RMR
Mifflin-St Jeor formula (most accurate for RMR):
Men: RMR = (10 x kg) + (6.25 x cm) - (5 x age) + 5
Women: RMR = (10 x kg) + (6.25 x cm) - (5 x age) - 161
Example: 40-year-old woman, 70kg, 165cm
RMR = (10 x 70) + (6.25 x 165) - (5 x 40) - 161
= 700 + 1031.25 - 200 - 161
= 1,370 kcal/day
This is the minimum calories this person burns daily
regardless of activity — the floor below which prolonged
restriction causes metabolic adaptation and muscle loss.
What Reduces RMR (Metabolic Adaptation)
Caloric restriction causes metabolic adaptation:
The body reduces energy expenditure beyond what fat/muscle
loss alone would predict.
In significant caloric restriction:
RMR can drop 10-20% below predicted values.
This "adaptive thermogenesis" is one reason diets plateau.
Factors that reduce RMR:
- Prolonged caloric restriction (-10-20%)
- Aging (approximately -1-2% per decade after 20)
- Hypothyroidism (significant effect — thyroid regulates RMR)
- Sleep deprivation (-5-20% in acute sleep deprivation)
- Muscle loss (each kg lost: -13-15 kcal/day)
Factors that increase RMR:
- Resistance training (muscle gain)
- Higher protein intake (higher thermic effect raises daily expenditure)
- Cold exposure (modest effect — brown fat activation)
- Hyperthyroidism