RMR
RMR — Resting Metabolic Rate (RMR) is the number of calories the body burns at rest, measured under standard but less stringent conditions than BMR. RMR includes minor energy costs of digestion, postural muscle tone, and recent activity, and is typically 5-10% higher than true BMR. Most clinical calorimetry measures RMR rather than BMR.
What is RMR?
Resting Metabolic Rate (RMR) is a clinically practical measure of resting energy expenditure. While BMR requires 12+ hour fasting and undisturbed pre-rest, RMR is typically measured after only 4-6 hours fasting, with the patient resting comfortably in a chair or supine for 15-30 minutes.
Because RMR conditions allow some residual digestion, postural muscle tone, and recent low-level activity, RMR values run 5-10% higher than true BMR. In most clinical and research contexts, “RMR” and “BMR” are used interchangeably, but careful sources distinguish them.
How is RMR measured?
RMR is most accurately measured by indirect calorimetry: a metabolic cart measures inhaled oxygen and exhaled carbon dioxide over 10-30 minutes. Energy expenditure is calculated from the Weir equation:
REE (kcal/day) = (3.94 × VO₂) + (1.11 × VCO₂) in L/min × 1440
Indirect calorimetry has typical accuracy of ±3-5% when performed under proper conditions. The Academy of Nutrition and Dietetics recommends measured RMR (rather than predicted BMR) for clinical populations including post-bariatric patients, athletes, post-weight-loss maintainers, and patients with suspected metabolic adaptation.
For consumer use, predictive equations such as Mifflin-St Jeor are more practical, with ±10% typical error.
Why RMR matters
RMR is the most metabolically meaningful resting expenditure measurement for clinical practice. It is the value:
- Bariatric surgery centers use to set post-operative calorie prescriptions
- Sports nutrition consultants use to calibrate athlete energy needs (especially for Relative Energy Deficiency in Sport screening)
- Some “metabolic testing” clinics measure for individualized calorie targets
The modest 5-10% difference between BMR and RMR rarely matters for general weight management, but it can matter for athletes near the edge of energy availability and for clinical populations with metabolic adaptation. See also TDEE for how RMR fits into total energy expenditure.