BMR
BMR — Basal Metabolic Rate (BMR) is the minimum number of calories the human body requires to sustain vital functions while completely at rest, fasted, and in a thermoneutral environment. BMR represents the energy cost of organ function, cellular maintenance, and core body temperature, and accounts for 60-75% of total daily energy expenditure in most adults.
What is BMR?
Basal Metabolic Rate (BMR) is the energy required to keep a fully resting, fasted, awake human alive in a thermoneutral environment. It represents the calorie cost of breathing, circulation, neural activity, organ function (the liver alone consumes ~20% of BMR), thermoregulation, and cellular maintenance.
Strict BMR measurement requires:
- 12+ hours fasted
- Lying still, awake, undisturbed for 30+ minutes prior
- Thermoneutral room (~25°C / 77°F)
- Indirect calorimetry (measuring O₂ consumption and CO₂ production)
Because these conditions are difficult to replicate clinically, the closely related Resting Metabolic Rate (RMR) is more often measured in practice. RMR is typically 5-10% higher than true BMR.
How is BMR estimated?
The most widely used predictive equation is Mifflin-St Jeor (Mifflin et al., 1990):
Men: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5 Women: BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161
For a 35-year-old woman, 65 kg, 165 cm:
BMR = 650 + 1031 − 175 − 161 = 1,345 kcal/day
Mifflin-St Jeor has approximately ±10% error compared with measured RMR in adults of normal body composition. Older equations (Harris-Benedict, 1919) systematically overestimate BMR by 5-15% in modern populations and have largely fallen out of clinical use.
Why BMR matters
BMR is the largest component of TDEE and the foundation for most calorie target calculations. It is influenced by:
- Lean body mass — the strongest determinant; muscle tissue is more metabolically active than fat
- Sex — men have ~5-10% higher BMR than women of equal mass, mostly via lean mass differences
- Age — declines ~1-2% per decade in adulthood
- Genetics — substantial individual variation (±200 kcal at the same body composition)
- Thyroid status — hypothyroidism lowers BMR; hyperthyroidism raises it
For accurate weight management, equation-based BMR estimates should be refined by tracking actual weight change over 2-4 weeks. See RMR and NEAT.