Basal metabolic rate.
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Here is a comprehensive overview of Basal Metabolic Rate (BMR).
Basal Metabolic Rate (BMR)
Definition
BMR is the minimum energy expenditure required to maintain life — the functioning of the lungs, kidneys, brain, heart, maintenance of ionic gradients across membranes, and the reactions of biochemical pathways — measured in a person who is:
- Mentally and bodily at rest
- In a thermoneutral environment
- 12–18 hours after the last meal
The resting metabolic rate (RMR), measured after awakening (rather than during sleep), is numerically very similar but slightly higher than BMR, and is more practical to measure clinically.
BMR as a Component of Daily Energy Expenditure
BMR accounts for 50–70% of daily energy expenditure in sedentary individuals. The three major components of total daily energy expenditure are:
- BMR / Resting energy expenditure (~60%) — sleeping + arousal cost
- Thermic effect of food (~8%) — digestion, absorption, and metabolism of nutrients
- Physical activity (~32%) — exercise and non-exercise activity thermogenesis
Standard Measurement Conditions
Strict criteria for measuring BMR (Guyton & Hall):
- No food for at least 12 hours
- After a night of restful sleep
- No strenuous activity for at least 1 hour before the test
- All psychic and physical sources of excitement eliminated
- Ambient temperature 68–80°F (20–27°C)
- No physical activity during the test
Oxygen utilization is the usual measure. BMR normally averages 65–70 Calories/hour in a 70-kg adult man.
Measurement Methods
| Method | Description |
|---|---|
| Direct calorimetry | Measures heat production directly |
| Indirect calorimetry | Measures O₂ consumption and CO₂ production; calculates respiratory quotient (RQ) |
| Simplified O₂ only | Measures O₂ consumption alone; less accurate but more practical |
| Predictive equations | Estimates from height, weight, age, sex — accurate to ±10% in ~90% of adults with BMI 18.5–45 |
Respiratory quotient (RQ): CO₂ produced / O₂ consumed
- Pure carbohydrate oxidation: RQ = 1.00
- Pure protein oxidation: RQ = 0.83
- Pure fat oxidation: RQ = 0.71
Estimating BMR: The Mifflin–St. Jeor Equation
The most widely validated predictive formula (Basic Medical Biochemistry, 6e):
| Sex | Formula |
|---|---|
| Males | (10 × W) + (6.25 × H) − (5 × A) + 5 |
| Females | (10 × W) + (6.25 × H) − (5 × A) − 161 |
W = weight in kg; H = height in cm; A = age in years; result in kcal/day
Quick estimate:
- Men: ~1 kcal/kg/hr (24 kcal/kg/day)
- Women: ~0.9 kcal/kg/hr (21.6 kcal/kg/day)
Factors Affecting BMR
| Factor | Effect |
|---|---|
| Lean/fat-free mass | ↑ muscle mass → ↑ BMR (muscle = 20–30% of BMR at rest) |
| Sex | Males > females (due to higher muscle, lower adipose %) |
| Age | BMR ↓ with age (muscle atrophy, fat deposition) |
| Thyroid hormones | Hyperthyroidism ↑ BMR 50–100%; hypothyroidism ↓ 40–50% |
| Testosterone | ↑ BMR ~10–15% (anabolic effect on muscle) |
| Growth hormone | ↑ BMR ~20% in GH-deficient adults on replacement |
| Fever | ↑ ~10–12% per °C rise in body temperature |
| Sleep | ↓ 10–15% (decreased muscle tone + reduced CNS activity) |
| Malnutrition | ↓ 20–30% (starvation adaptation) |
| Pregnancy/lactation | ↑ BMR |
| Cold environment | ↑ slightly (thermogenesis activation) |
| Genetics | Accounts for up to ~10% of inter-individual variation |
BMR vs. Age and Sex
Organ Contributions to BMR
Although skeletal muscle is the largest body mass contributor, at rest it accounts for only 20–30% of BMR. The major resting energy consumers are the brain, heart, kidneys, and liver. During maximal exercise, however, muscle can account for 80–90% of total energy expenditure.
Adipose tissue contributes very little — only ~3 kcal/kg of body fat per day. An exception is brown adipose tissue (BAT), which expresses uncoupling protein-1 (UCP-1), allowing mitochondrial proton leak that generates heat instead of ATP. While only present in small amounts in adults, it is a potential therapeutic target for obesity.
Clinical Relevance
- Caloric restriction can reduce BMR disproportionately to fat-free mass loss (mediated by ↓ T3 conversion from T4 and ↓ sympathetic drive) — a key mechanism of weight-loss plateau
- Overfeeding transiently raises BMR slightly above expected for lean mass
- BMR ranges from <1200 to >3000 kcal/day among sedentary adults; ~80% of this variability is explained by lean vs. fat tissue composition
- Clinical measurement: Portable indirect calorimetry is used in hospitalized patients requiring precise energy needs (e.g., ICU nutrition planning)
Sources: Guyton and Hall Textbook of Medical Physiology; Basic Medical Biochemistry: A Clinical Approach, 6e; Goldman-Cecil Medicine
Basal metabolic rate. Pleural Cavity & intrapleural pressure. total body water, avenues by which water leaves and enters body, types of body fluids general principles of fluid balance how body fluids maintain Homeostasis, Electrolytes & ions Function of electrolytes, how electrolyte imbalance leads to fluid imbalance. Different in each topics
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