Physiology · Temperature, exercise and metabolism
Energy metabolism and basal metabolic rate !
- Measurement
- Exam priority: medium
The basal metabolic rate is the energy the body burns at complete rest just to stay alive: most of what it spends each day.
Updated 2026-11-01
Also called: BMR, Calorimetry, Respiratory quotient
See it in 3D: Thyroid gland
What it is
All the energy in food ends up as work or heat. The basal metabolic rate (BMR) is measured fasting, at rest, awake and at a thermoneutral temperature, and is ≈60–70% of a sedentary person's daily expenditure. It is estimated by indirect calorimetry: O₂ consumption and CO₂ production.
Key facts
- Energy per gram: carbohydrate and protein ≈4 kcal; fat ≈9 kcal; alcohol ≈7 kcal.
- BMR depends mostly on lean mass: muscle, liver, brain, heart and kidney are expensive tissues.
- Only ≈25–30% of ATP's chemical energy becomes muscle work; the rest is heat.
- An RQ >1 means net lipogenesis (carbohydrate overfeeding).
Hormone axes and feedback
Hypothalamic–pituitary–thyroid axisLesion None (normal)
Tap a box to see the hormone, or choose a lesion to see the lab pattern.
- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓
Equations
- VCO₂
- CO₂ produced
- VO₂
- O₂ consumed
In ketoacidosis or prolonged fasting it approaches 0.7 (fat is burned).
Approximate values (adult)
| Parameter | Value | Note |
|---|---|---|
| BMR | ≈1200–1800 kcal/day | by sex, age and lean mass |
| Resting O₂ consumption | ≈250 mL/min | |
| Caloric equivalent of O₂ | ≈4.8–5.0 kcal/L | |
| Thermic effect of food | ≈10% of expenditure | highest with protein |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Graves disease: high BMR, weight loss despite a big appetite, heat intolerance and tachycardia.
- Hypothyroidism: low BMR, modest weight gain, cold intolerance and bradycardia.
- Fever, sepsis, burns and trauma raise expenditure; nutritional support is calculated from these figures.
Test yourself
Think of the answer, then tap the question to see it.
Why can energy expenditure be estimated from O₂ consumption alone?
Each litre of O₂ releases ≈4.8–5 kcal whatever the substrate; the difference between fuels is small, so VO₂ predicts energy well.
A patient has a respiratory quotient of 0.71. What fuel is mainly being used?
Fat (fatty acids and ketones), as in prolonged fasting or ketoacidosis; carbohydrate would give ≈1.0.
Why can two people of the same weight have very different BMRs?
BMR tracks lean mass, not weight: more muscle and active organs burn more than adipose tissue. Thyroid status, age and sex also matter.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Body temperature regulation
- Energy balance, food intake and obesity
- Fed and fasting states: the hormonal switch
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 24.6 Energy and Heat Balance (OpenStax (Rice University), 2022); 24.7 Nutrition and Diet (OpenStax (Rice University), 2022); 24.5 Metabolic States of the Body (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 73). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.