Biochemistry · Topic
Energy requirements and basal metabolic rate (BMR) !
The body spends energy to stay alive at rest (BMR), to handle food and to move. A person's energy requirement is the sum of these three parts.
Updated 2026-11-01
Key facts
- Total energy expenditure (TEE) = BMR (about 45–70 % in adults) + thermic effect of food (about 10 %) + physical activity (the most variable part).
- BMR is measured awake, lying at rest, after a 12-hour fast and in a comfortable room temperature; in an adult it is roughly 1 kcal per kg per hour.
- BMR rises with lean (muscle) mass, male sex, growth, pregnancy, fever, hyperthyroidism and catecholamines; it falls with age, starvation and hypothyroidism.
- Physical activity level (PAL) = TEE ÷ BMR: about 1.4–1.7 for sedentary or light lifestyles, 1.7–2.0 for active ones and up to about 2.4 for very heavy work.
- Energy requirement = TEE, plus the extra cost of growth in children and of pregnancy and lactation.
In detail
- 1. BMR is the cost of keeping the body running: ion pumps (Na⁺/K⁺-ATPase), protein turnover, heart and breathing muscles, liver, kidneys and brain (the brain alone uses about a fifth).
- 2. It is measured by indirect calorimetry: oxygen consumed and CO₂ produced. On a mixed diet each litre of O₂ corresponds to about 4.8 kcal.
- 3. In practice it is estimated with equations based on weight, height, age and sex (Harris–Benedict, Mifflin–St Jeor, or the Schofield equations used by FAO/WHO/UNU).
- 4. Classic textbooks express BMR per body surface area: about 35–40 kcal/m²/hour in adult men and a little less in women.
- 5. Fever raises BMR by roughly 10–13 % per °C; burns, sepsis and major trauma can raise it far more (hypermetabolism).
- 6. In starvation BMR falls as an adaptation: less thyroid hormone (T3), less sympathetic activity and less lean mass.
- 7. If intake exceeds expenditure, the surplus is stored mainly as fat; as an approximation, 1 kg of body fat holds about 7,700 kcal.
- Also called
- Basal metabolism, Total energy expenditure
Clinical notes
- In hyperthyroidism BMR rises: weight loss despite a good appetite, heat intolerance and tachycardia; in hypothyroidism it falls.
- Critically ill, burned or septic patients need their energy target estimated (or measured by indirect calorimetry) to avoid both underfeeding and overfeeding.
- After long fasting, BMR is low; restarting food too fast can cause refeeding syndrome (falls in phosphate, potassium and magnesium).
Related conditions
Related
- Specific dynamic action (thermic effect of food)
- Macronutrients and the energy value of food
- Nitrogen balance
- Na⁺/K⁺-ATPase (sodium pump)
- UCP1 (thermogenin)
- Uncouplers of oxidative phosphorylation
- Fed, fasting and starvation states
- Exercise metabolism
- Leptin, appetite and obesity
Test yourself
Think of the answer, then tap the question to see it.
Which component of total energy expenditure is the largest in a sedentary adult?
The basal metabolic rate, about 45–70 % of the total; the thermic effect of food is about 10 % and activity makes up the rest.
What conditions must be met to measure a true basal metabolic rate?
Awake, lying at rest, after about 12 hours of fasting and in a thermoneutral room, so that digestion, activity and cold do not add to it.
How does fever change the basal metabolic rate?
It raises it, roughly 10–13 % for each degree Celsius, because chemical reactions speed up with temperature.
What is the physical activity level (PAL)?
Total energy expenditure divided by BMR over 24 hours; about 1.4–1.7 in sedentary people and higher in active ones.
Why does BMR fall during prolonged starvation?
T3 and sympathetic tone fall and lean mass is lost, so the body spends less energy at rest: an adaptation to save fuel.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Human energy requirements (Food and Agriculture Organization of the United Nations (FAO/WHO/UNU expert consultation), 2004); Human energy requirements (Food and Agriculture Organization of the United Nations (FAO/WHO/UNU expert consultation), chapter 5: adults, 2004); Metabolism: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.