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[Biochemistry](https://durgical.com/en/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

- [Obesity](https://durgical.com/en/conditions/obesity/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)
- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)
- [Kwashiorkor](https://durgical.com/en/conditions/kwashiorkor/)
- [Marasmus](https://durgical.com/en/conditions/marasmus/)

## Related

- [Specific dynamic action (thermic effect of food)](https://durgical.com/en/biochemistry/topics/specific-dynamic-action/)
- [Macronutrients and the energy value of food](https://durgical.com/en/biochemistry/topics/macronutrients-and-the-energy-value-of-food/)
- [Nitrogen balance](https://durgical.com/en/biochemistry/topics/nitrogen-balance/)
- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)
- [UCP1 (thermogenin)](https://durgical.com/en/biochemistry/enzymes/ucp1/)
- [Uncouplers of oxidative phosphorylation](https://durgical.com/en/biochemistry/topics/uncouplers-of-oxidative-phosphorylation/)
- [Fed, fasting and starvation states](https://durgical.com/en/biochemistry/topics/fed-fasting-and-starvation-states/)
- [Exercise metabolism](https://durgical.com/en/biochemistry/topics/exercise-metabolism/)
- [Leptin, appetite and obesity](https://durgical.com/en/biochemistry/topics/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.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Human energy requirements](https://www.fao.org/4/y5686e/y5686e00.htm) (Food and Agriculture Organization of the United Nations (FAO/WHO/UNU expert consultation), 2004); [Human energy requirements](https://www.fao.org/4/y5686e/y5686e07.htm) (Food and Agriculture Organization of the United Nations (FAO/WHO/UNU expert consultation), chapter 5: adults, 2004); [Metabolism: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/002257.htm) (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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