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[Physiology](https://durgical.com/en/physiology/) · [Temperature, exercise and metabolism](https://durgical.com/en/physiology/#s-integrative)

# 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](https://durgical.com/en/anatomy/thyroid-gland/)

## Key points

- Daily expenditure = BMR (≈60–70%) + thermic effect of food (≈10%) + physical activity (the most variable part).
- 1 L of O₂ consumed releases ≈4.8–5 kcal, nearly the same whatever the fuel.
- Respiratory quotient (CO₂/O₂): carbohydrate 1.0; protein ≈0.8; fat ≈0.7; mixed diet ≈0.8.
- BMR rises with thyroid hormone, muscle mass, fever, growth and sympathetic tone; it falls with age, fasting and hypothyroidism.

## 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 axis **Lesion** None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓

Thyroid hormone is the main regulator of basal metabolism: try a lesion and watch T₄ and TSH.

## Equations

**Respiratory quotient**RQ = VCO2 / VO2

**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.

## Memory aid

«4, 4, 9 and 7»: carbohydrate, protein, fat and alcohol, in kcal per gram.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/energy-metabolism-and-basal-metabolic-rate/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Thyroid gland](https://durgical.com/en/anatomy/thyroid-gland/)

## Related topics

- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)
- [Energy balance, food intake and obesity](https://durgical.com/en/physiology/energy-balance-food-intake-and-obesity/)
- [Fed and fasting states: the hormonal switch](https://durgical.com/en/physiology/fed-and-fasting-states-the-hormonal-switch/)

## Related conditions

- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Graves disease (hyperthyroidism)](https://durgical.com/en/conditions/graves-disease/)

## In Biochemistry

- [ATP (adenosine triphosphate)](https://durgical.com/en/biochemistry/molecules/atp/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-6-energy-and-heat-balance) (OpenStax (Rice University), 2022); [24.7 Nutrition and Diet](https://openstax.org/books/anatomy-and-physiology-2e/pages/24-7-nutrition-and-diet) (OpenStax (Rice University), 2022); [24.5 Metabolic States of the Body](https://openstax.org/books/anatomy-and-physiology-2e/pages/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.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/energy-metabolism-and-basal-metabolic-rate/