Resting Energy Expenditure
Published February 2026
Educational Note: This article explains scientific concepts about energy metabolism. It is not personalised advice. Consult a healthcare professional for guidance related to your individual situation.
Introduction
Resting energy expenditure (REE)—also called basal metabolic rate or BMR—is the amount of energy your body requires to maintain basic physiological functions at rest. This includes processes like breathing, circulation, cell maintenance, protein synthesis, and thermoregulation. Understanding the biological factors that determine REE provides insight into individual variation in energy requirements.
The Biological Components of REE
Several organs and systems account for the largest proportion of resting energy expenditure:
- Brain: Accounts for approximately 20% of REE despite being only 2% of body mass, due to its continuous neural activity and high metabolic demand.
- Heart: Continuously beats and maintains circulation, requiring significant energy.
- Kidneys and Liver: Perform continuous metabolic and detoxification functions.
- Skeletal Muscle: Even at rest, maintains cellular integrity and readiness for activity.
Factors That Influence REE
Body Composition
Lean muscle tissue is metabolically more active than adipose tissue. Two people of the same weight but different muscle-to-fat ratios will have different REE, with the person having greater muscle mass typically having higher REE.
Age
REE generally decreases with age due to natural changes in muscle mass and cellular metabolism. This is one reason energy requirements may change across the lifespan.
Sex
On average, males have higher REE than females, partly due to typically greater muscle mass. However, individual variation is substantial.
Genetics
Genetic factors influence how efficiently your body processes energy and maintains cellular functions. This contributes to natural variation in energy requirements between individuals.
Hormonal Status
Thyroid hormones significantly influence metabolic rate. Variations in thyroid function affect how much energy the body expends at rest. Hormonal changes due to stress, sleep, or reproductive factors also influence REE.
Environmental Temperature
Maintaining body temperature requires energy. Exposure to cold increases metabolic activity, whilst warm environments may reduce it slightly.
REE and Total Energy Expenditure
REE comprises approximately 60-75% of total daily energy expenditure in sedentary individuals. The remainder comes from activity-related energy expenditure and the thermic effect of food. Understanding REE helps contextualise how individual energy requirements are determined.
Measurement Considerations
REE can be measured through indirect calorimetry (measuring oxygen consumption) or estimated using predictive equations. Predictive equations provide reasonable estimates but may be less accurate than direct measurement, particularly for individuals with unusual body compositions or metabolic situations.
Conclusion
Resting energy expenditure is determined by complex interactions between biological systems, body composition, genetics, and environmental factors. Individual variation in REE is substantial and normal. This foundational component of total energy expenditure provides context for understanding how different factors influence metabolic function.