Resting energy expenditure (REE), also known as resting metabolic rate, represents the calories your body burns to maintain essential life-sustaining functions whilst at complete rest. Understanding what resting energy means is fundamental to metabolic health, as it accounts for 60–75% of your total daily energy expenditure. Even during sleep, your body continuously performs vital processes—maintaining your heartbeat, breathing, regulating temperature, and supporting cellular repair—all of which require energy. This article explores how resting energy works, the factors that influence your individual metabolic rate, and when professional assessment may be warranted.
Quick Answer: Resting energy expenditure is the amount of calories your body burns to maintain essential physiological functions whilst at complete rest, typically accounting for 60–75% of total daily energy expenditure.
Resting energy expenditure (REE), also known as resting metabolic rate (RMR), refers to the amount of energy your body requires to maintain essential physiological functions whilst at complete rest. This represents the calories your body burns to keep you alive when you are not engaged in any physical activity. REE accounts for the largest proportion of your total daily energy expenditure, typically representing 60–75% of the calories you burn each day.
The concept of resting energy is fundamental to understanding human metabolism and energy balance. Even when you are sleeping or lying completely still, your body continuously performs vital functions that require energy. These include maintaining your heartbeat, breathing, regulating body temperature, synthesising proteins, and supporting cellular repair and renewal. Your brain alone consumes approximately 20% of your resting energy, despite representing only about 2% of body weight.
REE differs slightly from basal metabolic rate (BMR), though the terms are often used interchangeably. BMR is measured under more stringent conditions—after an overnight fast, in a darkened room, and following complete rest. REE measurements are typically conducted following manufacturer or local protocol guidance, usually requiring an overnight fast (≥6–8 hours), avoidance of strenuous exercise for 24 hours, no caffeine or nicotine, and 20–30 minutes of quiet rest in a thermo-neutral environment.
Understanding your resting energy expenditure is clinically relevant for several reasons. It helps healthcare professionals develop appropriate nutritional plans, particularly for patients with metabolic disorders, obesity, or malnutrition. It also provides insight into metabolic health and can identify conditions where metabolism is abnormally high or low, such as hyperthyroidism or hypothyroidism.
Your body allocates resting energy across multiple organ systems and physiological processes, each with distinct energy demands. The distribution of energy consumption reflects the metabolic activity of different tissues, with some organs being far more energy-intensive than others despite their relatively small size.
Major energy-consuming processes include:
Cardiovascular function – Your heart beats approximately 100,000 times daily, pumping blood throughout your circulatory system. This continuous muscular work requires substantial energy to maintain blood pressure and ensure oxygen delivery to all tissues.
Respiratory function – The muscles involved in breathing, including the diaphragm and intercostal muscles, work constantly to facilitate gas exchange. This process becomes more energy-demanding during illness or in individuals with respiratory conditions.
Neurological activity – Your brain and nervous system maintain consciousness, regulate involuntary functions, process information, and coordinate bodily activities. Neural tissue is metabolically expensive, requiring glucose and oxygen continuously.
Renal function – The kidneys produce around 180 litres of filtrate from plasma each day via glomerular filtration, with substantial energy required for tubular reabsorption and regulatory functions.
Hepatic metabolism – The liver performs over 500 functions, including protein synthesis, detoxification, glucose regulation, and bile production. It is one of the most metabolically active organs.
Protein synthesis and cellular repair – Your body continuously breaks down and rebuilds proteins, repairs damaged DNA, and replaces old cells. This turnover is essential for maintaining tissue integrity.
Thermoregulation – Maintaining a core body temperature of approximately 37°C requires energy, particularly in cold environments or during fever when metabolic rate increases significantly.
The efficiency of these processes can be affected by various factors, including age, hormonal status, and underlying medical conditions. Certain medications, particularly those affecting thyroid function or metabolism, can also alter how efficiently your body uses resting energy.
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Start HereResting energy expenditure varies considerably between individuals due to multiple physiological, genetic, and environmental factors. Understanding these variables helps explain why two people of similar size may have different caloric requirements.
Body composition is the most significant determinant of REE. Lean body mass (muscle, organs, and bone) is metabolically active tissue that burns more calories at rest compared to adipose (fat) tissue. Individuals with greater muscle mass typically have higher resting energy expenditure. This explains why men generally have higher REE than women of similar weight, as they typically possess more lean muscle mass.
Age significantly influences metabolic rate. REE typically peaks during childhood and adolescence when growth demands are high, then gradually declines with age. This decline is primarily attributed to loss of lean muscle mass (sarcopenia) and changes in hormonal status. Older adults may require fewer calories to maintain their weight, though individual variation is substantial.
Sex affects resting energy needs, with men typically having 5–10% higher REE than women of equivalent weight and age. This difference is largely explained by body composition differences, with men generally having more lean tissue and less body fat. Hormonal differences, particularly testosterone levels, also contribute to this variation.
Genetic factors account for a significant portion of the variation in REE between individuals. Some people are genetically predisposed to have faster or slower metabolic rates, independent of other factors. Family studies have demonstrated that metabolic rate tends to cluster within families.
Hormonal status profoundly affects energy expenditure. Thyroid hormones are primary regulators of metabolic rate:
Hyperthyroidism (overactive thyroid) increases REE, often causing unintentional weight loss, heat intolerance, and increased appetite.
Hypothyroidism (underactive thyroid) decreases REE, contributing to weight gain, cold intolerance, and fatigue.
Other hormones including cortisol, growth hormone, and reproductive hormones also influence metabolic rate. Women experience fluctuations in REE throughout the menstrual cycle, with slight increases during the luteal phase.
Pregnancy and lactation increase energy requirements. REE rises progressively during pregnancy, with NHS guidance suggesting an additional 200 kilocalories (kcal) daily needed only in the third trimester. Breastfeeding requires approximately 330–400 additional kcal daily for exclusive breastfeeding, though individual needs vary.
Environmental temperature affects energy expenditure through thermoregulation. Exposure to cold environments increases REE as the body generates heat through shivering and non-shivering thermogenesis. Conversely, very hot environments may slightly increase metabolic rate due to increased cardiovascular work and sweating.
Nutritional status influences metabolic rate. Prolonged caloric restriction or starvation decreases REE as an adaptive mechanism to conserve energy—a phenomenon known as metabolic adaptation. Conversely, overfeeding temporarily increases energy expenditure through the thermic effect of food.
Medical conditions can significantly alter resting energy needs. Conditions associated with increased REE include:
Fever (increases metabolic rate per degree Celsius above normal)
Cancer, particularly advanced disease
Chronic obstructive pulmonary disease (COPD)
Heart failure
Burns and major trauma
Inflammatory conditions such as rheumatoid arthritis
Medications may affect metabolic rate. The effects are drug- and dose-specific: beta-blockers may reduce REE slightly, whilst stimulant medications may increase it. Thyroid replacement therapy normalises metabolic rate in hypothyroid patients.
Unexplained unintentional weight loss (>5% over 6–12 months) should prompt a review with your GP, as this may indicate an underlying condition affecting your metabolism.
Several methods exist for measuring or estimating resting energy expenditure, ranging from simple predictive equations to sophisticated laboratory techniques. The choice of method depends on the clinical context, required accuracy, and available resources.
Indirect calorimetry is considered the gold standard for measuring REE in clinical practice. This technique measures oxygen consumption and carbon dioxide production whilst the patient rests quietly. From these gas exchange measurements, energy expenditure is calculated using established equations. The procedure typically requires the patient to lie still for 20–30 minutes whilst breathing through a mask or ventilated hood. Indirect calorimetry is available in specialist metabolic units and some hospital settings. It provides accurate, individualised measurements and is particularly valuable for patients with complex metabolic needs, such as those in intensive care, with eating disorders, or requiring specialised nutritional support.
Predictive equations offer a practical alternative when direct measurement is unavailable. Several validated formulas estimate REE based on readily available parameters:
The Harris-Benedict equation (revised in 1984) uses weight, height, age, and sex to predict REE. Whilst widely used, it may overestimate energy needs in obese individuals.
The Schofield equation is commonly used in UK clinical practice, with separate formulas for different age groups.
The Henry (2005) equation is widely used in UK clinical practice and was developed using a more diverse population sample.
In the UK, Henry (2005) and Schofield equations are commonly used; choice depends on population and clinical context. Indirect calorimetry is preferred where available, as recommended in NICE Clinical Guideline 32 on nutrition support in adults.
These equations provide reasonable estimates for healthy individuals but may be less accurate in certain populations, including the very obese, elderly, or those with metabolic disorders. Predicted values typically have a margin of error of ±10–15%.
Bioelectrical impedance analysis (BIA) devices estimate body composition by passing a small electrical current through the body. Some advanced BIA devices incorporate REE estimation based on lean body mass calculations. Whilst convenient and non-invasive, accuracy varies considerably between devices and may be affected by hydration status.
When to seek professional assessment:
Consider consulting your GP or a registered dietitian if you:
Experience unexplained weight changes despite consistent eating habits (particularly unintentional weight loss >5% over 6–12 months)
Have symptoms suggesting thyroid dysfunction (fatigue, temperature intolerance, changes in appetite)
Require specialised nutritional support due to medical conditions
Are planning significant dietary changes and want personalised guidance
Have concerns about metabolic health or energy balance
Your healthcare provider can arrange appropriate investigations, including thyroid function tests if indicated, and refer you to specialist metabolic or nutrition services when necessary. Understanding your resting energy expenditure can inform evidence-based approaches to weight management, nutritional support, and overall metabolic health.
Whilst often used interchangeably, basal metabolic rate (BMR) is measured under more stringent conditions—after overnight fasting, in a darkened room, following complete rest—whereas resting energy expenditure (REE) is measured under less restrictive resting conditions. Both represent the energy required for essential physiological functions at rest.
Resting energy expenditure typically peaks during childhood and adolescence when growth demands are high, then gradually declines with age. This decline is primarily due to loss of lean muscle mass (sarcopenia) and hormonal changes, meaning older adults generally require fewer calories to maintain their weight.
Consult your GP if you experience unexplained unintentional weight loss exceeding 5% over 6–12 months, symptoms suggesting thyroid dysfunction (fatigue, temperature intolerance, appetite changes), or unexplained metabolic changes despite consistent eating habits. Your GP can arrange appropriate investigations including thyroid function tests and specialist referrals if necessary.
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