Resting energy expenditure (REE), also known as resting metabolic rate, represents the energy your body requires to maintain essential physiological functions whilst at complete rest. This baseline energy demand accounts for 60–75% of your total daily energy use, supporting vital processes such as breathing, circulation, cellular metabolism, and organ function. Understanding REE is clinically important for nutritional planning, weight management, and identifying metabolic disorders. Healthcare professionals use REE measurements to tailor dietary recommendations for patients with metabolic conditions, those recovering from illness, and individuals managing their weight. This article explains what resting energy is, how your body uses it, and the factors that influence your metabolic rate.

What Is Resting 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 energy your body uses to keep vital systems operating when you are awake but inactive—typically measured after several hours of fasting and in a thermoneutral environment. REE accounts for the largest proportion of total daily energy expenditure in most individuals, typically representing 60–75% of total energy used each day.

The concept differs slightly from basal metabolic rate (BMR), though the terms are often used interchangeably in clinical practice. BMR is measured under more stringent conditions—immediately upon waking, after 12–14 hours of fasting, and in a completely relaxed state. REE measurements are less restrictive and more practical for clinical assessment, requiring only that the individual be rested and fasted for 4–6 hours beforehand.

Understanding your resting energy expenditure is clinically relevant for several reasons. Healthcare professionals use REE calculations to develop appropriate nutritional plans for patients with metabolic disorders, those recovering from illness or surgery, and individuals managing their weight. In hospital settings, accurate REE estimation helps determine appropriate energy intake for patients receiving nutritional support, whether oral, enteral, or parenteral. For the general population, knowing your REE provides insight into your baseline energy requirements and can inform dietary choices and weight management strategies. The measurement serves as a foundation for calculating total daily energy expenditure when combined with activity levels and the thermic effect of food.

How Your Body Uses Resting Energy

Your body continuously expends energy to maintain life-sustaining processes, even during complete rest. The majority of resting energy supports cellular metabolism—the biochemical reactions occurring in every cell throughout your body. These reactions include protein synthesis, DNA replication and repair, ion transport across cell membranes, and the production of adenosine triphosphate (ATP), the primary energy currency of cells. These fundamental processes operate constantly, regardless of physical activity.

Specific organ systems consume varying amounts of resting energy based on their metabolic activity. The brain is remarkably energy-intensive, accounting for approximately 20% of REE despite representing only 2% of body weight. This high demand reflects the constant electrical activity required for neurological function, neurotransmitter synthesis, and maintenance of ion gradients across neuronal membranes. The liver similarly demands substantial energy (approximately 20% of REE) due to its role in glucose metabolism, protein synthesis, detoxification processes, and bile production.

The heart and kidneys together account for another 15–20% of resting energy expenditure. The heart requires continuous energy to maintain cardiac muscle contraction and circulation, whilst the kidneys expend energy filtering blood, reabsorbing nutrients, and maintaining fluid and electrolyte balance. Skeletal muscle contributes significantly to REE as well, accounting for approximately 20–30% of resting energy use. Even at rest, muscles maintain tone, support posture, and undergo protein turnover.

Additionally, resting energy supports thermogenesis—the production of body heat necessary to maintain core temperature at approximately 37°C. The respiratory system, gastrointestinal tract, and endocrine organs also require energy for their baseline functions. This distribution of energy use varies with body composition and can be altered in certain disease states, but explains why REE remains relatively stable from day to day, as these essential functions continue regardless of your activity level or dietary intake.

Factors That Affect Resting Energy Expenditure

Numerous physiological and demographic factors influence an individual's resting energy expenditure, creating substantial variation between people. Body composition is perhaps the most significant determinant—lean body mass (muscle, organs, bone) is metabolically active tissue that requires considerable energy to maintain, whilst adipose (fat) tissue is relatively metabolically inert. Consequently, individuals with greater muscle mass typically have higher REE values. This explains why men generally have higher REE than women of similar weight, as men typically possess proportionally more lean tissue.

Age significantly impacts resting energy expenditure. REE is highest during infancy and childhood when rapid growth and development occur. Following peak values in early adulthood, REE gradually declines with advancing age—approximately 1–2% per decade after age 30. This decline primarily reflects the progressive loss of lean body mass (sarcopenia) that accompanies ageing, alongside reductions in organ metabolic activity. This age-related decrease in REE contributes to weight gain in older adults who maintain previous dietary habits without adjusting energy intake.

Hormonal status profoundly affects metabolic rate. Thyroid hormones (thyroxine and triiodothyronine) are primary regulators of metabolism—hyperthyroidism increases REE substantially, whilst hypothyroidism reduces it. Growth hormone, cortisol, and sex hormones (oestrogen, testosterone) also influence energy expenditure. Women experience cyclical variations in REE throughout the menstrual cycle, with modest increases during the luteal phase. Pregnancy and lactation significantly elevate energy requirements to support foetal development and milk production.

Genetic factors account for approximately 20–30% of inter-individual variation in REE, independent of body composition. Environmental temperature affects REE as well—exposure to cold increases energy expenditure through shivering and non-shivering thermogenesis. Certain medical conditions (fever, infection, trauma, burns) and medications (beta-blockers, stimulants) can alter metabolic rate. Severe energy restriction triggers metabolic adaptation, reducing REE as a protective mechanism against starvation—a phenomenon that complicates long-term weight management.

If you experience unexplained weight loss (5% or more of your body weight within 3–6 months) or weight gain, alongside symptoms such as fatigue, heat or cold intolerance, or palpitations, consult your GP as these may indicate an underlying metabolic disorder.

Measuring Your Resting Energy Expenditure

Several methods exist for determining resting energy expenditure, ranging from direct measurement to predictive equations. Indirect calorimetry represents the gold standard for REE measurement in clinical practice. This technique measures oxygen consumption (VO₂) and carbon dioxide production (VCO₂) whilst the individual rests quietly, typically for 20–30 minutes. These gas exchange measurements allow calculation of energy expenditure based on established metabolic equations. Indirect calorimetry requires specialised equipment (metabolic cart) and trained personnel, making it primarily available in hospital metabolic laboratories, research facilities, and some specialist clinics. The test requires fasting for at least 4–6 hours beforehand, avoiding caffeine, nicotine, and strenuous exercise for 24 hours prior.

Predictive equations offer a practical alternative for estimating REE without specialised equipment. The Henry (Oxford) equations are widely used in UK clinical practice, particularly by dietitians. The Schofield equations are commonly used for children and historical comparisons in the UK. The Mifflin-St Jeor equation, developed in 1990, is also used in UK practice and may be more accurate for some contemporary adult populations. These equations provide reasonable estimates for healthy individuals but may be less accurate for those with obesity, significant muscle wasting, or metabolic disorders. In critical illness, indirect calorimetry is preferred when available, as predictive equations are less reliable in these complex cases.

For individuals interested in understanding their resting energy expenditure, consulting a registered dietitian or healthcare professional is advisable. They can determine whether formal measurement is warranted or whether predictive equations suffice for your circumstances. Some fitness facilities and private clinics offer indirect calorimetry testing, though accuracy depends on proper equipment calibration and testing protocols.

It is important to recognise that REE represents only baseline energy needs. Total daily energy expenditure also includes the thermic effect of food (energy required for digestion and nutrient processing, approximately 10% of intake) and physical activity energy expenditure, which varies considerably based on lifestyle. Healthcare professionals can help interpret REE measurements within the context of your overall health goals, whether managing weight, optimising athletic performance, or addressing medical conditions affecting metabolism. If you have concerns about unexplained weight changes, fatigue, or suspect a metabolic disorder, contact your GP for appropriate assessment and investigation.

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