Metabolic rate is affected by many interacting factors, with body size and composition among the most important determinants of resting energy expenditure. Larger bodies generally require more energy for maintenance than smaller bodies, although the relationship is not perfectly proportional. Lean tissue, particularly organs and other metabolically active tissues, generally contributes more to resting energy expenditure per unit mass than adipose tissue.
This means that two people with identical body weight can have different resting metabolic rates if their proportions of lean mass and fat mass differ. Organ size and tissue composition also matter. This is why simple comparisons based only on body weight can provide incomplete explanations for differences in energy expenditure.
Age is associated with changes in energy expenditure, although age itself is not necessarily the sole cause. During childhood and adolescence, energy expenditure supports growth and development. In adulthood, resting energy expenditure can gradually change as body composition, organ function, physical activity and other physiological characteristics change. Older adults often have lower total energy expenditure partly because of reductions in lean mass and activity, but there is substantial individual variation. Sex is also associated with average differences in resting energy expenditure, largely because average body size and lean-mass distribution differ between males and females. These are population-level tendencies rather than precise predictions for an individual.
Physical activity can have a very large effect on total daily energy expenditure. Structured exercise such as running, cycling, swimming or resistance training directly increases energy use, while occupational movement and everyday activity contribute through non-exercise activity thermogenesis. The latter can vary considerably between individuals and can change unconsciously when activity levels or energy availability change. Someone who spends several hours standing and walking during work can therefore expend substantially more energy than someone who sits for most of the day, even if neither performs formal exercise. Physical activity also influences body composition over time, which can indirectly affect resting expenditure.
Food intake affects energy expenditure through the thermic effect of food. Digestion, absorption, transport, processing and storage of nutrients all require energy. Protein generally has a higher thermic effect than carbohydrate or fat, carbohydrate has an intermediate effect and fat has a lower thermic effect, although actual values vary with meal composition and individual circumstances. Total food intake also influences expenditure because prolonged energy restriction can produce adaptive reductions in expenditure, while increased intake can raise energy expenditure through greater food processing and other mechanisms. The thermic effect is therefore not a fixed percentage independent of what or how much a person eats.
Hormonal and medical factors can significantly alter metabolic rate. Thyroid hormones are particularly important because they influence oxygen consumption and energy expenditure across many tissues. Hyperthyroidism can increase resting energy expenditure, whereas hypothyroidism can reduce it, although the size of the effect varies and symptoms involve much more than metabolism alone. Fever and certain illnesses can increase energy requirements because the body is operating under altered physiological conditions. Some medications can affect appetite, activity, body weight or metabolic processes. Because unexplained substantial changes in body weight or energy expenditure can have medical causes, they should not automatically be attributed to a supposedly slow or fast metabolism.
Environmental temperature can affect energy expenditure because maintaining core body temperature requires physiological responses. Cold exposure can increase thermogenesis through shivering and non-shivering mechanisms, while heat can increase cardiovascular and thermoregulatory work. However, ordinary changes in environmental temperature generally have much smaller practical effects on daily calorie expenditure than sustained differences in physical activity or body composition. Sleep and circadian biology can also influence metabolic regulation. Sleep deprivation can alter appetite, glucose regulation and behaviour, and may affect energy expenditure, but the relationship is complex and cannot be reduced to a simple claim that sleeping more directly burns a particular number of calories.
Genetics contribute to individual variation in metabolic traits, including body composition, appetite regulation, spontaneous activity and resting energy expenditure. Genetic differences can affect enzymes, hormones, receptors, mitochondrial function and many other biological systems. However, genetic influence does not mean metabolic rate is predetermined and unchangeable. Environment, diet, physical activity, sleep, illness and body composition interact with genetic predisposition. Measurement error also complicates attempts to compare small metabolic differences between individuals. A reported difference in resting metabolic rate may reflect genuine biology, differences in body composition, measurement conditions or estimation error rather than a single inherited characteristic.
Metabolic rate is therefore best understood as a dynamic quantity rather than a permanent personal attribute. Weight loss can reduce expenditure because a lighter body costs less energy to move and maintain, and adaptive responses can further change expenditure. Muscle gain can modestly increase resting expenditure while also changing physical performance and activity capacity, although muscle does not burn enormous quantities of calories at rest.
Increasing everyday movement can have a substantial effect on total expenditure without necessarily changing resting metabolic rate. Consequently, when investigating why two people have different calorie requirements, it is more informative to consider body composition, activity, food intake, age, sex, physiological state and health together than to attribute everything to a single concept of metabolic speed.
