Metabolic rate—the energy your body expends at rest and during activity—varies significantly across different population groups. Understanding which group has a higher metabolic rate is essential for tailoring nutritional advice, managing weight, and identifying metabolic disorders. Infants and young children exhibit the highest metabolic rates relative to body weight, driven by rapid growth and development. Men typically have higher absolute metabolic rates than women, primarily due to greater lean muscle mass. Age-related decline becomes apparent after 60, whilst various medical conditions can substantially alter energy expenditure. This article examines the key factors determining metabolic rate across age, sex, and clinical contexts.

What Is Metabolic Rate and Why Does It Matter?

Metabolic rate refers to the amount of energy your body expends to maintain basic physiological functions at rest and during activity. It is typically measured in kilocalories (kcal) or kilojoules (kJ) per day. The basal metabolic rate (BMR) represents the minimum energy required to sustain vital processes such as breathing, circulation, cell production, and temperature regulation when the body is at complete rest. The resting metabolic rate (RMR) is measured under less stringent conditions and is typically slightly higher than BMR, though the terms are sometimes used interchangeably in clinical practice.

Understanding metabolic rate is crucial for several reasons. It influences body weight regulation, nutritional requirements, and overall energy balance. Individuals with higher metabolic rates burn more calories at rest, which can affect their susceptibility to weight gain or loss. From a clinical perspective, metabolic rate assessment helps healthcare professionals tailor dietary advice, manage obesity, and identify underlying metabolic disorders.

Metabolic rate is not static—it varies significantly between individuals and population groups. Key determinants include:

• Body composition – lean muscle tissue is metabolically more active than adipose (fat) tissue

• Age – metabolic rate changes throughout the lifespan

• Sex – biological differences influence baseline energy expenditure

• Hormonal status – thyroid hormones, growth hormone, and sex hormones all modulate metabolism

• Genetic factors – inherited variations in metabolic efficiency

• Physical activity levels – regular exercise increases total energy expenditure, with modest effects on resting metabolism primarily through changes in lean body mass

Clinicians may assess metabolic rate through indirect calorimetry or use predictive equations such as the Henry equations (commonly used in UK practice), Harris-Benedict or Mifflin-St Jeor formulas. Accurate assessment supports evidence-based management of conditions ranging from malnutrition to metabolic syndrome, making it a cornerstone of personalised nutritional and medical care as outlined in NICE Clinical Guideline CG32 on nutrition support in adults.

Which Age Groups Have Higher Metabolic Rates?

Age is one of the most significant determinants of metabolic rate, with substantial variation across the lifespan. Infants and young children exhibit the highest metabolic rates relative to body weight. During the first year of life, rapid growth and development demand considerable energy, with BMR per kilogram of body weight approximately twice that of adults. This elevated metabolic activity supports cell division, organ maturation, and thermoregulation in a body with a high surface area-to-volume ratio.

Adolescents also demonstrate elevated metabolic rates, particularly during puberty when growth spurts occur. Hormonal changes, including surges in growth hormone, thyroid hormones, and sex steroids, drive increased energy expenditure. Boys typically experience a greater rise in metabolic rate than girls during this period, largely due to greater gains in lean muscle mass.

Recent research indicates that metabolic rate peaks in infancy, declines to adult levels by around age 20, and then remains relatively stable until approximately age 60, when adjusted for body composition. After age 60, a gradual decline becomes more apparent. This age-related change is multifactorial:

• Loss of lean muscle mass (sarcopenia) – muscle tissue is metabolically active, and its reduction lowers resting energy expenditure

• Hormonal changes – declining levels of growth hormone, thyroid hormones, and sex hormones

• Reduced physical activity – sedentary lifestyles become more common with age

• Changes in organ function – metabolically active organs such as the liver and kidneys may show reduced function

Older adults (aged 65 and above) typically have lower metabolic rates. However, maintaining muscle mass through resistance exercise and adequate protein intake can partially offset age-related metabolic decline. NICE guidance on nutrition support (CG32) emphasises the importance of assessing energy requirements and malnutrition risk in older adults to prevent malnutrition and support healthy ageing, with tools such as the BAPEN Malnutrition Universal Screening Tool (MUST) recommended for clinical practice.

How Gender Affects Metabolic Rate

Biological sex significantly influences metabolic rate, with men typically having higher absolute metabolic rates than women. This difference is primarily attributable to variations in body composition. Men generally possess greater lean muscle mass and lower body fat percentages compared to women of similar age and weight. Since muscle tissue is metabolically more active than adipose tissue—requiring more energy for maintenance even at rest—this compositional difference translates directly into higher energy expenditure in men.

On average, men's BMR is approximately 5–10% higher than women's when matched for age and body weight. When adjusted for lean body mass, however, this difference narrows considerably, suggesting that body composition rather than sex per se is the primary driver. Hormonal factors also contribute: testosterone promotes muscle protein synthesis and increases metabolic rate, whilst oestrogen influences fat distribution and storage patterns.

Key sex-related metabolic considerations include:

• Menstrual cycle effects – women may experience slight fluctuations in metabolic rate across the menstrual cycle, with a modest increase (approximately 2–10%, often around 3–5%) during the luteal phase due to elevated progesterone levels

• Pregnancy and lactation – metabolic rate increases during pregnancy (with NHS guidance recommending approximately 200 kcal/day extra in the third trimester) and lactation (with BDA guidance suggesting approximately 330–400 kcal/day additional intake) to support foetal development and milk production

• Menopause – the decline in oestrogen levels is associated with reduced metabolic rate and changes in body composition, including increased central adiposity

From a clinical perspective, these differences inform nutritional recommendations and weight management strategies. Healthcare professionals should account for sex-specific metabolic characteristics when calculating energy requirements, prescribing dietary interventions, or investigating unexplained weight changes. Importantly, individual variation within each sex is substantial, and personalised assessment remains essential for optimal care.

Medical Conditions That Influence Metabolic Rate

Numerous medical conditions can significantly alter metabolic rate, either increasing or decreasing energy expenditure. Thyroid disorders are among the most common endocrine causes of metabolic disturbance. Hyperthyroidism (overactive thyroid) elevates metabolic rate, sometimes dramatically, leading to symptoms such as unintentional weight loss, heat intolerance, tachycardia, and increased appetite. Conversely, hypothyroidism (underactive thyroid) reduces metabolic rate, resulting in weight gain, cold intolerance, fatigue, and bradycardia. NICE guideline NG145 recommends thyroid function testing (TSH, free T4) in patients presenting with unexplained metabolic changes.

Cushing's syndrome, characterised by excess cortisol production, can alter body composition through central fat deposition and muscle wasting, though the resting energy expenditure may be increased or unchanged due to the catabolic effects of hypercortisolism. Growth hormone deficiency in adults similarly alters body composition and can affect metabolic rate. Diabetes mellitus, particularly when poorly controlled, can affect metabolic rate through various mechanisms, including insulin resistance and altered substrate utilisation.

Conditions that increase metabolic rate include:

• Fever and infection – metabolic rate increases by approximately 10–13% for each degree Celsius rise in body temperature

• Cancer – certain malignancies, particularly advanced disease, elevate metabolic rate through tumour metabolism and inflammatory mediators

• Chronic obstructive pulmonary disease (COPD) – increased work of breathing raises energy expenditure

• Heart failure – compensatory mechanisms increase metabolic demands

• Burns and major trauma – hypermetabolic states can persist for weeks

Medications also influence metabolic rate. Beta-blockers may reduce it, whilst certain psychiatric medications (e.g., some antipsychotics) can alter metabolism and promote weight gain, as noted in their electronic Medicines Compendium (emc) Summaries of Product Characteristics. Patients should seek medical advice if experiencing unexplained weight changes (particularly rapid weight loss), persistent fatigue, new palpitations or irregular heartbeat, severe hyperglycaemia, or temperature intolerance. Appropriate investigations may include thyroid function tests, glucose monitoring, and assessment of adrenal function. Early identification and management of metabolic disorders can prevent complications and improve quality of life. Patients who suspect medication side effects should report them through the MHRA Yellow Card Scheme.

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