Energy Expenditure Overview

The body expends energy through basal metabolic rate, physical activity, and thermogenic processes. Understanding energy expenditure provides context for how total daily energy balance is determined.

Components of Total Daily Energy Expenditure

Total daily energy expenditure comprises three primary components: basal metabolic rate, activity energy expenditure, and thermic effect of food.

Basal Metabolic Rate (BMR)

Basal metabolic rate represents energy expenditure at complete rest under standardised conditions. It primarily reflects the energy costs of maintaining cellular function, protein synthesis, circulation, and nervous system activity. BMR is influenced by body composition (muscle tissue is more metabolically expensive than fat tissue), age, sex, genetics, and hormonal status. BMR typically comprises 60-75% of total daily energy expenditure in sedentary individuals.

Activity Energy Expenditure

Energy expended during structured exercise and daily movement (occupational activity, incidental movement) constitutes activity energy expenditure. This component is highly variable between individuals and within individuals across days, depending on activity frequency, intensity, and duration. In active individuals, activity energy expenditure can comprise 30-40% of total daily expenditure. In sedentary individuals, it may be only 15-20%.

Thermic Effect of Food (TEF)

The thermic effect of food represents energy expended during digestion, absorption, and processing of consumed food. It comprises approximately 10% of total daily energy expenditure in most diets. Protein has the highest thermic effect (20-30% of protein calories), carbohydrates moderate (5-10%), and fats the lowest (0-3%). This difference contributes to different metabolic effects of macronutrient-varied diets, though total energy intake remains the primary determinant of weight change.

Non-Exercise Activity Thermogenesis (NEAT)

NEAT represents energy expended for all activity outside structured exercise: occupational activity, fidgeting, maintaining posture, and daily movement. NEAT varies substantially between individuals—some people are naturally more active throughout the day, expending hundreds of additional calories compared to others with similar exercise patterns. NEAT is a substantial contributor to total daily energy expenditure and varies considerably with occupation and lifestyle.

Metabolic Adaptation

During prolonged energy restriction, metabolic rate decreases beyond what would be expected from weight loss alone. This adaptive thermogenesis reduces energy expenditure, partly through decreased sympathetic nervous system activity and hormone adjustments. The magnitude of metabolic adaptation varies between individuals, influenced by genetics and the severity of restriction. Upon return to normal eating, metabolic rate typically returns to baseline, though the time course varies.

Factors Influencing Energy Expenditure

Age decreases metabolic rate (approximately 2-8% per decade after age 30). Sex influences metabolic rate partly through differences in muscle mass distribution. Genetic variation in metabolic rate between individuals is substantial. Hormonal status (thyroid hormones, cortisol, growth hormone) significantly affects energy expenditure. Environmental temperature requires increased energy expenditure for thermoregulation. Fitness level and regular exercise training can increase metabolic rate.

Energy Balance and Body Weight

Body weight changes reflect the balance between energy intake and total daily energy expenditure over time. However, the relationship is not perfectly linear—many factors influence both intake and expenditure simultaneously. Individual variation in response to identical energy deficits is substantial, reflecting differences in metabolic adaptation and adherence patterns.

Important Context