What are the components of energy expenditure and factors that affect BMR?

January 3, 2026 •

5 min read

Over 60% of an average adult's total daily calorie burn is attributed to their Basal Metabolic Rate (BMR). Understanding what are the components of energy expenditure and factors that affect BMR is crucial for effective weight management and overall health.

Quick Summary

This article details the three main components of total energy expenditure: Basal Metabolic Rate (BMR), the thermic effect of food (TEF), and physical activity. It explores the various intrinsic and extrinsic factors that influence an individual's BMR, such as genetics, age, and body composition.

Key Points

Three Components of Energy Expenditure: Your body's total calorie burn comprises Basal Metabolic Rate (BMR), the Thermic Effect of Food (TEF), and Physical Activity Thermogenesis (PAT), which includes NEAT and EAT.
BMR is the Largest Component: The Basal Metabolic Rate accounts for the majority of your daily energy use, fueling essential bodily functions at rest.
Muscle Mass Affects BMR: Because muscle tissue is more metabolically active than fat, individuals with more lean muscle mass have a higher BMR.
BMR Decreases with Age: Starting around age 20, BMR typically declines by approximately 2% per decade, largely due to a reduction in muscle mass.
Hormones Regulate Metabolism: Thyroid hormones (T3 and T4) are critical regulators of BMR, with imbalances causing it to increase or decrease significantly.
Stress and Sleep Impact Metabolism: Chronic stress and poor sleep can elevate cortisol levels, negatively affecting metabolism and increasing abdominal fat storage.
Diet and TEF Vary by Macronutrient: The Thermic Effect of Food (TEF) is highest for protein, followed by carbohydrates, meaning your body uses more energy to process these foods.
Genetics Play a Role: Genetic predisposition influences your inherent metabolic rate and body composition, impacting how your body burns calories.

Total daily energy expenditure (TDEE) is the total number of calories a person burns over a 24-hour period. Rather than a single number, this value is a composite of several distinct metabolic processes. Understanding these components provides a comprehensive view of how the body uses energy, which is fundamental for any wellness goal, from weight loss to optimizing athletic performance.

The Three Components of Total Energy Expenditure

Total daily energy expenditure is primarily composed of three elements, though their relative contribution can vary significantly among individuals.

Basal Metabolic Rate (BMR): This is the largest and most foundational component, accounting for approximately 60-70% of total energy expenditure for most adults. BMR represents the energy your body requires to maintain essential physiological functions while at complete rest. These functions include breathing, blood circulation, cell production, and nutrient processing. BMR is measured under strict conditions, including being in a thermoneutral environment and having fasted for at least 12 hours.
Thermic Effect of Food (TEF): TEF, also known as diet-induced thermogenesis, is the energy expended to digest, absorb, and metabolize the food you eat. It typically accounts for about 10% of total daily energy expenditure and can be influenced by the type and amount of food consumed. The thermic effect of protein is significantly higher than that of carbohydrates and fats, meaning the body burns more calories processing protein-rich meals.
Physical Activity Thermogenesis (PAT): This is the most variable component of energy expenditure. It includes two sub-components: Non-Exercise Activity Thermogenesis (NEAT) and Exercise Activity Thermogenesis (EAT).
- NEAT: This accounts for the energy expended for everything we do that is not sleeping, eating, or sports-like exercise. Activities like walking to work, fidgeting, household chores, and even typing contribute to NEAT. The variance in NEAT can be as high as 2,000 kilocalories per day between two individuals of similar size, making it a critical factor for weight management.
- EAT: This is the energy burned during intentional exercise and planned physical exertion, such as going to the gym, running, or playing sports. The contribution of EAT to TDEE depends entirely on an individual's level of physical fitness and dedication to working out.

Factors that Affect BMR

While BMR is the energy burned at rest, it is not a fixed number. Many physiological and environmental factors can influence an individual's BMR. This is why BMR calculations online can only provide an estimate, as they do not account for every variable.

Intrinsic Factors Affecting BMR

Body Composition: This is the single most important determinant of BMR. Muscle tissue is far more metabolically active than fat tissue, meaning it burns more calories at rest. A person with a higher percentage of lean muscle mass will have a higher BMR than a person with a higher body fat percentage, even if they weigh the same.
Age: BMR decreases with age. This is primarily due to a gradual loss of muscle mass, a process known as sarcopenia, and changes in hormonal balance. After age 20, BMR may drop by 2% per decade.
Sex: Men generally have a higher BMR than women due to having a naturally greater muscle mass and lower body fat percentage. This difference is largely influenced by hormonal makeup, including higher testosterone levels in men which promote muscle growth.
Genetics: An individual's inherited genetic makeup plays a significant role in determining their metabolic rate. Some people are simply predisposed to have a faster or slower metabolism. Genetic factors also influence body composition and fat distribution.
Hormones: The endocrine system, particularly the thyroid gland, is a major regulator of BMR. Thyroid hormones (T3 and T4) control the speed of metabolic activity. An overactive thyroid (hyperthyroidism) can increase BMR, while an underactive thyroid (hypothyroidism) can cause it to decrease significantly. Hormones like insulin, cortisol, and leptin also play critical roles in regulating metabolism.
Illness and Fever: Sickness and fevers increase BMR because the body needs to work harder to fight off infection and build new tissues. For every 0.5°C increase in internal body temperature, BMR can increase by about 7%.

Extrinsic Factors Affecting BMR

Diet and Nutritional Status: Drastic calorie reduction, crash dieting, or starvation can force the body to slow down its metabolism to conserve energy, causing BMR to drop by up to 15%. The thermic effect of food is also influenced by macronutrient composition, with protein having the highest TEF.
Environmental Temperature: Extreme external temperatures, both hot and cold, cause the body to expend more energy to maintain its core temperature. In cold environments, the body works harder to produce heat, increasing BMR. In hot environments, energy is used to cool the body down. Modern climate control has reduced this effect for many people.
Sleep and Stress: Lack of sufficient or quality sleep can increase stress hormones like cortisol, which have a negative impact on metabolism and can increase fat storage. Chronic stress and sleep deprivation can disrupt hormonal balance, leading to metabolic dysfunction.

BMR vs. TDEE: A Comparison


Feature	Basal Metabolic Rate (BMR)	Total Daily Energy Expenditure (TDEE)
Definition	Minimum energy needed to maintain basic bodily functions at complete rest and fasting.	The total number of calories burned in a full 24-hour period.
Components	BMR is a single component within TDEE.	TDEE is the sum of BMR, TEF, and PAT (NEAT + EAT).
Measurement	Measured under strict, controlled conditions in a lab.	Calculated or estimated based on BMR and daily activity level.
Variation	Relatively fixed but influenced by age, sex, and body composition.	Highly variable, dependent on daily physical activity.
Contribution	The largest single contributor to TDEE (60-70%).	The overall total, with the relative proportion of its components changing with activity levels.

Conclusion

Total energy expenditure is a dynamic process influenced by several key components, most notably the basal metabolic rate. BMR itself is not static, as it is molded by a wide array of intrinsic and extrinsic factors, from genetics and age to diet and stress levels. Gaining a deeper understanding of these metabolic processes allows for more informed lifestyle choices. For those seeking better health and weight management, focusing on controllable factors like building lean muscle mass, consuming a balanced diet rich in protein, and managing stress and sleep can significantly influence metabolic function for the better. These small, consistent changes can lead to meaningful, long-term improvements in overall metabolic health.

Sources

Longevity.Technology: 7 Factors Influencing Your Basal Metabolic Rate
NCBI: Factors Affecting Energy Expenditure and Requirements
Better Health Channel: Metabolism
Everyday Health: How Many Calories Does a Fever Burn?
Cleveland Clinic: Non-Exercise Activity Thermogenesis Is NEAT!

Frequently Asked Questions

Basal Metabolic Rate (BMR) is the energy expenditure measured under very strict laboratory conditions: a full night's sleep, complete physical and mental rest, and a 12-hour fast. Resting Metabolic Rate (RMR) is measured under less restrictive, more common conditions, making it a more practical estimate. The two terms are often used interchangeably, but BMR is technically more precise and marginally lower than RMR.

Genetics can influence BMR by affecting an individual's inherent metabolic rate and their body composition, including the ratio of muscle to fat. Some people are genetically predisposed to have a faster or slower metabolism. Certain genetic disorders can also impact metabolic function.

Yes, you can increase your BMR by increasing your lean muscle mass through strength training and exercise. Muscle tissue is more metabolically active than fat tissue, so building more muscle will cause you to burn more calories at rest. Eating a high-protein diet can also temporarily increase metabolic rate via the thermic effect of food.

The thyroid gland produces hormones like T3 and T4, which are key regulators of the body's metabolic rate. An underactive thyroid (hypothyroidism) slows down metabolism and decreases BMR, while an overactive thyroid (hyperthyroidism) can speed it up. Thyroid imbalances can significantly affect an individual's weight and energy levels.

Exposure to extreme cold or heat can increase your BMR. In cold environments, the body expends extra energy to generate heat and maintain core temperature. In very hot conditions, the body works to cool itself down. However, modern climate control has reduced the metabolic impact of environmental temperature fluctuations.

Spicy foods, particularly those containing capsaicin (like chili peppers), can cause a temporary, small increase in metabolic rate. While the effect is not significant enough for major weight loss, it can contribute to a slightly higher energy expenditure. This effect is part of the thermic effect of food.

The portion of total energy expenditure from intentional exercise (EAT) can vary greatly, from a small percentage in sedentary individuals to a larger share in highly active people. For most people in industrialized countries, the cumulative calories from non-exercise activity (NEAT) contribute more to daily energy expenditure than planned exercise.