Factors That Affect How Many Calories You Use

October 12, 2025 •

3 min read

Genetics can account for up to 80% of the difference in an individual's metabolic rate, demonstrating that calorie usage is not solely dependent on conscious behaviors like diet and exercise. Many interconnected factors affect how many calories you use, impacting your overall energy balance and weight management goals.

Quick Summary

Calorie usage depends on a combination of factors, including your basal metabolic rate, physical activity level, and the thermic effect of food. Age, sex, body composition, genetics, and hormonal changes also play significant roles in determining your daily energy expenditure. Understanding these influences is crucial for effective weight management.

Key Points

Basal Metabolic Rate (BMR): BMR accounts for the majority of your daily calorie use, representing the energy needed for basic bodily functions at rest.
Physical Activity Matters: All movement, including formal exercise and subconscious fidgeting (NEAT), contributes to your total daily calorie burn.
Age and Metabolism: Metabolism doesn't drastically slow down in middle age, with a more noticeable, yet gradual, decline occurring after age 60, largely due to muscle mass loss.
Body Composition's Role: Individuals with higher lean muscle mass have a higher BMR, as muscle tissue burns more calories at rest than fat tissue.
Hormonal Influence: Hormones like insulin, cortisol, and leptin regulate metabolism, appetite, and fat storage, with imbalances potentially impacting energy expenditure.
The Thermic Effect of Food (TEF): Digesting food requires energy, and consuming protein has a higher thermic effect than consuming fat or carbohydrates.

The Components of Total Daily Energy Expenditure (TDEE)

Your total daily energy expenditure (TDEE) is the number of calories your body burns in a 24-hour period. TDEE is comprised of three main components: your basal metabolic rate (BMR), the thermic effect of food (TEF), and your physical activity level (PAL). Understanding each component is key to knowing what factors affect how many calories you use.

Basal Metabolic Rate (BMR)

Your BMR accounts for the largest portion of your TDEE, often comprising 60–75% of your daily calorie expenditure. It represents the energy your body needs to perform essential, life-sustaining functions at rest, such as breathing, blood circulation, and cell production. Unlike common misconceptions, BMR does not drastically slow down in middle age. A 2021 study in Science found that metabolism remains relatively stable between the ages of 20 and 60, with a more gradual decline of about 0.7% per year after age 60.

Thermic Effect of Food (TEF)

TEF is the energy your body uses to digest, absorb, and metabolize the food you eat. It accounts for approximately 10% of your daily energy expenditure and varies depending on the macronutrient composition of your meals. For instance, protein has a higher thermic effect than fat, meaning your body burns more calories processing protein-rich foods.

Physical Activity Level (PAL)

This component encompasses all movement, from structured exercise to subconscious fidgeting. It is often the most variable part of your TDEE. PAL includes:

Exercise-related activity: Intentional physical exertion, such as running, weightlifting, or swimming.
Non-exercise activity thermogenesis (NEAT): Energy expended from spontaneous daily activities that are not formal exercise, like walking, gardening, or standing. Studies have shown that NEAT can vary by up to 2,000 calories per day between individuals of a similar size, significantly impacting TDEE.

Influences on Your Metabolic Rate

Several uncontrollable and controllable factors contribute to your metabolic rate, shaping your overall energy needs.

Uncontrollable Factors

Age: While the decline isn't as steep as once thought, age affects muscle mass and hormonal balance, which can influence BMR. After 60, the average metabolic rate declines gradually.
Genetics: Your genes play a powerful role, with studies showing significant genetic components influencing resting metabolic rate. This can explain why some people can eat more without gaining weight than others.
Sex: Men typically have a higher metabolic rate than women of the same age and weight. This is largely due to men's higher muscle mass and lower body fat percentage.

Controllable Factors

Body Composition: The ratio of lean muscle mass to fat mass is a major determinant of BMR, as muscle tissue burns more calories at rest than fat tissue. Increasing muscle mass through strength training is one of the most effective ways to increase your BMR.
Hormones: Hormones like insulin, cortisol, and thyroid hormones regulate appetite and metabolism. Chronic stress, for example, can lead to elevated cortisol, which may slow metabolism and increase fat storage.
Dietary Choices: What you eat impacts your TEF. A diet rich in protein, for example, can increase thermogenesis, helping you burn more calories during digestion. Avoiding crash diets is also crucial, as severe calorie restriction can cause metabolic slowdown as the body tries to conserve energy.

Comparison of Calorie Expenditure Factors


Factor	Primary Function	Impact on Calorie Use	Variability	Influence	Control
BMR	Energy for resting functions	Highest daily contribution (60-75%)	Low-moderate (influenced by age, muscle mass)	High	Limited (can increase via muscle mass)
Physical Activity	Energy for movement	Variable daily contribution	High (depending on lifestyle)	High	Controllable
Thermic Effect of Food (TEF)	Energy for digestion	Relatively small contribution (~10%)	Low-moderate (influenced by food choices)	Moderate	Controllable
Genetics	Sets metabolic predisposition	Variable baseline metabolic rate	Fixed	High	Uncontrollable
Hormones	Regulates appetite & metabolism	Influences hunger and expenditure	High (influenced by diet, stress, sleep)	High	Controllable (via lifestyle choices)
Age	Influences muscle mass and hormones	Gradual decline over time	Fixed	High	Uncontrollable

Conclusion

While diet and exercise are cornerstones of weight management, a multitude of factors affect how many calories you use. Your body's baseline energy needs, or BMR, are influenced by elements like age, sex, and genetics, while your day-to-day expenditure is determined by your physical activity and the food you consume. Recognizing these diverse and interconnected variables is essential for developing a holistic and effective strategy for managing your energy balance. By focusing on optimizing what you can control, such as building lean muscle and managing stress, and understanding what you cannot, like your genetics, you can build a more sustainable and successful path toward your health and wellness goals.

Frequently Asked Questions

The single most important factor is your Basal Metabolic Rate (BMR), which accounts for the energy your body needs just to exist at rest. Your BMR is influenced by age, sex, and body composition.

Yes, but not as dramatically as often believed. Research indicates that metabolism remains stable between the ages of 20 and 60, with a more gradual decline starting after 60, primarily linked to a decrease in muscle mass.

Muscle tissue is more metabolically active than fat tissue, meaning it burns more calories at rest. Therefore, a higher percentage of lean muscle mass increases your BMR and overall calorie usage.

While acute stress can temporarily increase energy expenditure, chronic stress can have the opposite effect. Chronically elevated cortisol can slow metabolism due to muscle loss, potentially leading to increased appetite and fat storage.

The thermic effect of food (TEF) is the energy used for digestion. Foods high in protein require more energy to process than fats or carbohydrates, meaning your dietary choices can slightly influence your calorie expenditure.

NEAT stands for Non-Exercise Activity Thermogenesis, which is the energy burned from all movement outside of formal exercise, such as walking, cleaning, or fidgeting. NEAT can account for a significant portion of your daily calorie expenditure.

Yes, on average. Due to differences in body composition, men typically have a higher muscle mass and lower body fat percentage, which results in a higher BMR compared to women of the same weight and age.