How the Human Body Uses Calories for Energy and Essential Functions

December 4, 2025 •

4 min read

The average person's body uses roughly 60-75% of its total daily calories just to maintain basic life-sustaining functions while at rest. Far from being simply fuel for exercise, these energy units are the foundation for every cellular process, from breathing and blood circulation to digestion and cell repair.

Quick Summary

Calories are the body's energy units, used for three main purposes: basal metabolism, the thermic effect of food, and physical activity. Most calories fuel fundamental functions like breathing and circulation, while others power digestion and all forms of movement.

Key Points

Basal Metabolic Rate (BMR): Your body's largest calorie expenditure, covering essential involuntary functions like breathing, circulation, and cell maintenance while at rest.
Thermic Effect of Food (TEF): The energy required to digest and process food, accounting for about 10% of daily calorie use. Protein has the highest thermic effect.
Physical Activity: The most variable component of calorie expenditure, including both structured exercise (EAT) and non-exercise movement (NEAT).
Cellular Respiration: The fundamental process where calories are converted into ATP, the body's main energy currency, to power all cellular functions.
Macronutrient Differences: Proteins and carbohydrates offer 4 calories per gram, while fats provide 9 calories per gram, making fat a more energy-dense fuel source.

The Three Pillars of Calorie Expenditure

Your body's total daily energy expenditure (TDEE) can be broken down into three primary components, each responsible for consuming the calories you ingest from carbohydrates, proteins, and fats.

1. Basal Metabolic Rate (BMR)

This is the largest portion of your daily calorie use, accounting for 60% to 75% of your total energy expenditure. Your BMR represents the energy your body requires to perform its most fundamental, life-sustaining functions while at complete rest. These include:

Circulation: Pumping blood throughout your body.
Respiration: The continuous process of breathing.
Cell Production and Repair: Building new cells and fixing damaged ones.
Nutrient Processing: Breaking down nutrients for use by the body.
Temperature Regulation: Maintaining a stable body temperature.

2. The Thermic Effect of Food (TEF)

Also known as diet-induced thermogenesis, TEF is the energy your body expends to digest, absorb, transport, and store the food you eat. This process accounts for approximately 10% of your total daily calorie burn. The amount of energy required to process food varies by macronutrient:

Protein: Has the highest thermic effect, using 20-30% of its calories for processing.
Carbohydrates: Have a moderate thermic effect, at 5-10%.
Fats: Have the lowest thermic effect, requiring only 0-3% of their calories for digestion.

3. Physical Activity

This is the most variable component of your daily energy expenditure. It includes all movement beyond basic resting functions, from structured exercise to daily activities. Physical activity can be further divided into:

Exercise Activity Thermogenesis (EAT): The calories burned during planned, structured exercise like running, weightlifting, or swimming.
Non-Exercise Activity Thermogenesis (NEAT): The energy expended for everything else that isn't eating, sleeping, or formal exercise. This includes walking, fidgeting, doing chores, and even typing. NEAT can account for a significant portion of daily calorie burn, ranging from 15-30% depending on a person's lifestyle.

A Comparison of Calorie Use Components


Feature	Basal Metabolic Rate (BMR)	Thermic Effect of Food (TEF)	Physical Activity (EAT + NEAT)
Proportion of TDEE	60-75% (Largest)	~10% (Fixed)	Highly Variable (Remaining %)
Function	Keeps vital organs and processes running at rest	Digests, absorbs, and metabolizes food	Powers all bodily movements
Primary Influencers	Age, gender, weight, muscle mass, genetics	Macronutrient composition of food	Activity level, duration, intensity, weight
Controllability	Mostly involuntary, though modifiable by muscle mass	Partly controllable via dietary choices	Highly controllable through conscious effort
Example Task	The energy needed for your heart to beat while you sleep	Processing a protein-rich meal	Walking to your car or a vigorous workout

How the Body Spends Energy at the Cellular Level

At its core, the use of calories is a cellular process known as cellular respiration. This metabolic pathway breaks down energy-rich molecules, primarily glucose, to produce a high-energy compound called adenosine triphosphate (ATP). ATP is the universal energy currency of the body, powering almost all cellular functions.

Glycolysis: Glucose is broken down in the cell's cytoplasm, producing a small amount of ATP and pyruvate.
Citric Acid Cycle (Krebs Cycle): Pyruvate enters the mitochondria and is further oxidized, producing more energy-carrying molecules.
Oxidative Phosphorylation: The majority of ATP is produced here. The energy from the previous steps is used to add a phosphate group to ADP, creating ATP.

Beyond simply providing energy, these metabolic intermediates are also used as building blocks for essential molecules like proteins, lipids, and nucleic acids. Thus, calories not only fuel action but also build and maintain the very structure of the body.

Conclusion

Understanding what the human body uses calories for provides crucial insight into weight management and overall health. It's a complex, multi-layered process, with the majority of calories fueling the involuntary, life-sustaining functions of your basal metabolic rate. A smaller, yet still significant, portion is dedicated to digesting and processing the food you eat. Finally, the calories burned through physical activity are the most variable and controllable, playing a key role in energy balance.

By appreciating this full picture, you can move beyond thinking of calories solely in terms of exercise. Optimizing your nutrient intake to support your BMR and choosing foods that have a higher thermic effect can be just as important as increasing your physical activity. The balance between calorie intake and expenditure is what ultimately determines weight and metabolic health, a balance influenced by many factors beyond just your workout routine. For a deeper dive into metabolic health, you might consult reputable resources like the National Institutes of Health for their extensive guides on nutrition.

Note: The energy requirements and metabolic rates can be influenced by various factors, including genetics, age, sex, and body composition. Consulting with a healthcare provider or a registered dietitian is always recommended for personalized dietary and health advice.

Frequently Asked Questions

The primary thing the human body uses calories for is its Basal Metabolic Rate (BMR), which powers the essential functions required to keep you alive, such as breathing, pumping blood, and cell production. This accounts for the majority of your daily energy expenditure.

While physical activity increases calorie usage significantly, the majority of calories are still used for your basal metabolic rate (BMR). Your body burns energy constantly to stay alive, even when you're completely still. A highly active person will use more calories overall, but BMR is the largest component for everyone.

Fats, proteins, and carbohydrates contain different amounts of energy and require different amounts of energy to process. Fats are the most calorie-dense (9 kcal/gram), while protein and carbs have fewer (4 kcal/gram). Your body also uses more energy to digest protein than it does fat, known as the thermic effect of food.

If you consistently consume more calories than your body uses for energy, the excess energy is stored. Most of this excess energy is stored as body fat for future use, which can lead to weight gain over time.

No, BMR is only one component of your overall metabolism. Metabolism is the entire chemical process of converting food into energy. BMR specifically refers to the minimum number of calories needed to sustain life at rest, while metabolism includes all three areas of calorie expenditure: BMR, TEF, and physical activity.

Yes, you can increase your calorie burn. While you can't dramatically alter your BMR, you can increase your physical activity (both structured exercise and NEAT), build more muscle mass (which raises BMR), and eat a balanced diet with adequate protein (which has a higher thermic effect).

While some people appear to have a 'fast metabolism,' studies show that overweight or obese individuals often have a higher metabolism because their bodies require more energy to function. Weight is influenced by genetics, hormones, and lifestyle, not just metabolism speed. The key is energy balance: consuming and burning a similar number of calories to maintain weight.