What is the science behind burning calories?

Burning calories is the process of converting the energy from food into energy that can be used by the body. Calories refer to the amount of energy in food, and burning calories refers to the body using that energy for things like breathing, digesting, regulating body temperature, and exercising. Understanding the science behind how the body burns calories can help people make informed choices about diet, exercise, and weight management.

How are calories measured?

A calorie is a unit of measurement for the amount of energy in food. Specifically, one calorie is the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. Food calories are actually kilocalories. One food calorie is equal to 1,000 calories (4.184 kilojoules).

The number of calories in a given amount of food represents the total potential energy contained in that food that is available for the body to use. The calories in food come from three main macronutrients:

  • Carbohydrates – 4 calories per gram
  • Protein – 4 calories per gram
  • Fat – 9 calories per gram

Nutrition labels provide information on the number of calories per serving of a food. This allows people to track how many calories they are consuming from the foods they eat.

How does the body use calories?

The body uses calories from food to fuel all of its functions. Here is an overview of the main ways the body uses calories:

  • Basal metabolism – This is the minimum number of calories needed to keep the body functioning if a person is at rest. It powers critical functions like breathing, blood circulation, nutrient processing, and cell repair.
  • Digestion – Digesting and absorbing nutrients from food requires calories. About 10% of daily calorie expenditure goes toward digestion.
  • Maintaining body temperature – The body needs calories to maintain a core temperature of 98.6°F (37°C). More calories are needed if external temperatures are very hot or cold.
  • Physical activity – Any movement of the body that uses the muscles requires calories. The intensity of the movement determines how many calories are needed.
  • Growth and repair – Calories are used to build new cells and tissues and maintain or repair existing cells.
  • Pregnancy and lactation – Additional calories are required to support fetal development and breast milk production.

If the body does not get enough calories to meet this energy demand, it will start burning stores of energy in muscle and fat tissue. Likewise, if the body receives more calories than needed, the excess is stored as fat.

What affects the number of calories burned?

Several factors influence the number of calories a person’s body burns each day:

Basal metabolic rate

Basal metabolic rate (BMR) is the number of calories required to perform the basic biological functions. It makes up about 50-70% of total daily calorie expenditure. BMR is determined by:

  • Age – Metabolism tends to slow as people age.
  • Sex – Men tend to have a higher BMR because they have more muscle mass.
  • Body size and composition – More calories are needed to maintain more body tissue. Muscle burns more calories than fat.
  • Genetics – Some people inherit a naturally faster or slower metabolism.

Physical activity level

Calories burned through activity and exercise make up 20-40% of total daily expenditure. Types of physical activity that burn calories include:

  • Aerobic exercise – Jogging, cycling, swimming.
  • Strength training – Weight lifting, resistance bands, pilates.
  • Everyday activities – Cleaning, gardening, climbing stairs.

The more strenuous and frequent the activity, the more calories are burned. Muscle mass and fitness level also impact activity-driven calorie expenditure.

Diet induced thermogenesis

The calories required to process and digest food make up about 10% of total expenditure. This varies based on the macronutrient composition of meals. Protein requires the most calories to digest, followed by carbohydrates and then fat.

Health conditions

Certain health conditions like hyperthyroidism, diabetes, and autoimmune disorders can accelerate metabolism. Others like hypothyroidism and rheumatoid arthritis may slow it down. Medications may also impact the metabolic rate.

External temperature

Colder environments require calories to maintain body heat. Hot weather and environments can accelerate water loss and prompt more calories to be burned as well.

How many calories should people aim to burn?

The number of calories that a person should aim to burn each day depends on their goals:

Weight maintenance – Burn the same number of calories as are consumed.

Weight loss – Burn more calories than are consumed through diet and increased activity. A calorie deficit of 500 per day can lead to about 1 pound of weight loss per week.

Weight gain – Burn fewer calories than are consumed through diet and less activity. A calorie surplus of 250-500 per day can lead to about 0.5-1 pound of weight gain per week.

Here are some general guidelines for calorie requirements based on age, sex, and activity level:

Age Sedentary Moderately Active Active
Child (2-8 years) 1000 – 1400 1000 – 1600 1000 – 1800
Child (9-13 years) 1200 – 1600 1400 – 1800 1600 – 2000
Teen (14-18 years) 1400 – 2000 1600 – 2200 1800 – 2400
Adult man (19-30 years) 1800 – 2600 2000 – 2800 2400 – 3000
Adult woman (19-30 years) 1600 – 2400 1800 – 2600 2000 – 2800
Adult man (31-50 years) 1600 – 2400 2000 – 2800 2200 – 3000
Adult woman (31-50 years) 1400 – 2200 1800 – 2400 2000 – 2800

However, calculating exact daily calorie needs can be tricky. Various online calculators and mobile apps are available to help estimate calorie requirements based on individual stats like height, weight, age, and activity level.

How does exercise impact calorie burning?

Physical activity and exercise are the most variable components of calorie expenditure. Exercise has the following impacts on calories burned:

  • Increases BMR – Exercise, especially strength training, builds muscle mass. This increases resting metabolism.
  • Burns calories directly – The body uses ATP and glycogen to fuel muscle movements. The intensity and duration determine how many calories are burned during the activity.
  • Increases excess post-exercise oxygen consumption (EPOC) – After exercise, the body enters recovery mode and continues to burn extra calories for hours or days. Higher intensity exercise results in more EPOC.
  • Triggers fat burning – Exercise helps trigger the release and burning of stored body fat for energy. This does not occur during sedentary behavior.

Here are estimates for calories burned per hour with various exercises:

Exercise Calories burned (hourly estimates)
Walking – 2 mph 204
Walking – 3.5 mph 280
Jogging – 5 mph 544
Running – 6 mph 661
Bicycling – 12-14 mph 544
Swimming laps – vigorous 612
Aerobic dance 423
Jumping rope 700
Weight lifting 211

In addition to the direct calorie burn during exercise, excess post-exercise oxygen consumption (EPOC) will lead to extra calories being burned for hours afterward. The exact amount depends on the workout intensity and duration.

What causes plateaus in calorie burning?

It is common for people to hit plateaus during weight loss journeys where it seems like calorie expenditure decreases and weight loss stalls. Some potential causes include:

Metabolic adaptation

As people lose weight, their BMR declines because their body mass has decreased. A decline of 20-25% in BMR can happen after substantial weight loss. The body is adapting to the new lower weight by needing fewer calories for basic functions.

Changes in NEAT

Non-exercise activity thermogenesis (NEAT) refers to calories burned through daily movement like standing, walking, and fidgeting. As people lose weight, everyday activities may become less strenuous. The body has become more efficient, so NEAT declines.

Loss of fat-free mass

Weight loss through diet alone often includes the loss of fat-free or muscle mass. With less metabolically active tissue, the body requires fewer calories to maintain itself.

Hormonal adaptations

Hormones like leptin and thyroid hormones play a role in metabolic rate. Drops or changes to circulating hormone levels during weight loss can therefore slow metabolism.

Inaccurate tracking

It is also possible that calorie expenditure is not actually decreasing, but inaccurate tracking is leading people to think they are burning fewer calories than expected. This underscores the importance of objective measurement.

Tips for boosting calorie burning

Some strategies to help boost calorie expenditure and break through a weight loss plateau include:

  • Increasing activity through steps, exercise, sports, or lifestyle activities
  • Building muscle mass through strength training
  • Eating more protein to help preserve muscle during weight loss
  • Trying high intensity interval training to maximize calorie burn
  • Drinking caffeine, green tea, or other thermogenic foods and beverages
  • Getting adequate sleep to support metabolic function
  • Managing stress, which can increase cortisol and slow metabolism
  • Being patient and keeping perspective, as plateaus are normal

However, if stalled weight loss lasts for many weeks or months, consulting a doctor or dietitian may be warranted to identify any underlying issues.


Calories provide the energy that powers all of the body’s functions. Burning calories refers to the body using this energy for basic survival, completing physical work, and powering the digestion and assimilation of foods. Calories in versus calories out determines weight maintenance or weight loss. Several factors like age, sex, body composition, health, and behaviors impact the number of calories a person burns each day. Understanding the science of calorie expenditure can empower people to make dietary and lifestyle choices that support their health and fitness goals.

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