Do You Know How Your Body Burns Calories?

The 2 Week Diet
Metabolism and Energy Needs

Metabolism is the group of biochemical processes that occur within an organism in order to maintain life.  However, the term is commonly used to refer to the breakdown of food and its transformation into energy. But how does food become energy?

Energy Intake

It is common sense that the human body needs some kind of fuel in order the work properly. This fuel is obtained from food sources. Everything we eat and drink is broken down into to smaller units that are absorbed by the digestive system, and then, released into the bloodstream. Travelling through blood vessels, these small units are distributed to all body cells where they will be used to create energy.

The body can use carbohydrates, fats and proteins from the diet to produce energy. However, during exercise, the primary nutrients used for energy are carbohydrates and fats; protein contribution for energy is small. This energy is measured in kilocalories (kcal), or just calories[1].

For example, the breakdown of one gram of carbohydrate releases four calories. Proteins, fats, and alcohol are also energy sources; vitamins and minerals are non-caloric nutrients. See the table below for detailed information.

Nutrients Energy yielded
Carbohydrates 4 kcal/g
Proteins 4 kcal/g
Alcohol 7 kcal/g
Fats 9 kcal/g
Vitamins, minerals and water

Energy intake or caloric intake is the total amount of calories consumed during the day through foods and drinks.

Energy Output

Energy output is the opposite of energy intake. It refers to the total amount of calories burned during the day through metabolism, digestive processes, thermogenesis, and physical activity. The amount of energy a person expends daily varies with age, weight, height, and physical activity levels. Genetics and hormonal differences can also affect the demand for energy.

Everyone needs a minimal amount of energy just to stay alive (to maintain heart, lung, kidney, liver, and brain function). This is known as basal metabolism[2], and in a sedentary person, can contribute to as much as 60% to 70% of the total energy output. People with great percentages of lean body mass[3] usually have higher metabolic rates[4] at rest than people with more fat tissue. This is because muscle burns more calories at rest than fat tissue. As we age, we tend to lose muscle mass which can drop our basal metabolic rate considerably. However, regular exercise can maintain muscle mass and contribute to a higher metabolic rate in older adults.

The body also burns energy during digestion. This is known as the thermic effect of food (TEF). Different foods have different TEFs. The principle behind TEF is simple: the harder it is for the body to breakdown a nutrient, the more energy it will burn during the digestive process. For example, protein rich meals take longer to digest than sugar and fat laden foods. This means that protein requires more energy to be digested, thus having a higher TEF value. Also, a large meal has a higher TEF value than a small meal or a snack, because large meals will take longer to digest.

Meal rich in TEF (% of calories consumed)
Proteins 20% to 30%
Carbohydrates 5% to 10%
Fats 0% to 3%

Another way the body expends energy is through thermogenesis. Thermogenesis is the amount of energy needed to maintain body temperature. However, its contribution to the energy output is rather small.

The best way to boost energy expenditure is through physical activity. Exercise increases the demand for energy on the heart, lungs, brain and working muscles. Therefore, calorie burning is enhanced both during and after an exercise session. The table bellow shows estimated caloric expenditure for a 150 pounds person during physical activities of several intensities[5].

Walking the dog 204 kcal/hour
Walking at a moderate pace 224 kcal/hour
Walking at a brisk pace 340 kcal/hour
Backpacking 476 kcal/hour
Jogging 476 kcal/hour
Running at 5 mph 544 kcal/hour

Energy Balance

Energy balance is the state in which the energy intake matches the energy output. This means that all calories consumed are burned. It is important to keep the energy equation balanced in order to maintain a stable weight throughout our lives.

Energy Intake = Energy Output

When the energy intake is greater than the energy output, a positive energy balance occurs, meaning that there is more energy available than your body can actually use. According to the law of conservation of energy, energy may neither be created nor destroyed[6]. So, because this extra energy can’t be just thrown away, it is stored mostly as fat, and weight gain occur.

Energy Intake > Energy Output → Positive Energy Balance

On the other hand, when the energy output is greater than the energy intake, there is a negative energy balance. This means that the body has to figure out a way to balance the energy equation in order to survive. One way of doing it is using the energy stored in the fat tissue. That’s what happens when we exercise. The body uses fat as fuel, leading to weight-loss.

Energy Intake < Energy Output → Negative Energy Balance

However, when the caloric intake is dramatically reduced for extended periods of time (such as when you engage on a crash diet), the body has no alternative but to shift to a conservation mode. It will try to reduce the energy demand by slowing down the metabolism. This means you’ll be burning fewer calories at rest, and the low levels of blood sugar will make you feel sluggish and cranky. In this case, weight-loss is impaired, because to burn fat as fuel, the body needs energy, which in this situation, you won’t have any to spare.

A safer approach to weight-loss is to modestly reduce the caloric intake by 250 kcal to 500 kcal per day, while increasing the energy expenditure by 250 kcal to 500 kcal per day (through exercise). These steps will create a modest energy deficit with which the body can deal. Following this guideline may lead to a weight-loss of one to two pounds per week[7].


[1] Kilocalorie, popular known as calories, is the unit of heat energy. Calorie is defined as “The energy needed to raise the temperature of 1 gram of water through 1 °C.”

[2] “Basal metabolism is the minimal amount of energy expended in a fasting state to keep a resting, awake body alive in a warm, quiet environment.” Contemporary Nutrition, seventh edition.

[3] Lean body mass is the total mass of the body minus fat tissue. It is usually used to refer to the amount of muscle mass in the body.

[4] “Metabolic rate is the amount of energy expended in a given period.” Available at

[5] Available at

[6] Available at

[7] ACE Personal Trainer Manual, third edition.