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Energy Metabolism – II: Energy Expended by the Body

Energy metabolism should be considered in two aspects; the energy spent and the energy produced in the body. The body expends energy on all the activities it performs. Even when no physical or mental activity is performed, the body must spend a certain amount of energy.

From the point of view of vital activities, even maintaining the integrity of cells is a task in itself and the body has to expend energy to achieve this.

As a matter of fact, the inside of the cell is denser than the blood. As a simple physics rule, while the osmotic pressure acts in the direction of the passage of substances from the cell with high density to the less dense blood, the cell prevents this transition by expending energy. Thus the integrity of the cell is preserved.

Therefore, the body expends energy to maintain its physical structure, even if there is no anabolic or catabolic event and no activity is performed. Oxygen and glucose are required to provide this energy.

Breathing takes place for oxygen supply; the lungs take in oxygen from the breath and give it to the blood. Glucose needs to be taken with food, digested and absorbed. The supplied oxygen and glucose must be delivered to the cells; the heart works non-stop for this.

After the cells use oxygen and glucose, waste is produced. Waste must also be removed from the body. The brain coordinates all these activities. Separate energy is required for each of these activities.

From this point of view, more than one work is carried out collectively and systematically to perform a simple and basic mission. As a matter of fact, this systematic work also creates an extra load and energy need.

When cell production and destruction, recovery from disease, growth and development are added to these events, the body’s activities will become more complicated and the amount of energy spent will increase accordingly.

When the energy consumed by the individual in physical and mental activities for his work and occupations is added to the energy expended in vital activities, the total amount of energy spent by the individual emerges.

The total energy expended in the body is evaluated in three main activities;

1. Basal metabolism

2. Physical activities

3. Thermic effect of food

Schema showing ways the body expends energy
ways the body expends energy

1. Basal Metabolism

Basal metabolism includes all metabolic activities except digestion. By definition, all metabolic activities that an individual performs at rest are called “basal metabolism.”

The organs with the highest share of the energy spent in basal metabolism are the liver and the brain. Although the brain is only 2% of the body’s weight, it uses 20% of the energy expended in basal metabolism. The diagram below shows the share of organs in the energy expended in basal metabolism.

Schema showing energy expenditure in the body during basal metabolism
energy expenditure in the body during basal metabolism

In general, 60-75% of the total energy that an individual spends daily is spent within the scope of basal metabolism activities. This rate corresponds to 1000 – 2500 kilocalories per day on average.

The average daily amount of energy expended in basal metabolism has a wide range because “the basal metabolic rate” varies significantly from person to person. In fact, the basal metabolic rate of an individual differs in different times, environments and seasons.

The factors affecting the basal metabolic rate

1. Age; Basal metabolic rate is higher during infancy, childhood and development. The basal metabolic rate is highest during infancy. As getting older, the basal metabolic rate decreases; decreases to its lowest level in old age.

2. Gender; Basal metabolic rate is higher in men than in women. One of the reasons for this is the proportionally higher amount of adipose tissue in women and the difference in hormone metabolism. In general, the basal metabolic rate in men is 5-10% higher than in women.

3. Body Structure; As the amount of adipose tissue in the body increases, the basal metabolic rate decreases. On the other hand, as the muscle tissue increases, the basal metabolic rate increases. However, lean individuals have a higher basal metabolic rate than overweight individuals.

For dieters, it makes sense to aim to increase the basal metabolic rate. The way to do this is to increase muscle mass. Muscle mass is increased by physical activity.

4. Illness and Injury; Conditions such as illness and injury in the body increase the basal metabolic rate. The degree of this elevation varies according to the severity of the disease or injury. In fever, a 1-degree increase in body temperature causes a 12% increase in basal metabolic rate. Similarly, the basal metabolic rate can increase by 50% after a major surgical intervention.

5. Pregnancy and Lactation; Basal metabolic rate increases during pregnancy and lactation. Basal metabolic rate can increase by 20%, especially in the last pregnancy period.

6. Nutrition and Foods; This factor is very important for dieters and those who want to do it. As a matter of fact, in diets made for weight loss, the individual is often unconsciously hungry. However, prolonged fasting reduces the basal metabolic rate. Therefore, the body adapts to work with lower energy. This situation causes the targeted weight loss not to be achieved.

Similarly, another mistake frequently seen in individuals who are on a diet to lose weight is skipping breakfast. Breakfast in the first hours of the day can increase the basal metabolic rate by 10%.

Therefore, breakfast should be made in small quantities. Skipping breakfast; It acts as an incentive to gain weight rather than lose weight. Other problems that individuals who aim to lose weight without eating anything will encounter are also mentioned in related topics.

Today, it is an urban legend that some foods increase the basal metabolic rate and causes more weight loss, especially for individuals on a diet. Coffee, peppers and some spices can slightly increase the basal metabolic rate.

However, this increase is also minimal; and its effect on metabolic rate is very short-lived. Therefore, it is out of the question to target weight loss based on the fact that these and similar foods increase the basal metabolic rate.

Another issue that should be known in nutrition is that protein-based food increases the basal metabolic rate.

7. Geography and Climate; Based on 20 – 37oC, basal metabolic rate increases as the ambient temperature increases and decreases. In a cold environment, while energy is spent to keep the body temperature in balance; Wearing thick clothes also increases the amount of energy expended.

At temperatures above 37oC, the body still has to expend energy to maintain heat balance. Therefore, the basal metabolic rate of individuals living in cold and hot climates is higher.

8. Stress; Stress can have different effects on the body. Some individuals lose weight under stress; Some people gain weight. This depends on the duration of the stress and the body’s response to the stress. While some individuals lose their appetite during stress, stress causes an increase in appetite in some individuals.

Considering the effect of stress on basal metabolic rate, the body secretes some hormones under stress. These hormones, including epinephrine and cortisol, prepare the body to respond to stress with responses that can be classified as “fight or flight”.

Cortisol enters the glycogen, fat and protein tissues in the body and allows these structures to be burned. At this stage, the basal metabolic rate increases. However, the burning of proteins means the breakdown of muscle tissue.

As it is known, muscle tissue is the tissue that accelerates basal metabolism. Therefore, while stress initially accelerates basal metabolism; In the future, it causes a slowdown in basal metabolism.

Short-term stress may not have much effect on basal metabolism; however, chronic stress can lower the basal metabolic rate due to muscle breakdown.

Calculation of energy expended in basal metabolism

Many factors affect basal metabolic rate. However, gender, age and body weight are the most important factors affecting the amount of energy consumed by basal metabolism.

There are two different ways to calculate the amount of energy expended by basal metabolism.

In the first method, the calculation is simple;

For men; body weight (kg) x 24

For women; 0.9 x body weight (kg) x 24

With these formulas, the individual can calculate the amount of energy spent in basal metabolism in a day.

For example, the amount of energy spent by basal metabolism in a day in a man weighing 80 kg;

80 x 24 = 1920 kcal.

The amount of energy spent by basal metabolism in a day in a 57 kg woman;

0,9 x 57 x 24 = 1231 kcal.

What should be known here is that the values ​​found are average values ​​for healthy individuals.

To calculate by the second way, it is necessary to know the surface area according to body weight and the standard metabolic rate according to age and gender. In this way, the individual can calculate the amount of energy expended in an hour with basal metabolism by multiplying the body surface area he has with the standard metabolic rate. This method is not useful in daily life as there are many parameters to be known.

2. Physical Activities

Physical activities are the activities in which the body spends the most energy after basal metabolism. It would be more beneficial to examine physical activities in 2 different branches, namely the occupation and other activities during the day.

 Occupations are evaluated in four different classes in terms of energy expended;

Light works; On average, they are activities that require 10 – 100 kcal of energy per hour. The total daily (basal metabolism + physical activity + thermic effect of food) energy need of individuals engaged in these works is 2300 – 2700 kcal.

  • Advocacy,
  • Accountancy,
  • office work,
  • Deskwork,
  • Doctor,
  • Architect,
  • Housework with tools

Moderate works; These are jobs that require 100 – 300 kcal of energy per hour on average. The total daily energy need of individuals dealing with these works is 3000 – 3500 kcal.

  • Shop assistant,
  • Teacher,
  • Student,
  • Driver,
  • the light industry works,
  • Bakery,
  • Assembly works,
  • Housework without tools

Heavy works; On average, these jobs require 300 – 500 kcal of energy per hour. The average daily energy need of individuals dealing with these jobs is 3500 – 3800 kcal.

  • Farming,
  • mining,
  • construction work,
  • industrial work,
  • Carpentry

Very heavy works; These jobs require 500 – 800 kcal of energy per hour on average. The average daily energy need of individuals dealing with these jobs is 3800 – 4200 kcal.

  • Porters,
  • Blacksmithing,
  • Heavy construction work,
  • Mining,
  • Forestry,
  • Military service,
  • Athlete

The body spends different amounts of energy depending on the difficulty of physical activity. The amount of energy expended in some physical activities is given in the table below.

Table showing energy expended in physical activities
energy expended in physical activities

However, energy is also spent on mental activities, although not as much as physical activities. Depending on the intensity of mental activities such as thinking, making decisions and solving problems, the energy between 50 and 300 kcal can be spent in 1 hour.

3. Thermic Effect of Foods

The body gets the energy it needs from food. However, food must be digested and absorbed first to obtain energy from food. The energy expenditure for digesting and absorbing food is called the thermic effect of food.

To put it briefly, the thermic effect of food is the amount of energy expended in digestion. The thermic effect of proteins is higher than that of carbohydrates and fats. While the basal metabolic rate increases by 30% during the digestion of proteins, it increases by 15% during the digestion of carbohydrates and 7% during the digestion of fats.

In general, 10% of the total energy consumed daily is spent on the thermic effect of food. An individual consuming a total of 2500 kcal of energy, spends 250 kcal per day for the thermic effect of food.


Here’s an article that might interest you; 

Energy Metabolism – I: Energy Production in the Body

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