Energy metabolism has two aspects: the production of energy and its expenditure by the body. The body expends energy in every activity it performs. Even when no physical or mental activity is taking place, it still needs to expend 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 supplies oxygen: the lungs take oxygen from inhaled air and transfer it to the blood. Glucose must be obtained from food through digestion and absorption. The oxygen and glucose supplied must reach the cells, and the heart works continuously to transport them.
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.
Even a simple, basic task therefore requires several processes to work together in a coordinated way. This coordination itself creates an additional workload and energy requirement.
When cell formation and breakdown, recovery from illness, growth and development are added, the body’s activities become more complex and its energy expenditure increases accordingly.
An individual’s total energy expenditure is the sum of the energy needed for vital functions and the energy expended in physical and mental activities associated with work and other pursuits.
Total energy expenditure is considered under three main components:
1. Basal metabolism
2. Physical activities
3. Thermic effect of food
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.
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 wide range of average daily energy expenditure for basal metabolism reflects considerable differences in basal metabolic rate between individuals. Even in the same person, it varies with time, surroundings and season.
The factors affecting the basal metabolic rate
1. Age: Basal metabolic rate is higher during infancy, childhood and development, reaching its highest level in infancy. It declines with age and reaches its lowest level in old age.
2. Sex: Basal metabolic rate is higher in men than in women. Reasons include the proportionally higher amount of adipose tissue in women and differences in hormone metabolism. In general, men’s basal metabolic rate is 5–10% higher than women’s.
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 breastfeeding. It can increase by 20%, particularly in late pregnancy.
6. Dietary pattern and foods consumed: This factor is particularly important for people following or considering a weight-loss diet. People often go hungry while dieting without understanding the consequences. Prolonged fasting reduces basal metabolic rate, allowing the body to adapt to a lower energy supply. This can prevent the intended weight loss.
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%.
For this reason, breakfast should be eaten even if the amount is small. Skipping breakfast encourages weight gain rather than weight loss. Other problems associated with trying to lose weight by eating nothing are discussed in related articles.
Claims aimed particularly at dieters that certain foods greatly increase basal metabolic rate and thereby cause more weight loss are urban myths. Coffee, red peppers and some spices can increase basal metabolic rate slightly.
However, the increase is very small and short-lived. Weight loss therefore cannot be targeted simply by relying on the effect of these foods on basal metabolic rate.
Another point to know about nutrition is that a protein-rich diet increases basal metabolic rate.
7. Geography and climate: Taking 20–37 °C as the reference range, basal metabolic rate increases as the surrounding temperature rises or falls beyond it. In cold conditions, energy is expended to maintain body temperature, and wearing thick clothing also increases energy expenditure.
At temperatures above 37 °C, the body must also expend energy to maintain its thermal balance. Basal metabolic rate is therefore higher in people living in cold and hot climates.
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 its effect on basal metabolism, stress causes the body to release hormones including epinephrine and cortisol. These prepare the body to respond through reactions described as “fight or flight.”
Cortisol promotes the breakdown of the body’s glycogen, fat and protein stores. At this stage, basal metabolic rate increases. However, protein breakdown also means the breakdown of muscle tissue.
Muscle tissue increases basal metabolic rate. Stress may therefore initially speed up basal metabolism but cause it to slow down over time.
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, sex, age and body weight are the most important factors affecting the amount of energy expended through 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 × 57 × 24 = 1231 kcal.
These calculated values are averages for healthy individuals.
The second method requires the body surface area corresponding to body weight and the standard metabolic rate for the person’s age and sex. Multiplying body surface area by the standard metabolic rate gives the energy expended through basal metabolism in one hour. Because several parameters must be known, this method is less practical for everyday use.
2. Physical Activities
After basal metabolism, physical activity accounts for the greatest energy expenditure. It is useful to examine it in two categories: occupational activity and other activities during the day.
Occupations are divided into four categories according to the energy expended while performing them:
Light work: These activities require an average of 10–100 kcal per hour. People doing this work have a total daily energy requirement of 2300–2700 kcal, including basal metabolism, physical activity and the thermic effect of food.
- Legal practice,
- Accountancy,
- office work,
- Deskwork,
- Doctor,
- Architect,
- Housework with appliances
Moderate work: These jobs require an average of 100–300 kcal per hour. Total daily energy requirements for people doing this work are 3000–3500 kcal.
- Shop assistant,
- Teacher,
- Student,
- Driver,
- Light industrial work,
- Bakery,
- Assembly work,
- Housework without appliances
Moderately heavy work: These jobs require an average of 300–500 kcal per hour. Average daily energy requirements for people doing this work are 3500–3800 kcal.
- Farming,
- mining,
- construction work,
- industrial work,
- Carpentry
Heavy work: These jobs require an average of 500–800 kcal per hour. Average daily energy requirements for people doing this work are 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.
Energy is also expended on mental activities, although less than on physical activities. Depending on their intensity, mental activities such as thinking, decision-making and problem-solving can expend 50–300 kcal in one 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, the thermic effect of food accounts for 10% of total daily energy expenditure. Someone expending a total of 2500 kcal per day therefore uses 250 kcal for this purpose.
References
Raymond, J. L., & Morrow, K. (2023). Krause ve Mahan Besin ve Beslenme Bakım Süreci (16. baskı; G. Akbulut, Çev. Ed.). Nobel Tıp Kitabevleri.
Aksoy, M. (Ed.). (2022). Beslenme Biyokimyası (7. baskı). Ankara Nobel Tıp Kitabevleri.
Sencer, E., & Orhan, Y. (2005). Beslenme. İstanbul Medikal Yayıncılık.
Gökalp, H. Y., Nas, S., & Certel, M. (2002). Biyokimya-I: Temel Yapılar ve Kavramlar. Pamukkale Üniversitesi Mühendislik Fakültesi.
Food and Agriculture Organization of the United Nations, World Health Organization, & United Nations University. (2004). Human energy requirements: Report of a Joint FAO/WHO/UNU Expert Consultation. FAO. FAO Food and Nutrition Technical Report Series, 1.
Food and Agriculture Organization of the United Nations. (2003). Food energy: Methods of analysis and conversion factors. FAO. FAO Food and Nutrition Paper, 77.
Here’s an article that might interest you; Energy Metabolism – I: Energy Production in the Body
Halil İbrahim Akgül
Halil Ibrahim Akgul, Ph.D., is an Assistant Professor of Food Engineering at Bayburt University, where he works in the Division of Food Sciences within the Faculty of Engineering. He received his B.Sc. in Food Engineering from Akdeniz University in 2010, followed by an M.Sc. and a Ph.D. in Food Engineering from Atatürk University in 2015 and 2020, respectively. His formally recognised academic fields are Food Sciences and Engineering, Food Technologies, Dairy and Dairy Products Technology, and Food Microbiology.
Dr. Akgul has more than fifteen years of professional, research, and academic experience in food science. Between 2011 and 2013, he worked as the food engineer responsible for operations at the Pilot Dairy and Bread Factory of Atatürk University’s Directorate of Agricultural Enterprises. This industrial experience, combined with his subsequent academic research, gave him extensive practical and scientific knowledge of milk processing, dairy production, food quality, hygiene, and product development.
His research is primarily focused on milk and dairy products, dairy microbiology, starter cultures, lactic acid bacteria, exopolysaccharides, butter technology, and the physicochemical, microbiological, sensory, and nutritional properties of foods. His studies have examined the quality and fatty-acid composition of traditional butter, aroma formation in butter produced with different starter cultures, raw-milk quality, the characterisation and antioxidant activity of bacterial exopolysaccharides, traditional dairy products such as çökelek and karın kaymağı, colostrum powder, and the use of fruit-derived ingredients in ice cream production.
His academic record includes peer-reviewed research articles published in national and international journals, including Mljekarstvo, Acta Scientiarum Technology, the Journal of Food Safety and Food Quality, the International Journal of Food Engineering, the Atatürk University Journal of Agricultural Faculty, and ATA Food Journal. Four of the research articles currently listed in his university profile were published in SCI-Expanded-indexed journals. He has also contributed to twelve national and international conference papers and presentations covering dairy products, food microbiology, butter quality, fatty-acid composition, starter cultures, exopolysaccharide-producing bacteria, colostrum, traditional foods, and functional ice-cream formulations.
Dr. Akgul has participated as a researcher or adviser in eleven completed scientific and applied research projects. These projects have addressed the modernisation of pilot dairy-processing facilities; the production and quality characteristics of colostrum powder; the physicochemical and microbiological quality of butter and raw milk; the identification of exopolysaccharide-producing lactic acid bacteria; the development and production optimisation of starter cultures for mould-ripened cheese; the standardisation of traditional dairy products; and the use of sloe berry and persimmon fibre in ice-cream production.
His teaching experience spans undergraduate, associate-degree, and graduate education. The courses he has taught include Food Chemistry, Analytical Chemistry, General Chemistry, Organic Chemistry, Fermented Dairy Products Technology, Butter Production and Processing Technology, Healthy Living and Nutrition, Organic Agriculture and Food, Popular Science, Digital Literacy, and Information Technologies. Through these courses, he combines fundamental food chemistry and engineering principles with dairy technology, nutrition, scientific literacy, and the practical communication of scientific knowledge.
He is the founder and Editor-in-Chief of Gıda ve Sağlık, a multilingual digital publication devoted to food, nutrition, health, food chemistry, food microbiology, and culinary science. Through this platform, he draws upon his academic background, teaching experience, research activity, and practical knowledge of food production to present scientifically grounded information in a detailed and accessible form for an international readership.