Metabolism and energy balance

Metabolism is the fundamental process by which our body converts food into useful energy and building materials needed for growth, renewal, and daily functioning. The concept of "calorie balance" (known as "calories in" versus "calories out") is closely related to metabolic processes and influences weight regulation and overall health. In this article, we will discuss three essential elements of metabolism and energy balance:

• Basal Metabolic Rate (BMR): the minimum energy required at rest.
• Calories in vs. calories out: how to understand the balance of energy intake and expenditure that drives weight changes.
• The role of macronutrients: how carbohydrates, proteins, and fats contribute to energy production and health.

By the end of the article, you will have a comprehensive understanding of why these principles are important and how to apply them to optimize body composition, improve results, and maintain long-term well-being.

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Basal Metabolic Rate (BMR): what it is and why it matters

1.1 What is BMR?

Basal Metabolic Rate (BMR) – the amount of calories (energy) the body needs to maintain basic vital functions for 24 hours at rest. These functions include:

• Heart function and blood circulation
• Breathing and oxygen transport
• Body temperature regulation
• Brain activity
• Cell renewal and hormone secretion

BMR usually accounts for about 60–75% of total daily energy expenditure in sedentary individuals. Therefore, people with a higher BMR can consume more calories without gaining weight because their body burns more energy even at rest.

1.2 Factors determining BMR

Although each person has a unique metabolic rate shaped by both genes and environment, there are several common factors that influence BMR:

• Age: Over time, lean body mass (especially muscle) gradually decreases, and hormonal changes further slow metabolism. As a result, BMR usually decreases with age.
• Sex: Men often have more muscle and less fat than women of similar weight, so their BMR is generally higher. For women, especially after menopause, BMR may decrease further due to hormonal changes.
• Body composition: Muscle tissue metabolizes more energy than fat, so individuals with a higher muscle-to-fat ratio usually have a higher BMR.
• Genetics: Some individuals inherit genes that lead to a faster metabolic rate, others to more efficient energy storage.
• Hormone balance: Thyroid hormones (T3, T4), insulin, cortisol, and other hormones have a significant impact on metabolic rate. An underactive thyroid (hypothyroidism) often slows BMR, while hyperthyroidism speeds it up.
• Environmental temperature: Extreme heat or cold can force the body to work harder to maintain a constant temperature, slightly increasing energy expenditure.

Understanding these factors helps explain why two people of the same weight can have different calorie needs. Practically, to promote a higher BMR, it is worth increasing muscle mass, maintaining a healthy hormone balance, and overall body composition.

1.3 BMR vs. RMR

Sometimes the term RMR (resting metabolic rate) is used. Although it is very close to the concept of BMR, RMR is more often measured under less strict conditions (e.g., minimal movement, after a short fast), while BMR requires highly standardized conditions. RMR is usually slightly higher because it may include a small amount of digestion or minimal activity expenditure. However, in practice, BMR and RMR are often equated and considered close indicators—showing the minimal daily energy requirement.

1.4 Impact on weight management

Many people who want to control their weight focus mostly on exercise and diet composition, but it is the BMR that sets the basic "floor" level of how many calories the body needs per day. If the BMR is relatively low and a person's food energy intake often exceeds this norm (plus calories burned during exercise), weight will increase over time.

"By understanding your approximate BMR, you can better tailor your diet and workouts to individual body needs, more accurately setting goals for weight loss, gain, or maintenance."

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2. Calories In vs. Calories Out

2.1 Energy Balance Equation

Weight regulation is often summarized by the energy balance principle:

Weight change = calories consumed – calories expended

Calories In – total energy obtained from consumed food and drinks. Calories Out – total energy expended by the body:

• BMR/RMR: Basal metabolic rate at rest
• Physical Activity: Energy used for exercise and daily movements (NEAT – non-exercise activity thermogenesis)
• Thermic Effect of Food (TEF): Energy expended to digest, absorb, and process food

Although the body's energy balance is a complex interaction (involving hormones, food quality, gut microbiota), the basic logic remains: if we create a calorie surplus, weight increases, and a deficit leads to weight loss. If intake roughly equals expenditure, weight remains stable.

2.2 Surplus, Deficit, and Maintenance

• Calorie Surplus: When more calories are consumed than burned. The body stores this excess energy, usually as fat; if strength training is intense, part of the surplus is used to build muscle. Over time, a constant surplus leads to weight gain.
• Calorie Deficit: When more calories are expended than consumed. The body compensates for the deficit by using reserves (fat or, in worse cases, muscle), so weight decreases. A long-term deficit can significantly change body composition.
• Maintenance: Calorie intake matches expenditure, so weight remains roughly stable. Minor fluctuations may occur daily, but the overall trend does not change.

2.3 Nutritional Composition and Weight Changes

Although the energy balance formula essentially describes weight change, food quality is also significant. A lot of refined sugar and saturated fats can promote fat accumulation and disrupt feelings of fullness and hunger, while nutritious foods (containing protein, fiber, micronutrients) help maintain more stable blood sugar levels and more effectively control appetite.

Additionally, different macronutrients have varying thermic effect of food (TEF). Proteins generally have the highest TEF, so digesting them requires more energy than digesting fats or carbohydrates. Therefore, a diet higher in protein can provide a slight "metabolic advantage" due to increased energy expenditure. However, overall energy balance is the most important factor.

2.4 The Role of Physical Activity

Increasing physical activity burns more calories but can also affect appetite and body composition. Strength training helps maintain or increase muscle mass, which over time can raise BMR and facilitate weight maintenance. Aerobic exercises (e.g., running, cycling, swimming) create an additional calorie deficit, accelerating fat loss when combined with proper nutrition.

"Calories in vs. calories out is a cornerstone in explaining weight changes. However, it is important to consider food quality, hormonal health, and exercise type, as these affect how the body uses or stores energy."

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3. The role of macronutrients in energy production

3.1 Carbohydrates

Carbohydrates are often called the body's main energy source because each gram contains 4 kcal. They are especially important for high-intensity physical work, supplying muscles with glucose. The body stores carbohydrates as glycogen in muscles and liver, from where they can be quickly accessed during training.

• Simple carbohydrates: Found in fruits (fructose), dairy products (lactose), table sugar (sucrose), and many processed foods. They are quickly broken down, providing a rapid energy boost but may cause fluctuations in blood sugar levels.
• Complex carbohydrates: Starch and fiber found in whole grain products, legumes, vegetables, and some fruits. They are digested more slowly, providing longer-lasting satiety and more stable energy metabolism.

Carbohydrate needs depend on the intensity of physical activity. Endurance athletes often require a higher proportion of carbohydrates in their diet to replenish glycogen stores, while those aiming to lose weight or stabilize blood glucose levels may choose a lower amount, focusing more on complex, fiber-rich carbohydrates.

3.2 Proteins

Proteins are important for tissue building and repair (muscles, enzymes, hormones), as well as for supporting immunity. They also provide 4 kcal/g, but unlike carbohydrates, they are usually used first for structural and functional tasks rather than energy. However, when carbohydrates or overall calories are lacking, the body can break down certain amino acids to produce glucose (gluconeogenesis).

• Amino acids: Proteins are digested into amino acids. Essential amino acids must be obtained from food, while non-essential ones can be produced by the body.
• Muscle preservation and growth: Adequate protein intake combined with strength training promotes muscle protein synthesis and helps maintain or increase lean mass, thereby contributing to a faster BMR.

Many health and sports organizations recommend 1.2–2.0 g of protein per kilogram of body weight per day for active individuals, although the exact requirement depends on age, training intensity, and individual health characteristics.

3.3 Fats

Fats are the most calorie-dense macronutrient (about 9 kcal/g). However, this is not necessarily bad – fats are needed for hormone production, cell membrane structure, and absorption of fat-soluble vitamins (A, D, E, K).

• Unsaturated fats: Generally considered "good," found in avocados, nuts, seeds, and fatty fish. They include monounsaturated and polyunsaturated fats (such as omega-3 and omega-6 fatty acids).
• Saturated fats: Mostly found in animal products (meat, dairy) and some plant oils (coconut, palm). While moderate consumption can be part of a balanced diet, excessive amounts may raise cholesterol levels in some people.
• Trans fats: Mostly artificial (during hydrogenation), significantly increasing "bad" LDL cholesterol – they should be limited.

Fats also act as a reserve or long-term energy source, where less intense aerobic exercise allows burning a greater proportion of fatty acids. An adequate amount of fat is essential for hormone production. Too little fat can harm health, while too much increases the risk of cardiovascular diseases.

3.4 Macronutrient Balance

The best ratio of carbohydrates, proteins, and fats depends on a person's goals and conditions. Endurance athletes may need more carbohydrates to replenish glycogen, while those aiming to lose weight might emphasize higher protein to support satiety and muscle mass. However, regardless of macronutrient distribution, the final outcome depends on energy balance: if calories consumed exceed calories burned, weight will increase, even if the diet is "ideally" balanced.

"Macronutrients are like a toolkit: carbohydrates, proteins, and fats each play important roles. Balancing them according to activity, goals, and health status can significantly improve nutrition strategy."

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4. Beyond Metabolism Basics: Hormones and Individual Differences

Although the "calories in vs. calories out" model forms the basis of weight regulation, hormones like leptin, ghrelin, insulin, and cortisol can alter appetite, fat storage, and energy use. Chronic stress, poor sleep, or endocrine disorders (e.g., hypothyroidism) can also change metabolic rate and the tendency to store/use energy.

Another important area is individual differences, including genetics and gut microbiota. Some people process carbohydrates more efficiently, while others feel better with more protein or fat in their diet. So you can experiment with general principles – energy balance, timing, and amounts of macronutrients – to find what works best for your unique body.

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5. Practical strategies to regulate energy balance

With knowledge of BMR, calorie balance, and macronutrients, effective strategies can be created to achieve specific goals (health, physique, sports performance). Here are some tips:

5.1 Approximate calorie norm determination

• Equations: Formulas like Harris-Benedict or Mifflin-St Jeor help roughly determine BMR. Multiplying the result by an activity factor (sedentary, light, moderate, intense) gives the total daily calorie requirement.
• Use of technology: Smart devices and apps can roughly estimate daily energy expenditure. Although there may be errors, this helps establish a starting point and later adjust nutrition.

5.2 Nutritional adjustments to achieve goals

• Weight loss: The goal is a moderate calorie deficit: about 250–500 kcal less than needed per day. This way, fat is gradually burned and muscle mass is better preserved.
• Weight (muscle) gain: A slight surplus, e.g., 200–300 kcal, emphasizing adequate protein intake (1.2–2.0 g/kg body weight) and progressive strength training, promotes muscle growth.
• Maintenance: Consumed calories match daily energy needs. It's worth monitoring weight changes and adjusting food portions if necessary.

5.3 Macronutrient balance

• Carbohydrates: Choose complex ones (whole grains, fruits, legumes) and limit refined sugar for better energy balance. The amount needed depends on the type and intensity of training.
• Proteins: Distribute throughout the day to ensure steady muscle protein synthesis. Proteins can be of animal origin (lean meat, dairy products, fish) or plant origin (legumes, soy products).
• Fats: Priority – unsaturated fats from avocados, olive oil, nuts, seeds, and fatty fish. Saturated fats – in moderation, and trans fats are better avoided.

5.4 Integration of physical exercises

• Strength training: Increases muscle mass and thus raises BMR. Compound exercises, such as squats or deadlifts, help engage multiple muscle groups simultaneously.
• Aerobic exercises: Running, cycling, swimming, etc. help create a calorie deficit and strengthen the cardiovascular system. High intensity (e.g., interval training) can effectively develop both aerobic and anaerobic capacities.
• The importance of daily movement (NEAT): Don't rely solely on formal exercise – movement at the workplace, climbing stairs, short breaks, and active activities help burn a significant portion of energy.

5.5 Progress monitoring

• "Body composition: Regularly assess fat percentage or waist circumference to distinguish fat loss from muscle changes. Weight alone can be misleading."
• "Results and energy levels: Monitor changes in training ability, endurance, and well-being. This can indicate whether the nutrition plan is effective."
• "Adaptation and improvement: Over time, both metabolism and lifestyle change, so periodically review your plan. If progress stalls, adjust calorie intake, training frequency, or macronutrient ratio."

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Conclusion

"Metabolism and energy balance are extremely important for health and weight regulation. Basal Metabolic Rate (BMR) determines the basic energy "need," while the calories in vs. calories out principle shows whether weight will increase, decrease, or remain stable. Macronutrients – carbohydrates, proteins, and fats – have unique roles in the context of energy and health, but their overall balance and total calorie amount determine the final outcome regarding body mass."

"Indeed, energy balance is only part of the whole picture. Hormonal characteristics, nutrition quality, genetics, gut microbiota, stress levels, and sleep also influence how the body expends or stores energy. Nevertheless, understanding these basic principles makes it easier to experiment by adjusting calorie intake and macronutrient distribution until the most sustainable and effective strategy is found."

"Once you master the basics of BMR, energy balance, and macronutrients, you can discard short-term fad diets and create a well-founded nutrition and training plan that supports a strong, healthy, and balanced body."

 

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• Anatomy and Functions of the Muscular System
• Physiology of Physical Exercise
• Principles of Physical Fitness
• Body Composition
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