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Metabolism and Energy Balance

Metabolismus

For medical students2 min readUpdated 2026-10-10

Metabolism is a fundamental and essential property of all living organisms. It consists of the continuous transformation of ingested nutrients into specific components of the body's own tissues, as well as the generation of waste products of vital activity.

EnergyGenerated primarily from carbohydrates and fats
Anabolism / PlasticProteins provide the building blocks for new tissues
BalanceNutrient intake equals output
CarbohydratesMake up the bulk of the diet — about 500 g/day

Physiological Significance and Energy Aspect

Metabolism is inextricably linked with energy exchange. Any modification of molecules within cells is accompanied by either the absorption or release of energy. This continuous cycle forms the basic metabolic demands of the organism, ensuring its capacity for survival, growth, and adaptation.

Functional Role of Nutrients

Nutrients ingested with food perform strictly defined tasks in the body, which can be divided into two main categories:

  1. Plastic (Structural) Role. Involves the construction, renewal, and regeneration of structural elements of tissues. This function is predominantly carried out by proteins.
  2. Energy Role. Aims to supply the body with fuel for all biochemical and mechanical processes. The primary sources of energy are carbohydrates and fats. Proteins can also be utilized for energy, but under normal conditions, this occurs to a minimal extent.

Factors Determining Nutrient Requirements

Metabolic demands are not constant. The intensity of metabolism and, consequently, the need for nutrients are influenced by a complex set of conditions:

Components of Metabolism and Daily Allowances

To maintain proper metabolism, the body requires a comprehensive set of elements. In addition to macronutrients (proteins, fats, and carbohydrates), water, mineral salts, vitamins, and trace elements are critically important.

Average daily requirements for an adult are distributed as follows:

Under adequate functioning of life-support systems, a strict overall metabolic balance is maintained. This means that the total amount of consumed nutrients mathematically equals the amount of substances excreted from the body (taking into account all intermediate transformations).

Mnemonic

To quickly remember the daily requirement, use the proportion: proteins and fats are needed in roughly equal amounts (70–100 g each), while carbohydrates are needed in an amount 5–6 times greater (500 g).

Frequently asked questions

What is the basal metabolic rate (BMR) and what factors determine its value?

Basal metabolic rate is the minimum energy expenditure required to maintain vital body functions under standardized conditions: physiological rest and the waking state.

Factors influencing BMR include:

  • age;
  • height and body mass;
  • sex;
  • ecological and geographical living conditions;
  • environmental conditions and lifestyle: at identical body surface areas, energy expenditure can vary (e.g., a wolf has a higher BMR per unit surface area than a dog of the same size);
  • climatic conditions: tropical inhabitants have a lower BMR than polar animals of equivalent surface area;
  • nature of thermal insulation: fur or human clothing;
  • type of protoplasmic organization: differences between homeotherms and poikilotherms.
What are the physiological caloric coefficients (energy values) of proteins, fats, and carbohydrates?

The physiological energy values (caloric coefficients) are:

  • proteins — 4.1 kcal/g;
  • fats — 9.3 kcal/g;
  • carbohydrates — 4.1 kcal/g.

For fats and carbohydrates, the breakdown products in a calorimeter and in the body are identical (H₂O and CO₂), so their energy yield matches.

Proteins burn to end products like N₂, CO₂, and H₂O in a bomb calorimeter, but are oxidized incompletely in the body. The end products of protein metabolism—urea, creatinine, uric acid—retain some energy, which is why oxidizing 1 g of protein in the body yields less heat than in a calorimeter.

Through which organ systems are metabolic waste products excreted?

Metabolic waste excretion is carried out by excretory organs:

  • kidneys — primary pathway; urine formation;
  • skin / sweat glands — perspiration;
  • gastrointestinal tract — excretion of water and salts;
  • lungs — evaporation of water during respiration.

Products of fat and carbohydrate oxidation are CO₂ and H₂O, excreted via the lungs, GI tract, and sweat glands.

For protein metabolism, the end products are nitrogenous compounds, including urea and uric acid; high concentrations of these compounds are toxic to metabolic processes.

What is the plastic function of nutrients?

It is the use of nutrients (primarily proteins) as building blocks to form and renew the body's own tissues.

Which factors have the greatest impact on energy requirements?

Requirements vary depending on physical activity, body size, age, sex, as well as endocrine system activity and thermoregulation processes.

What is the principle of overall metabolic balance?

In a healthy organism, the mass of all substances consumed over a given period must equal the mass of end products excreted during the same period, despite all chemical transformations.

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