Human Anatomy and Physiology - N. I. Fedyukovich 2003

Metabolism and Energy Balance
Energy Production and Expenditure

The vital activity of an Organism is maintained through the continuous supply of energy generated during The oxidation of complex organic molecules via the Cleavage of chemical bonds. These molecules are broken down into three-carbon compounds, which enter the Krebs cycle (Citric Acid Cycle) and are further oxidized to CO₂ and H₂O. All energy-yielding processes occurring with the participation of oxygen constitute the aerobic METABOLISM system. The release of energy without oxygen is referred to as Anaerobic Metabolism. Energy is stored primarily in the high-energy phosphate bonds of adenosine triphosphate (ATP). ATP also serves as an energy carrier, as it diffuses to the sites where Energy is required. In turn, the formation and breakdown of ATP are coupled with processes that require an input of energy. When energy is needed, the bond of the phosphate group is cleaved via Hydrolysis, releasing the chemical energy stored within it. The resulting potential energy is then converted into kinetic energy—mechanical, chemical, osmotic, and electrical work. Part of this energy is used to maintain the constancy of the organism's internal environment, synthesize new substances, renew and build Cells, contract Muscles, and conduct nerve impulses.

The amount of energy released during the combustion of any substance is independent of the stages of its breakdown. CARBOHYDRATES and Proteins are known to yield an average of approximately 17.16 kJ/g (4.1 kcal/g) of energy. Fats possess the highest energy density: 1 g of fat yields 38 kJ/g (9.1 kcal/g) of energy, which exceeds the amount of energy released by the Oxidation of proteins and carbohydrates combined.

The Energy Metabolism of a living organism consists of the basal metabolic rate and the working increment above the basal rate. The amount of energy expended by an organism at rest and in a fasting state is termed the basal metabolic rate.

The basal metabolic rate is determined in the morning (with the patient at rest in a lying position), under conditions of thermal comfort (18–20 °C), in a fasting state 12 hours after a meal, and with the exclusion of proteins from the diet 2 to 3 days prior to the study. Basal metabolism is expressed in kilocalories (kcal) or kilojoules (kJ) expended by the organism under the specified conditions per 1 kg of body mass or per 1 m² of body surface area per hour or per day.

Basal metabolism largely depends on the Functions of the nervous and endocrine systems, the physiological state of Internal Organs, and external influences on the organism. The level of basal metabolism can change under conditions of inadequate or excessive Nutrition, prolonged physical exertion, climatic variations, and other factors. In different individuals, the magnitude of basal metabolism depends primarily on age, body mass, sex, and height. In a healthy adult, the basal metabolic rate averages 4.2 kJ (1 kcal) per 1 kg of body mass per hour, being 10–15% lower in women than in men. It is higher in children than in adults and decreases in elderly individuals.

The working increment is the elevation of energy metabolism above the basal level. Factors that increase Energy Expenditure include food intake, changes in ambient Temperature, and muscular work.

Basal metabolism is disrupted in Diseases of the Endocrine glands. For example, in Hyperfunction of the Thyroid Gland, it can increase up to 150% of the normal value, whereas it decreases in hypofunction. Significant changes are also observed in Pathologies of the Pituitary gland, which regulates The activity of peripheral endocrine glands.

Direct and Indirect calorimetry Methods are used to determine the intensity of substance and energy metabolism. The direct calorimetry method is based on the direct measurement of heat released during the vital activity of an organism. To achieve this, a person is placed in a special calorimeter chamber that accounts for the total amount of heat emitted by The Human Body. This method is complex and used exclusively in research institutions.

In practice, the indirect calorimetry method is more commonly used. Its essence lies in first determining the volume of pulmonary ventilation and subsequently measuring the amount of oxygen consumed and carbon dioxide produced. The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed by the organism is called the respiratory quotient. The magnitude of the latter allows one to evaluate The Nature of the substrates undergoing oxidation in the body.

Thus, during the oxidation of carbohydrates, the respiratory quotient is equal to 1, since the Complete oxidation of 1 molecule of glucose to carbon dioxide and Water requires 6 molecules of oxygen, while releasing 6 molecules of carbon dioxide:

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During the oxidation of proteins, the respiratory quotient is 0.8, and during the oxidation of fats, it is 0.7. Owing to the low content of intramolecular oxygen in fats and proteins, more oxygen is required for their oxidation: 0.97 L of oxygen is needed to oxidize 1 g of proteins, and 2.03 L for 1 g of fats.

Energy expenditure can also be determined from gas exchange. The amount of heat released in the organism per 1 L of oxygen consumed (the oxygen caloric equivalent) depends on which substances were oxidized using that oxygen. The oxygen caloric equivalent for the oxidation of carbohydrates is 21.13 kJ (5.05 kcal), for proteins 20.1 kJ (4.8 kcal), and for fats 19.62 kJ (4.686 kcal). There is a relationship between the respiratory quotient and the amount of energy generated upon the consumption of 1 L of oxygen (Table 4).

Table 4 Relationship between respiratory quotient values and oxidation energy

The intensity of metabolic processes depends largely on the magnitude of physical exertion. The metabolic rate at very low activity (an individual's "relative rest") is approximately 9600 kJ/day (2300 kcal/day) for men. The workload level during physical labor can be estimated by the energy expended and expressed using the so-called stepped energy scale, whose adjacent steps are separated by 2000 kJ. Thus, during light work, the intensity of metabolic processes reaches 12,000 kJ/day (2800 kcal/day); during moderate work, 22,000 kJ/day (5200 kcal/day); and during heavy work, 42,000 kJ/day (10,000 kcal/day).

Balanced nutrition is understood as a diet that is sufficient in quantity and nutritionally complete in quality. The foundation of balanced nutrition is dietary balance and optimal ratios of food components (Amino Acids, polyunsaturated Fatty acids, phosphatides, sterols, fats, sugars, Vitamins, mineral salts, organic acids, etc.). There are about 60 dietary substances that require balancing. Rational nutrition is ensured by the optimal intake of energy-yielding, plastic, and regulatory substances necessary for the normal functioning of the organism. A monotonous diet that excludes individual components of a balanced food ration causes Metabolic Disorders. For humans, A balanced diet includes proteins, fats, and carbohydrates in a mass ratio of 1:1:4. This makes it possible to regulate the daily caloric intake of the diet through proteins: 15% of the daily caloric value (half of animal origin). Fats should constitute approximately 30% of the daily caloric value (70–80% animal fat). The energetic share of carbohydrates at such ratios should be 55%. If body mass reduction is necessary, the amount of ingested carbohydrates should be restricted. During heavy muscular work, a large amount of protein is degraded, so its intake through food into the human body must be increased.