Energy Expenditure Structure and Influencing Factors
Even when a person is completely relaxed, internal organs continue to consume energy. Basal metabolism includes expenditures that cannot be eliminated in a living organism. These include cardiac activity and circulation, continuous respiration, excretory organ function (kidneys and liver), and the functioning of endocrine glands.
The final value of basal energy expenditure is influenced by several key factors:
- Sex and age;
- Height and body weight;
- Ecological and geographical habitat conditions.
Age-Related Features and Metabolic Dynamics
The intensity of energy expenditure directly depends on the balance of two processes: tissue synthesis (anabolism) and tissue breakdown (catabolism).
- Childhood. Anabolic processes predominate; therefore, values per kilogram of body weight are significantly higher than in adults. Newborns expend about 53 kcal/kg per hour, dropping to 42 kcal by age one, 1.8 kcal at age 7, and 1.3 kcal by age 12.
- Adolescents. This period shows the absolute peak of total energy expenditure. The daily expenditure of 16-year-old boys can reach 3,500 kcal, and for 14-year-old girls, 2,800 kcal.
- Adults. With light labor, total expenditure stabilizes (around 2,420 kcal for men and 2,100 kcal for women). The basal expenditure of an adult male weighing 70 kg is approximately 1,680 kcal/day.
- Old Age. Metabolic intensity decreases as catabolic processes begin to dominate over anabolism.
Law of Body Surface (Rubner's Rule)
Comparative physiology shows that heat production per 1 kg of mass varies sharply among different animal species. For example, in a mouse it is 654 kcal, whereas in a horse it is only 11.3 kcal.
Based on these data, M. Rubner formulated the law of body surface: the energy expenditure of warm-blooded animals is proportional not to mass, but to body surface area. Heat loss directly depends on the surface area in contact with the environment. From 1 square meter of surface, different organisms dissipate approximately the same amount of heat (ranging from 900–1,200 kcal/day).
Limitations of the law: The rule is not absolute. Metabolism is influenced by species ecology (the metabolic rate in wolves is higher than in dogs of similar size), climate (polar animals have a higher metabolic rate than tropical ones), insulation type (fur, clothing), and the cellular protoplasmic organization type.
Measurement Conditions and Clinical Significance
In medical practice, this parameter is determined by direct or indirect calorimetry. To ensure data validity, the study is conducted under strictly controlled standard conditions:
- Time and State: In the morning, immediately after sleep, in a supine position. Complete physical and psycho-emotional relaxation is required.
- Diet: Fasting (12–14 hours postprandial). Protein-rich foods are restricted 2–3 days prior to the procedure to eliminate their specific dynamic action (digestion costs).
- Environment: Thermal comfort is maintained in the room (around 22 °C).
In clinical settings, assessing basal expenditure helps diagnose thyroid gland function. This is especially valuable when direct measurement of T3 and T4 hormone levels is difficult. With hyperfunction, metabolism increases significantly, while with hypofunction, it drops.
Neurohumoral Regulation
The main center regulating metabolic processes is the hypothalamo-pituitary system.
- Autonomic Nervous System: The sympathetic division accelerates metabolism, increasing energy expenditure, while the parasympathetic division decreases it.
- Hormonal Control: Secretion of thyroid, pituitary, and sex hormones modulates the ratio of anabolism to catabolism. The release of these hormones leads to a substantial increase in basal metabolism.