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 heart activity and circulation maintenance, continuous breathing, the work of excretory organs (kidneys and liver), as well as the functioning of endocrine glands.
The final value of basal energy expenditure is influenced by several key factors:
- Gender 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 breakdown (catabolism).
- Infancy and childhood. Anabolic processes prevail, so the values per kilogram of body weight are significantly higher than in adults. Newborns expend about 53 kcal/kg per hour; by one year of age, this figure drops to 42 kcal; at 7 years old, it is 1.8 kcal; and by 12 years old, it is 1.3 kcal.
- Adolescents. It is during this period that the absolute peak of total energy expenditure is observed. The daily expenditure of 16-year-old boys can reach 3500 kcal, and of 14-year-old girls, 2800 kcal.
- Adults. With light work, total expenditure stabilizes (about 2420 kcal in men and 2100 kcal in women). The basal expenditure of an adult man weighing 70 kg is about 1680 kcal/day.
- Old age. Metabolic intensity decreases as catabolic processes begin to dominate over anabolism.
Surface area law (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, while in a horse it is only 11.3 kcal.
Based on these data, M. Rubner formulated the surface area law: the energy expenditure of warm-blooded animals is proportional not to mass, but to body surface area. Heat dissipation 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 (in the range of 900–1200 kcal/day).
Limitations of the law: The rule is not absolute. Metabolism is influenced by the species' ecology (the metabolism of a wolf is higher than that of a dog of similar size), climate (polar animals have a more intense metabolism than tropical ones), the nature of thermal insulation (fur, clothing), as well as the type of cellular protoplasm organization.
Measurement conditions and clinical significance
In medical practice, this indicator is determined using direct or indirect calorimetry methods. To ensure the data are reliable, the study is carried out under strictly controlled standard conditions:
- Time and state: in the morning, immediately after sleep, in a lying position. Complete physical and psycho-emotional relaxation is required.
- Diet: on an empty stomach (12–14 hours after a meal). Protein food is restricted 2–3 days prior to the procedure to exclude its specific dynamic action (energy spent on digestion).
- Environment: thermal comfort is created in the room (about 22 °C).
In clinical practice, the assessment of basal expenditure helps diagnose the condition of the thyroid gland. This is especially valuable when direct measurement of T3 and T4 hormone levels is difficult. With hyperfunction of the gland, metabolism increases significantly, while with hypofunction, it drops.
Neurohumoral regulation
The main center regulating metabolic processes is the hypothalamic-pituitary system.
- Autonomic nervous system: the sympathetic division accelerates metabolism, increasing energy expenditure, while the parasympathetic division decreases it.
- Hormonal control: the secretion of thyroid, pituitary, and sex hormones modulates the ratio of anabolism and catabolism. The release of these hormones leads to a significant increase in basal metabolism.
Briefly: answers to topical questions
What are the differences between direct and indirect calorimetry when measuring basal metabolic rate?
Direct calorimetry consists of physically measuring the heat released by the body, whereas indirect calorimetry is based on calculating heat production from gas exchange indicators.
| Characteristic | Direct calorimetry | Indirect calorimetry |
|---|---|---|
| Principle | Measurement of body heat release | Indirect determination of heat production via gas exchange |
| Equipment | Special calorimetric chamber | Douglas bag (collection of expired air) |
| Measured parameters | Heat released | Consumed O₂ and expired CO₂ |
| Features | Methods are cumbersome and complex | Most common method (Douglas-Haldane method) |
Which specific hormones stimulate an increase in the basal metabolic rate?
According to the sources, the basal metabolic rate is increased by:
- Thyroid hormones — thyroxine (T4) and triiodothyronine (T3): sources indicate their effect on metabolism, enhancement of energy and oxidative processes.
- Adrenaline: enhances oxidative processes in tissues, increases oxygen consumption and heat production, and raises the basal metabolic rate.
- Pituitary hormones — indicated in the source as increasing the basal metabolic rate, but specific names are not provided.
- Sex hormones — indicated in the source as increasing the basal metabolic rate, but specific names are not provided.
Class and exam questions
What components make up total daily energy expenditure?
Total expenditure, or total metabolism, includes basal metabolism, the specific dynamic action of food (energy spent on digesting and assimilating nutrients), and working metabolism, which provides for daily physical activity.
Why should protein intake be restricted when determining basal metabolism?
Protein food has a pronounced specific dynamic action — it strongly accelerates metabolism during the digestion process. If it is not removed from the diet 2–3 days before the study, the results will be falsely elevated.
How to calculate basal metabolic rate for an adult male
For calculation, an averaged norm is used: 1 kcal per 1 kg of body weight per hour. Accordingly, to find the daily expenditure, you need to multiply body weight in kilograms by 24 hours.
What else to study on this topic
- Direct and indirect calorimetry: principles and methods
- Specific dynamic action of various nutrients
- Working metabolism and labor intensity classification
- Effect of thyroid hormones on cellular metabolism
- Thermoregulation features in warm-blooded and cold-blooded organisms
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