Basal metabolism is the rate of energy expenditure per unit time. The correctness of its measurement requires a strict set of criteria. They include being in a physically and psychologically calm state, a thermally neutral environment, and a postabsorbing state.
Description
Body heat release is known as thermogenesis. It can be measured to determine the amount of energy expended. Basal metabolism decreases with age. It can also increase due to muscle building. Diseases, nutrition, stress levels, and environmental temperatures affect the total energy expenditure.
Calculation Methods
Accurate calculation of basal metabolism requires that the human sympathetic nervous system is not stimulated. It can be measured by gas analysis using direct or indirect calorimetry.
You can also calculate basal metabolism using an equation using age, gender, height and weight. Studies using both methods provide evidence of the validity of the respiratory coefficient, which measures the inherent composition and use of carbohydrates, fats and proteins as they are converted to energy substrate units.
Phenotypic flexibility
Basal metabolism is a flexible feature. It can be reversibly adjusted within the body. For example, lower temperatures, which usually leads to higher metabolic rates. There are two models that explain how basal metabolism varies with temperature: variable maximum model (PMM) and variable fractional model (PFM).
PMM claims that the metabolic rate increases during the cold season. PFM says that the level of basal metabolism is a constant value.
Research
The early work of scientists J. Arthur Harris and Francis G. Benedict showed that approximate values of metabolism can be obtained using body surface area (calculated by height and weight), age and gender. In this case, oxygen and carbon dioxide parameters taken from calorimetry are taken into account.
Eliminating the gender differences that occur during the accumulation of adipose tissue by expressing the metabolic rate per unit of "fat-free" body weight, the value of calculating the level of basal metabolism (UBM) between the sexes is essentially the same.
Physiology
The main body responsible for regulating metabolism is the hypothalamus. It forms part of the side walls of the third ventricle of the brain. The main functions of the hypothalamus:
- Control and integration of the autonomic nervous system (ANS). It regulates the narrowing of the heart muscle and the secretion of many endocrine organs (thyroid gland).
- Regulation of feelings of rage and aggression.
- Body temperature control.
- Regulation of food intake.
The nutrition center (hunger) is responsible for the sensations that make a person seek food. With enough nutrition, leptin levels become high. The saturation center is stimulated. Impulses are sent that block the feeling of hunger. If there is not enough food, the level of ghrelin rises. Hypothalamic receptors are irritated. There is a feeling of hunger.
The thirst center works similarly. The increased osmotic pressure of extracellular fluid is stimulated by hypothalamic cells. If the thirst is satisfied, pressure will decrease. These functions form a survival mechanism that forces a person to support the body's processes, as measured by the level of basal metabolism.
Record Parameters
Basal metabolism formulas were first published at the beginning of the twentieth century. They used the following concepts:
- P - total heat production at rest;
- m is the mass (kg);
- h is the height (cm);
- and - age (years).
One popular valuation method is the Harris-Benedict formula:
- for women: UBM = 665 + (9.6 × m) + (1.8 × h) - (4.7 × a);
- for men: UBM = 66 + (13.7 × m) + (5 × h) - (6.8 × a).
Why calculate?
Basal metabolic rate can be used to gain, lose, or maintain weight. Knowing how many calories are burned, you can calculate how much to consume. For instance:
- to maintain weight, you should consume and burn an equal amount of calories;
- for recruitment, consumption must exceed combustion;
- for loss, you should consume fewer calories than burned.
How to calculate calories
The next step after assessing the level of basal metabolism is calculating calories based on lifestyle:
- Sedentary. No physical exertion. Multiply UBM by 1.2.
- Slightly active. Performing light exercises 1-3 times a week. Multiply UBM by 1.375.
- Moderately active. Exercise 3-5 times every seven days. UBM multiplied by 1.55.
- Active. Up to seven workouts per week. Multiply UBM by 1.725.
- Hard labour. Constant physical activity. UBM multiplied by 1.9.
The formula would be more accurate if it included body composition, weight history, and other factors.
Reasons for individual differences
In Scotland, a basal metabolic rate study was conducted in 150 adult subjects. Indicators ranged from 1020 to 2500 kcal / day. Researchers estimate that 62.4% of this variation is due to differences in the "fat-free mass." Other causes included fat mass (6.8%), age (1.8%), and experimental error (2.1%). The rest of the variation (26.6%) was inexplicable. It was not affected by either gender or the size of the tissues of such high-energy organs as the brain.
Biochemistry
An increase in postprandial thermogenesis in basal metabolism occurs depending on the composition of the food consumed. Almost 70% of the total expenditure of human energy is associated with life support processes that occur in the body. Approximately 20% of energy costs are in physical activity. About 10% - for the digestion of food. To carry out these processes, oxygen consumption with coenzymes is required. This provides energy for the survival and displacement of carbon dioxide.
The body spends most of its energy to maintain the required level of fluid in the tissues. About one tenth is given to mechanical work (respiration, digestion, and heartbeat).
The breakdown of large molecules into smaller ones is catabolism (for example, the breakdown of proteins into amino acids). Anabolism is the process of their creation (proteins are converted into amino acids). Metabolism is the result of these reactions.
Heartbeat hypothesis
In 1925, Raymond Pearl suggested that life expectancy is inversely related to basal metabolic rate. Support for this theory comes from the fact that large animals have longer periods of life. This hypothesis was supported by several new studies linking a lower basal metabolic rate with an increase in the life cycle in the animal kingdom, including humans. Calorie restriction and lower thyroid hormone levels have been associated with higher animal longevity.
However, the ratio of total daily energy expenditures to metabolic rate at rest can vary from 1.6 to 8.0 in different species of mammals.
With allometric scaling, the maximum potential life span is directly related to metabolic rate.
Medical aspects
The metabolism of a person depends on his activity and physical condition. Reducing food intake usually reduces its speed. The body is trying to conserve energy. Studies prove that a low-calorie diet (less than 800 calories per day) reduces the rate of metabolic processes by more than 10 percent. Menopause and illness also affect metabolism.