Excessive fat deposition in humans (obesity) is a complex process. According to modern ideas, it can develop under various conditions:
- Increased intake of food
- Insufficient use (mobilization) of spare fat as an energy source
- Excess fat formation from carbohydrates.
In a healthy person, balance of consumed and expended calories is very accurately regulated: when changing energy expenditures, appetite and food intake also change to an appropriate degree. This provides a stable weight under varying environmental conditions. In obese people, deviations occur in this perfect system of metabolism, resulting in violations of dynamic balance between the amount of energy coming from food and energy expenditures, thus energy consumption prevails over its expenditure.
Excessive fat deposition in adipose tissue is associated primarily with excessive intake of food and, first of all carbohydrates that as known, are easily turn into fats in the body.
Intake of food in continuously changing energy expenditures is regulated by feeding center, which is a functional union of nerve centers located in the cortex, subcortex and brain stem. One of the forms of function manifestation of this center is appetite – a feeling of hunger.
Activity of the feeding center is subject to a wide variety of influences. A great role is given to impact of nerve impulses (signals) coming from various receptor fields (olfactory, visual, taste analyzers, receptors of the digestive tract). Thus, predominance of excitatory process in the feeding center can be caused by excitation of taste nerve endings in the oral cavity or the stomach receptors, for example, in frequent food testing, consumption of spicy snacks and delicacies.
- This is often observed in people working at public catering establishments (bistro, restaurants).
Metabolic processes affect activity of the feeding center. So, all conditions that lead though to a small but persistent decrease in blood sugar level can be accompanied by a predominance of excitatory process in the feeding center and, consequently overeating and development of obesity.
Tendency to obesity appears in some people in transition from physical labor to a sedentary lifestyle. Herewith, energy expenditures are significantly reduced, but habit of consuming increased amount of food is preserved. This leads to overeating and obesity.
Obesity often develops in elderly, which can be explained by discrepancy between the same level of excitability of feeding center and lower energy expenditures. It is known that physical development of a person stops by the age of 25. The most intensive metabolic processes are characteristic for this period of life. In each subsequent decade, metabolism is reduced by 2-7.5 percent, while eating habits, appetite, and amount of food intake remain the same. As a result, relative overeating and weight gain occur.
In addition, many people very often cease to engage in physical exercises and sports after 25-30, as a result their muscle activity and energy expenditures are reduced. Such deviations in lifestyle lead to a corresponding decrease in appetite, so energy balance is set at a new, lower level in people with normal regulation. In violation of this regulation, adequate correction of appetite and metabolism does not occur, and intake of the same amount of food inevitably leads to increase in weight.
Obesity development can be associated with a prolonged stay on bed rest in various diseases. Reduction of energy expenditures in these patients often leads to rapid weight gain and appearance of obesity in excessive and in some cases – even in normal food consumption.
A special role in regulation of appetite and metabolic processes belongs to the subcortical formations and hypothalamus centers. Processes of digestion and absorption of fats, accumulation of fats, and transformation of carbohydrates into fats are constantly regulated by these parts of the brain.
Appetite is disturbed and metabolic diseases occur when hypothalamic region is affected. However, in usual obesity, changes in these parts of the brain are functional, rather than crude, organic.
Violation of dietary regime in addition to excessive food consumption leads to obesity. For example, in connection with professional or household characteristics, the most abundant food intake is transferred to the later hours of the second half of the day when motor activity of a person usually decreases. In addition, long intervals in food lead to excessive appetite, and in this case, the person eats fast, greedily, thus he overeats in most cases because of belated feeling of satiety.
Endocrine glands disorders play an important role in the mechanism of obesity development. Often, these disorders are not clearly expressed; they can be detected only after careful examination.
Endocrine glands synthesize and release hormones that affect fat metabolism in different ways. Some of them contribute to splitting and mobilization of fat (lipolysis); others on the contrary, increase its formation (lipogenesis). The most intensive influence on processes of lipolysis is caused by growth hormone produced by the anterior pituitary gland, as well as thyroid hormones. In prolonged starvation, production and release of growth hormone in the blood increase that leads to splitting of fat and entry into tissues as energy material of fatty acids. An increase in thyroid function is usually accompanied by decrease in body weight, but its reduction causes increase in body fat.
A significant place in regulation of fat metabolism belongs to hormone of pancreas – insulin. Excessive food intake, especially rich in carbohydrates, causes an increased release of this hormone by beta cells of the pancreas. As a result, absorption of glucose by fatty tissue increases, fatty acids release from adipose tissue is inhibited, lipolysis is inhibited, formation of glycogen in the liver is stimulated, and blood sugar level decreases that in turn may cause increase of activity of the feeding center and consequently appetite.
Other hormonal factors that affect fat metabolism are hormones of the adrenal cortex. Obesity also contributes to reduction and termination of function of the sex glands.
Deviations in function of the endocrine glands do not play a leading role in occurrence of simple forms of obesity, but in most cases, they lead to development of this disease in combination with other predisposing factors.
Effect of hereditary predisposition is established in approximately half of patients with obesity. However, importance of heredity role in obesity development should not be overestimated, since it can be caused by common external causes associated with unhealthy diet in the family, habit of overeating developed already in childhood. This can be confirmed by studies showing that 74 out of 100 children growing up with obese parents also become obese. If these children are grown by people with normal body weight, obesity occurs only in seven children.
Condition of tissues, in particular fatty also affects obesity development. It produces enzymes (lipases) that participate both in mobilization of fat and in its deposition. A number of researchers have found a decrease in activity of fat-mobilizing lipase in obese people comparing to healthy ones.
Thus, excess of normal weight arises because of excessive nutrition (overeating), reduction of fat expenditure for the energy needs of the body, mainly at the expense of carbohydrates.
All these unfavorable changes in the body are consequence of metabolic disorder: changes in the nervous regulation, shifts in the functional connections of the endocrine glands occur, and condition of the adipose tissue is disturbed. However, in some cases, increased appetite and overeating are brought to the forefront, in others – impaired function of the endocrine glands, and thirdly – the nervous system dysfunction. The predominance of those or other factors determines clinical form of obesity.