Visceral fat - a particular health risk
CELL FAT AND FAT TISS
Fat cells[9,16,20].
In the subcutaneous tissue there are numerous fat cells called adipocytes. The organism of a mature human is about 30 million adipocytes. Single cells have a diameter of about 80-100 microns, ovoid, round, sometimes polyhedral. They are formed successively during the maturation of the body from undifferentiated stem cells of loose connective tissue. First, the so-called pre-adipocyte, later - a mature cell that gradually fills up with fat. Adipocytes are able to accumulate in their interior a significant amount of fat in the form of triglycerides (triacylglycerols). The type of fatty acids contained in them is to a large extent a reflection of the composition of these acids in the diet. It is worth noting here at the beginning that while the unsaturated fatty acids stored in adipocytes are for the system alternatively a supply of energy for conversion to calories or cell building material, saturated fatty acids are used almost exclusively as an ad hoc or backup source of energy (mainly for muscles skeletal and cardiac muscle). Fat accumulates in fat cells cumulatively in the form of lipid particles. As the fat filling inside the adipocyte increases - the triglycerol particles merge into a swelling drop that can completely fill the cell and push its cytoplasm and nucleus to the periphery. The adipocyte's achievement of a critical cell mass of approximately 0.8 micrograms can result in a new cell in adipose tissue. In this context, it should be noted that the total number of fat cells in adult humans remains approximately unchanged, although the actual population of adipocytes is constantly transformed, including the aging, dying and renewal of cell populations through newly emerging ones. Using radioactive isotope labeling of adipocytes, it was established that every year, "exchange" is about 10 percent. adipocytes. Adipocytes in women are larger than in men and usually concentrated in the area of buttocks and thighs. In men, the abdominal cavity is a typical place of fatty tissue concentration.
Fat tissue[2,9,16,20].
Fatty cells - adipocytes - tend to group together in the subcutaneous tissue into small spatial modules, so-called payroll (lobules). A single fat pad is surrounded by a stabilizing mesh of connective tissue fibers. In addition to the adipocytes grouped in lobules, they contain less numerous nerve cells, immune cells and a network of blood vessels that feeds oxygen and nutrients. The lobelia syndromes form a considerable area of subcutaneous fat, locating itself most abundantly in the abdomen, buttocks and hips, as well as in the abdominal region around the visceral organs, mainly around the kidneys and liver. The space between the cells of this tissue is called the intercellular essence. The density of the intercellular matter is small compared to the total weight of adipocytes filled with fat. Fat cells over 90-95% fill adipose tissue. Importantly, adipocytes filled with fat are not only a storehouse of spare calories and thermal insulation of the body - but intensively metabolize food-derived substances (glucose, lipids, amino acids, etc.) and secrete numerous metabolites into the blood, including biologically active substances called adipokines (a type of cytokines ). There are many adipokines that act as hormones and signaling proteins necessary for the autoregulation of physiological functions.
The most important functions of adipose tissue:
- energy storage in the form of fat - release of fat into the blood in energy deficit situations - glandular-metabolic function - immunomodulatory role - body thermal insulation - cover and shock absorption of internal organs against shocks and injuries
Energy storage in the form of fat[1,2,3,9,20]
When the capacity of the liver to the food granule glucose in the form of glycogen is exhausted - then the pancreatic hormone insulin begins the transformation of this sugar into fatty compounds. The lipids synthesized from glucose in the liver are then dumped in adipocytes via the blood stream. The biochemical processes leading to this goal are: the pentose cycle, the synthesis of fatty acids and their coupling with glycerol to triglyceride molecules. Also, the excess of dietary lipids in the diet ultimately leads to their storage in the form of triglycerides in adipose tissue adipocytes. Fat is for the organism an indispensable source of energy reserves, because it is the most calorific. With unit weight, it delivers more calories than competitive carbohydrates. In one kilogram of triglycerides there is about 7,000 kcal, while in 1 kg of glucose only about 3,700 kcal, or 2.3 times less! That is why the pool of energy that the human body is capable of storing fat cells is about 84% - against about 1% of the energy reserve in the form of glycogen stored in the liver and muscles. Liver glycogen is consumed after several hours of strict fasting. Hence, it mainly serves the buffering of standard interprandial energy expenditure, so that the optimal blood glucose level of approx. 80-100mg% can be maintained. It is worth noting at this point that the relatively large mass of protein from which muscles and other body tissues are built, even though it contains about 15% of the calories available to the body - can not be "burned" to satisfy the energy needs between the for its systemic role of organs and tissues. The body protects the body proteins and amino acids against energy consumption. It uses the energy contained in protein body structures only when it no longer has any other source of energy, for example during extreme fasts. Then, figuratively speaking - "the body eats itself".
In an energy deficit situation - release of fat into the blood [1,9.17,20,21]
During intense weight loss, longer starvation, or due to a significant deficiency of calories in the diet preceding the significant exercise - triglycerides stored in fat cells are released into the blood. The signal for fat release from adipocytes in a process called lipolysis is the low blood glucose level, respectively. In response to this signal, a direct lipolytic initiator is activated, which is a specific lipase enzyme that breaks down triglycerides from adipocytes - to fatty acids and glycerol. This enzyme is also activated by the appropriate concentration in the blood of hormones such as insulin, glucagon, adrenaline, etc. The level of these hormones also affects the rate of lipolysis and thus the intensity of fat release into the blood. Blood-released lipids - glycerol and free fatty acids are then transported in the blood plasma to the target tissues. During this transport, they are partially hydrolysed. As a result, some of the free fatty acids are combined with globulins, creating the physiologically necessary lipoprotein fraction, especially cholesterol. Once the lipids reach the target cells, free fatty acids are "burned" in the cell mitochondria to generate energy that drives physiological processes, which is particularly important during the aforementioned slimming or physical exercise due to work or sports. It is worth noting that lipids present in the bloodstream are not only an energy resource, but also a potential building block of cells, skin epithelium and a component of many biologically active substances. For example, lipids in the form of glycerophospholipids and sphingolipids are the main component of cell membranes. Fatty acid derivatives are already mentioned cholesterol and numerous hormones and important for the metabolic and nervous processes so-called. signaling molecules. Tłuszczowce also participate in the processes of cellular protein synthesis - i.e. in the so-called covalent modification of proteins.
HORMONAL AND PROTECTIVE ROLE
Glandular-metabolic function of adipose tissue[1,9,20,21]
Currently, adipose tissue is perceived not only as an energy storage and thermal insulation, but also as an important endocrine gland. It has been documented in numerous studies that fat cells - adipocytes - have receptors thanks to which they actively react to signaling chemical stimuli from many organs and tissues and actively participate in the autoregulation of the hormonal system. In response to stimuli, adipocytes synthesise and release endocrine numerous biologically active compounds calledadipokines. Adipokines are peptides, proteins, glycoproteins, etc. Among them are many substances with the physiological role of hormones, e.g. adiponectin, leptin and others. (We will discuss their operation later in the article). The adipokine secretion-related enzymes work both within the fatty tissue, where they exhibit autocrine and paracrine effects, and collectively endocrine - as a specific endocrine gland. Autocrine operation means that the fat cell synthesises in its interior a specific autoregulatory hormone in the form of adipokine and the hormone works on itself. The paracrine activity consists in the fact that the hormone synthesized in the adipocyte acts locally on neighboring cells. The most important thing, however, is that adipokines of adipose tissue also show full endocrine activity, i.e. regulate the functions of distant organs and tissues! Obvious evidence of this is the recognition in adipocytes of receptors for such important hormones as: insulin, glucagon, growth hormone, some steroids, etc.
In addition, there were also receptors for signaling substances that transfer biochemical stimuli from the humoral and nervous systems. These adipocyte receptors enable the entire local adipose tissue to play the role of a "regulatory network" autoregulating both other hormonal glands, as well as regulating with the nervous and immune systems. The special role of metabolic and endocrine adipose tissue is the impact on energy management, including the granulation of lipids and the reversible release of them into the blood. Fat cells, thanks to their receptors for insulin and glucagon, actively react to changes in the levels of these hormones in the blood by their cellular metabolism, which allows them to synthesise and release into the blood the lipid and adipokine profile appropriate for the situation. For example, in response to critically low insulin levels, adipocytes inhibit intracellular lipolysis, i.e. the release of free fatty acids into the blood. Conversely, in response to high insulin levels, they enhance cellular respiration processes in which glucose and triglyceride synthesis coupled with it are burned. In addition, adipose tissue is the site of endogenous steroid transformation and therefore plays an important role in the regulation of the systemic steroid balance. Due to their complex specific hormone-secretory activity, adipose tissue is often considered to be the largest endocrine gland.
Immunomodulatory role of adipose tissue[1,9,20]
Adipokines synthesized by adipose tissue have the ability to stimulate the immune system. Of the more than 100 already identified human adipocytokines - numerous have been discovered that are actively involved in the regulation of some immune system functions. It should be emphasized that the adipokine releases exhibit the already mentioned ability of feedback - "on their own" - and thus on adipocytes and preadipocytes; as well as immune cells - monocytes and macrophages - permanently present in the structure of fat tissue. For example - preadipocytes under their influence and under certain conditions - have the ability to destroy germs and other antigens (phagocytosis) and transform into specialized immune cells - macrophages. Therefore, some researchers of endocrinologists and immunologists define adipose tissue and its metabolite group with immuno-modulating properties - as a dynamic system that co-controls some activities of immune cells.
Thermal insulation and body thermoregulation[2,20,21]
Fat tissue effectively shields the interior of the body from radiating heat. It provides its subcutaneous location and high thermal insulation properties of fat. Competitive advantage, but under physiological self-control is the ability to quickly give excess heat to the surroundings, thanks to the activity of sweat glands. In the tropics and hot weather situation, the sweat evaporation mechanism absorbs large amounts of calories and effectively cools the body.It is worth remembering that sweating intensifies when the ambient temperature increases significantly above 31ºC, as well as in the case of post-exercise overheating of the body. The operation of this mechanism of reducing body temperature by evaporation of water
