Adipose tissue is a special connective tissue that functions as the primary storage for fat in the form of triglycerides. In humans, it is present in two different forms: white and brown. Its quantity and distribution are individual for each.
White adipose tissue has three functions: thermal insulation, mechanical cushion and, most importantly, an energy source. Basically, it is located directly under the skin and is the main heat insulator of the human body, because it conducts heat three times worse than other tissues. The degree of insulation depends on the thickness of this layer. For example, a person with a 2 mm layer of subcutaneous fat will feel as comfortable as possible at 15 ° C, while with a 1 mm - 16 ° C. In addition, adipose tissue surrounds the internal organs and provides them with protection against shaking.
For example, it is located:
- around the heart;
- in the area of the kidneys;
- filling around the joints;
- inside the orbit, behind the eyeball, etc.
As the main storage of energy, it provides its reserve with excessive consumption. Therefore, more energy can be obtained from a gram of fat (9 Kcal) than from a gram of carbohydrate (4 Kcal) or protein (4 Kcal). In addition, if a person stored excess energy in the form of carbohydrates, an increase in mass would interfere with his mobility.
However, there are some restrictions on the use of fat as a "fuel". So, tissues that function mainly due to anaerobic processes (for example, red blood cells) must receive energy from carbohydrates and must have a sufficient supply of them. In addition, under normal conditions, the brain is dependent on glucose and does not use fatty acids. In unusual metabolic circumstances, he may use ketone bodies (a by-product of incomplete fat metabolism) if they are present in large enough quantities.
Brown adipose tissue got its name due to the color caused by rich vascularization and tightly packed mitochondria, which can be found in various places.
Instead of serving as a substrate, the lipids in it release energy directly in the form of heat. The mechanism of its generation is associated with metabolism in mitochondria.
The biochemical process of energy release in the form of heat is activated in those cases when the total body temperature begins to decline. In response to hypothermia, hormones are released in the human body that stimulate the release of fatty acids from triglycerides, which, in turn, activate thermogenin.
In humans, the formation of brown adipose tissue begins at the 20th week of fetal development. At birth, it is approximately 1% of body weight. Its layer is located around the blood vessels that supply oxygen to the brain and organs of the abdominal cavity, and also surrounds the pancreas, adrenal glands and kidneys. Thanks to brown adipose tissue, the vital organs of the newborn are not supercooled in an environment with low temperature.
After birth, the baby begins to develop white adipose tissue, and brown begins to disappear. An adult has no places of its accumulation at all, although it is present (about 1% of the mass of fat), but is randomly mixed with white.