Adipose tissue is specialized connective tissue that functions as the major storage site for fat in the form of triglycerides.
There are two different types of adipose tissues. They are
- White adipose tissue.
- Brown adipose tissue.
Most adipose tissue is white. White adipose tissue does three functions:
- Heat insulation,
- Mechanical cushion. (Adipose tissue also surrounds internal organs and provides protection for these organs.)
- A source of energy.
The adipose tissue which is directly below skin (subcutaneous adipose tissue) specially acts as the heat insulator of the body. It conducts heat only one third compared to other tissues. The degree of insulation is dependent upon the thickness of this fat layer.
Lipogenesis is the deposition of fat or accumulation of body fat. This process occurs in adipose tissue and in the liver .
The excess calories (consumed more than what actually needed for the current physical activity ) is stored in adipose tissue. Carbohydrate and protein consumed in the diet can be converted to fat. The carbohydrates can be stored as glycogen in the liver and muscle and can also be converted to triglycerides in the liver and transferred to adipose tissue for storage. Amino acids from proteins are used for new protein synthesis or they can be converted to carbohydrate and fat.
Fatty acids, in the form of triglycerides are consumed through the diet or synthesized by the liver . Very little synthesis of free fatty acids occurs in the cells of adipose tissue ( adipocytes ). Triglycerides are the most important source of fatty acids, because this is the form in which dietary lipids are assembled by the digestive system and liver.
Triglycerides are made up of long chain of fatty acids .These are hydrolyzed ( broken ) to glycerol and free fatty acids by an enzyme called lipoprotein lipase (LPL). The free fatty acids are taken up by cells of adipose tissues and stored again as triglycerides through a complex process.
Insulin, a hormone secreted by the cells of the pancreas, plays an important role in the lipogenic process. The net effect of insulin is to
- Increase storage,
- Block mobilization and oxidation of fatty acids.
Insulin stimulates LPL formation. The LPL breaks the circulating to free fatty acids which can enter the adipocyte. Insulin is also required for the transport of glucose, which is needed for conversion of free fatty acids to tryglycerides in adipocytes. The conversion of glucose to fatty acids is accomplished by insulin’s activation of several enzymes.