Adipose tissue, commonly known as body fat, has long been regarded as a storage depot for excess energy in the form of triglycerides.

However, recent research has demonstrated that adipose tissue is not just an inert storage site, but is also an active endocrine organ that plays a crucial role in maintaining metabolic homeostasis.

What is Adipose Tissue?

Adipose tissue is a specialized connective tissue that is composed of adipocytes or fat cells. There are two main types of adipose tissue: white adipose tissue (WAT) and brown adipose tissue (BAT).

White adipose tissue is the predominant form of adipose tissue and is primarily responsible for energy storage. Brown adipose tissue, on the other hand, is specialized for heat production and energy expenditure.

Endocrine Function of Adipose Tissue

Adipose tissue secretes a variety of bioactive molecules, collectively known as adipokines, that regulate numerous physiological processes such as appetite, glucose and lipid metabolism, insulin sensitivity, inflammation, blood pressure, and angiogenesis. Adipokines include leptin, adiponectin, resistin, visfatin, and numerous others.

Each of these adipokines has a specific function, and alterations in their levels have been associated with metabolic disorders such as obesity, type 2 diabetes, cardiovascular disease, and cancer.

Leptin

Leptin is perhaps the most well-known adipokine and plays a crucial role in regulating body weight and energy expenditure.

Leptin is secreted by adipocytes in proportion to their size and mass and acts as a satiety signal to the hypothalamus, which decreases food intake and increases energy expenditure. Leptin deficiency or resistance is associated with hyperphagia, reduced energy expenditure, and obesity.

Adiponectin

Adiponectin is an adipokine that is secreted exclusively by adipocytes and is involved in glucose and lipid metabolism and insulin sensitivity.

Adiponectin enhances insulin sensitivity, promotes glucose uptake and utilization by skeletal muscle, and inhibits hepatic glucose production. Low levels of adiponectin are associated with insulin resistance, type 2 diabetes, and metabolic syndrome.

Resistin

Resistin is an adipokine that was initially identified as a link between obesity and insulin resistance.

However, subsequent research has yielded inconsistent findings, and recent evidence suggests that resistin may not play a significant role in insulin resistance in humans. Resistin may have pro-inflammatory effects, and elevated levels have been associated with atherosclerosis, inflammation, and cardiovascular disease.

Visfatin

Visfatin is an adipokine that was initially identified as a novel insulin-mimetic adipokine. Visfatin enhances glucose uptake by peripheral tissues, such as skeletal muscle and adipose tissue, and reduces hepatic glucose production.

Recent evidence suggests that visfatin may also have anti-inflammatory and anti-apoptotic effects and may play a protective role in atherosclerosis and other inflammatory disorders.

Other Adipokines

In addition to the four adipokines described above, numerous other adipokines have been identified, each with a specific function.

For example, adipocyte fatty acid-binding protein (A-FABP) is an adipokine that is involved in fatty acid metabolism and may contribute to the development of insulin resistance and type 2 diabetes. Chemerin is an adipokine that is involved in adipogenesis and inflammation and may contribute to the development of obesity and metabolic disorders.

Omentin is an adipokine that has anti-inflammatory effects and may have a protective role in cardiovascular disease.

Conclusion

Adipose tissue is no longer regarded as an inert storage site for excess energy but is now recognized as a critical component of the endocrine system.

Adipokines secreted by adipose tissue play a crucial role in regulating appetite, glucose and lipid metabolism, insulin sensitivity, inflammation, blood pressure, and angiogenesis. Dysregulation of adipokines has been associated with metabolic disorders such as obesity, type 2 diabetes, cardiovascular disease, and cancer.