Adipokines

Clinical information

As an endocrine organ, the fatty tissue secretes a variety of hormones called adipokines. In severely overweight individuals, the adipocyte function is deregulated, resulting in altered levels of adipokines being produced. Leptin and adiponectin are the most frequent adipokines. In their role as hormone messengers, they represent a link between metabolic disorders and obesity.

Leptin is synthesised and secreted by fat cells in the white fatty tissue. However, small amounts of leptin have also been found in other body tissues, such as the placenta, muscles, bone marrow and brain. Leptin circulates in the blood in both free and protein-bound forms, but only the free form is biologically active. Leptin can cross the blood-brain barrier via receptor-mediated transport and enter the central nervous system (CNS). In the brain’s satiety centre, leptin influences the appetite and the metabolism by regulating the synthesis of certain neuropeptides. High levels of leptin activate the part of the brain that regulates satiety. Low leptin levels occur during fasting and signal the body to take in food.

This is different for adiponectin. The synthesis of adiponectin in the fatty tissue of obese people is downregulated, so blood levels are lower than in normal-weight people.

The higher the proportion of fatty tissue in the body, the more leptin is produced. In the case of leptin resistance, the satiety centre in the brain no longer responds to the hormone. Leptin resistance is both a cause and a consequence of overweight and obesity.

Adiponectin, on the other hand, is insulin-sensitising and anti-inflammatory. Low adiponectin and increased leptin levels in obesity cause a state of low-grade chronic inflammation, which promotes the development of insulin resistance and type 2 diabetes, high blood pressure, atherosclerosis and other cardiovascular diseases.

Diagnostics

Measurement of adiponectin levels can provide useful additional information in the diagnostics and treatment of lipid metabolism disorders, particularly in obesity-related diseases. In this context, some publications recommend looking at the ratio of leptin and adiponectin in particular, rather than just the concentration of one of the hormones.

The correlation between leptin levels and the amount of body fat also plays an important role in the diagnosis of lipodystrophy, which is abnormal growth of subcutaneous fatty tissue.

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