Leptin is secreted by adipocytes and has a dual role, acting as both a hormone and pro-inflammatory adipokine. It plays a key role in energy homeostasis (via receptors in the hypothalamus), by suppressing food intake and increasing energy expenditure, and it therefore acts as a negative feedback regulator of adiposity.
Although leptin levels are higher in obesity, high levels are often accompanied by leptin resistance in the hypothalamus, resulting in symptoms of low leptin.
Low levels (which are rare and usually congenital) are linked to increased appetite, poor satiety, rapid weight gain and difficulty losing weight plus decreased levels of reproductive, thyroid and adrenal hormones. Low levels have also been linked to increased risk of osteoporosis, frequent infections, hyperinsulinaemia, dyslipidaemia and non-alcoholic fatty liver disease.
Similarly, raised levels are linked to obesity, hypothyroidism, fatigue, diabetes, poor appetite regulation, fatty liver disease, cardiovascular disease, depression and poor cognition. Raised levels have also been linked to the development of autoimmunity, possibly through differential effects on T cells.