The big picture: Leptin is a hormone predominantly made by adipose (fat) cells and enterocytes in the small intestine. It helps to regulate energy balance by inhibiting hunger, which in turn diminishes fat storage in adipocytes. Often referred to as the “satiety hormone,” leptin communicates with the hypothalamus in the brain to signal when the body has enough energy stores, thus reducing appetite and promoting energy expenditure. Dysregulation of leptin production or signaling can lead to obesity, metabolic disorders, and other health issues.

Role in appetite and weight regulation: Leptin’s primary function is to inform the brain about the status of fat stores in the body. When fat stores increase, leptin levels rise, signaling the brain to reduce appetite and increase energy expenditure. Conversely, when fat stores are low, leptin levels decrease, leading to increased hunger and reduced energy expenditure to conserve energy. This feedback loop is crucial for maintaining energy homeostasis and a healthy body weight.

Leptin resistance and obesity: In many cases of obesity, leptin levels are elevated, but the body becomes resistant to its effects—a condition known as leptin resistance. This means that despite high levels of leptin, the brain does not receive the signal to reduce appetite or increase energy expenditure. Factors contributing to leptin resistance may include chronic inflammation, high levels of circulating fatty acids, and disruptions in leptin transport across the blood-brain barrier. Leptin resistance can perpetuate weight gain and make weight loss more challenging.

High leptin levels: Elevated leptin levels are commonly found in obese individuals due to increased fat mass. High leptin levels may indicate leptin resistance and are associated with an increased risk of cardiovascular diseases, insulin resistance, and metabolic syndrome.

Low leptin levels: Low levels are rare but can occur in conditions of low body fat, such as in extreme weight loss, anorexia nervosa, or lipodystrophy. Low leptin levels can lead to increased appetite, decreased energy expenditure, and can impact reproductive function, bone health, and immune responses.