Leptin is a hormone produced by fat cells that helps regulate energy balance by signaling the brain to reduce appetite and increase energy expenditure. When functioning properly, leptin levels rise after eating, signaling the body to feel full. However, in some individuals, the brain does not respond effectively to leptin, leading to a condition known as leptin resistance. In leptin resistance, despite high levels of leptin, the brain fails to recognize the hormone's signals, resulting in continued hunger and reduced energy use. This can contribute to weight gain and make it challenging for individuals to lose weight, as their bodies do not respond appropriately to the signals that indicate they should stop eating.