Leptin is released from fat cells in our bodies. Its job is to tell your brain when you have enough fat stored and that you don't need to continue eating. The more fat cells that are present, the more leptin is produced, so individuals who have a high BMI also have high levels of leptin.
However, overweight individuals do not respond to their bodies' leptin signals, despite having excess amounts of it. This is known as leptin resistance and is thought to be a biological contributor to obesity. Leptin resistance does not allow leptin's fullness cues to reach the brain, causing the body to think it is starving. This false information will increase hunger and decrease the number of calories burned at rest to save energy and prevent starvation.