The slimming hormone is a naturally occurring hormone called leptin. Leptin is secreted by fat cells and is responsible for signaling the brain that the body has enough energy stored in the form of fat. When the brain receives this signal, it suppresses feelings of hunger and encourages the body to use the fat as fuel instead of storing it. Leptin is also involved in regulating other aspects of metabolism, such as energy expenditure and fat storage. In some cases, people may have difficulty producing enough leptin, which can lead to obesity.