Ghrelin is the first natural appetite stimulant found to be made outside the brain. It is one of a complex array of natural substances that carry messages between the brain and the digestive system. Secreted by specialized cells in the stomach and the upper part of the small intestine, ghrelin acts on the brain. The hormone ghrelin is produced at high levels in the stomach when the organism is hungry and at decreased levels after meals. As such, this hormone plays a critical role in signaling the brain and has become an important focus of obesity research.