Ghrelin is a hormone produced by enteroendocrine cells of the gastrointestinal tract, especially the stomach. It is often called the "hunger hormone" because it increases the drive to eat. Ghrelin is released when the stomach is empty and signals the brain to increase appetite. It plays a role in regulating energy homeostasis, which adjusts both energy input and output, and is reflected in body weight. Ghrelin also promotes fat storage, stimulates the release of growth hormones from the pituitary gland, and plays a role in controlling sugars and how the body releases insulin. It has protective effects on the cardiovascular system and may play a key role in mediating memory and stress. Ghrelin levels are highest before meals when hungry, returning to lower levels after mealtimes.