Final answer:
The hormone ghrelin, produced in the stomach, signals the brain to start eating and is often referred to as the 'hunger hormone'.
Step-by-step explanation:
The hormone produced by the stomach that signals the brain to start eating is ghrelin. Ghrelin is known as the "hunger hormone" because it is released when our stomachs are empty, triggering hunger pangs and sending a chemical message to the brain that initiates feeding behavior. In contrast, hormones like leptin are involved in signaling satiety, which reduces the urge to eat when the stomach is full. Other gastrointestinal hormones, such as gastrin, cholecystokinin, and secretin, play various roles in the digestive process but are not directly responsible for inducing hunger.