Which of the following is NOT an official GI hormone?

Insulin is not classified as an official gastrointestinal (GI) hormone because it is primarily produced by the pancreas in response to blood glucose levels, rather than as a part of the digestive process directly associated with the gastrointestinal tract. In contrast, gastrin, secretin, and cholecystokinin (CCK) are recognized as GI hormones because they are secreted from the GI tract and play direct roles in digestive regulation.

Gastrin is released by the stomach and stimulates acid secretion, while secretin and CCK are produced in the small intestine and help regulate the functions of the pancreas and gallbladder. Therefore, insulin doesn't fall into the category of GI hormones since its primary action is related to glucose metabolism rather than the regulation of digestive processes.