It is now well established that gastrointestinal hormones regulate multiple functions of the gastrointestinal tract and its glands, eg, motility, secretion, and absorption. In addition, gastrointestinal and pancreatic hormones control the disposal of the absorbed nutrients.

Shortly after the turn of this century, it was postulated that gut factors influence blood glucose levels following a carbohydrate load. After the recognition of” secretin” as a regulator of pancreatic secretion, it was speculated that signals arising in the gut after ingestion of nutrients may affect endocrine responses and the disposal of carbohydrates.’Moore et aL2 unsuccessfully tried to treat overt diabetes mellitus by injection of gut extracts. The term” incretin” was introduced by Zunz and LaBarre3 to describe humoral activity of the gut that might enhance the endocrine secretion of the pancreas. These authors observed two active principles in small gut extracts: one component stimulated the exocrine pancreatic secretion and was therefore called” excretin”; the other component augmented the endocrine response of the pancreas and lowered the blood glucose levels. This factor was called “in~ retin.”~ At the same time Heller5fj