What hormone is likely responsible for the increased insulin secretion following the injection of fatty acids into the duodenum?

The hormone responsible for the increased insulin secretion following the injection of fatty acids into the duodenum is glucose-dependent insulinotropic peptide (GIP), also known as gastric inhibitory polypeptide. GIP is secreted by the K cells located in the small intestine in response to the presence of fatty acids and glucose in the intestinal lumen.

When fatty acids are introduced into the duodenum, they stimulate the release of GIP, which in turn enhances insulin secretion from the beta cells of the pancreas. This response is crucial as it helps to manage blood glucose levels by promoting glucose uptake by tissues. GIP plays a significant role in the 'incretin effect,' where oral intake of nutrients leads to greater insulin secretion compared to intravenous delivery of the same nutrients, largely due to the actions of hormones like GIP.

Gastrin primarily stimulates gastric acid secretion and has less direct influence on insulin secretion under these circumstances. Motilin is involved in enhancing gastrointestinal motility and does not significantly affect insulin secretion. Secretin, on the other hand, primarily regulates bicarbonate secretion in the pancreas and does not have a major role in stimulating insulin release in response to fatty acids. Thus, the action of GIP in promoting insulin secretion in the presence of fatty acids