*Geek Box: Incretin Hormones
The term “incretin hormones” describes GLP-1 and GIP, two hormones that are secreted by the gastrointestinal tract, specifically in the small intestine. These hormones are nutrient-sensing hormones and are secreted in response to the passing of food from the stomach to the small intestine. Both hormones stimulate the response of insulin to the elevation of glucose in the blood.
Incretin hormones make the most significant contribution to the signalling for the secretion of insulin, even more than blood glucose itself. Although both GLP-1 and GIP exert this effect, it is important to note that they both have independent mechanisms. GIP primarily stimulates insulin secretion in a manner that is dose-dependent to the level of glucose in the blood [which GLP-1 also does].
However, GIP has no effect on gastric emptying, whereas GLP-1 does by slowing the rate of gastric emptying, , i.e., the rate at which food leaves the stomach into the small intestine for further digestion and nutrient absorption. GLP-1 also reduces gastric acid secretions and slows the overall rate of intestinal transit. This helps control the overall digestive process, but importantly, can thus also result in a slower rate of glucose absorption into the blood.
It is thought that this role of GLP-1 provides a means of controlling overall gastrointestinal motility, to ensure the proper passage of food through the gastrointestinal tract. We also know that incretin hormones follow a circadian rhythm and are amplified in the morning, which may explain the beneficial effect on morning energy intake on postprandial glucose responses.