Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1) are incretin hormones secreted in response to food in the gut. These hormones are able to potentiate the glucose stimulated release of insulin in a response called the incretin effect. The incretin effect causes a higher insulin secretion from oral glucose when compared to intravenous glucose. Secretion of GIP and GLP occurs in response to food from enteroendocrine cells into the hepatic portal vein. The magnitude of incretin secretion is related to the energy content of the food, suggesting that other macronutrients can cause incretin release. Therefore although carbohydrate (glucose) causes the release of GLP-1 and GIP, fatty acids may also cause significant release. While the focus of most studies have been on the glucose stimulated release of the incretins, some studies have investigated the effects of fat and protein on incretin release.
For example, researchers1 have investigated the relative contributions of fat and protein to the incretin effect following a mixed meal, as well as individual fat and protein meal. The meals were fed to 8 healthy subjects and at the same time a hyperglycaemic clamp was used to assess the insulin release. The mixed meal consisted of a sandwich that contained dried meat, butter and white bread, and individual fat and protein was in the form of butter or dried meat. The control treatment consisted of the hyperglycaemic clamp in the fasted state. The incretin effect was calculated as the incremental increase in insulin seen after ingestion of the meal, relative to the control. The results showed that there was a significant increase in plasma insulin after ingestion of the sandwich and the butter (203 and 167.4%, respectively), but not following the dried meat.
This suggests that fat causes a significant incretin effect, whereas protein does not. The 2-fold increase in post meal plasma insulin secretion following the mixed meal was expected, and this caused an increase in the glucose rate of disappearance of 130%. The mixed meal GIP response had a peak at a similar time point to the peak for insulin, but it was surprising that the butter also caused a significant rise in GIP and an increase in the glucose rate of disappearance. While the protein (meat only) did not cause an increase in the secretion of insulin above the control, there was a slight increase in the glucose rate of disposal, but very little stimulation of GLP-1 or GIP. However, other types of protein (e.g. whey protein) may affect incretin secretion differently as large variations seem to exist between the way that protein is metabolised.
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