The incretin effect in healthy individuals and those with type 2 diabetes: physiology, pathophysiology, and response to therapeutic interventions

The incretin effect describes the phenomenon whereby oral glucose elicits higher insulin secretory responses than does intravenous glucose, despite inducing similar levels of glycaemia, in healthy individuals. This effect, which is uniformly defective in patients with type 2 diabetes, is mediated by the gut-derived incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). The importance of the incretin effect for the maintenance of glucose homoeostasis is clearly established, and incretin-based therapies are among the most promising new therapies for type 2 diabetes. However, despite the effectiveness of these therapies in many patients, the idea that they restore the incretin effect is a common misconception. In type 2 diabetes, the endocrine pancreas remains responsive to GLP-1 but is no longer responsive to GIP, which is the most likely reason for a reduced or absent incretin effect. Incretin-based drugs, including GLP-1 receptor agonists and dipeptidyl peptidase 4 (DPP-4) inhibitors, stimulate GLP-1 receptors and thus augment insulin secretion in response to both oral and intravenous glucose stimulation, thereby abolishing any potential difference in the responses to these stimuli. These drugs therefore do not restore the defective incretin effect in patients. By contrast, some bariatric surgical procedures enhance GLP-1 responses and also restore the incretin effect in obese individuals with type 2 diabetes. Thus, not all biological actions elicited by the stimulation of GLP-1 receptors lead to quantitative changes to the incretin effect.