Incretins

Incretins are gut-derived hormones that enhance insulin secretion from pancreatic beta cells in response to nutrient intake, particularly oral glucose. The two main incretins are glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP). Together they contribute to the incretin effect, the higher insulin response observed after oral glucose administration than after an equivalent intravenous glucose dose.

GLP-1 is produced by L cells scattered along the distal small intestine and colon, while GIP is produced by K cells in the duodenum and proximal jejunum. Both hormones are released in response to nutrients, with GLP-1 also exerting glucagon suppression, slowing gastric emptying, and promoting satiety. They act on their respective receptors on pancreatic beta cells to amplify insulin secretion in a glucose-dependent manner. Both incretins are rapidly inactivated by the enzyme dipeptidyl peptidase-4 (DPP-4), giving short circulating half-lives (GLP-1 about 1–2 minutes; GIP about 7 minutes).

Clinical relevance and therapeutics: In type 2 diabetes, the incretin effect is diminished, contributing to postprandial hyperglycemia. Therapeutic strategies include GLP-1 receptor agonists (e.g., exenatide, liraglutide) and DPP-4 inhibitors (e.g., sitagliptin, vildagliptin). These therapies improve glycemic control and may aid weight loss or cardiovascular risk reductions, though common side effects include nausea. Safety signals include debates about pancreatitis risk and, in rodent studies, potential thyroid C-cell tumor concerns with certain GLP-1–based agents.

Research continues into the full physiological roles of incretins and their potential in metabolic disease management.