Incretins

Incretins are gut-derived peptide hormones secreted in response to oral nutrient intake that potentiate glucose-stimulated insulin secretion from pancreatic beta-cells. The two principal human incretins are glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide 1 (GLP-1). Collectively, incretins account for the "incretin effect" — the observation that oral glucose produces substantially greater insulin secretion than intravenous glucose at matched plasma glucose.

The incretin effect

The phenomenon was formally described by Elrick, Perley, McIntyre, and others in the 1960s: when matched glucose doses were administered orally versus intravenously (with plasma glucose profiles clamped to equivalent trajectories), oral glucose produced 50-70% greater insulin secretion. The gap is attributable to incretin hormones released during oral but not intravenous glucose exposure. This effect is blunted in type 2 diabetes, contributing to postprandial hyperglycemia — a mechanistic observation that motivated incretin-based diabetes therapeutics.

GIP — glucose-dependent insulinotropic polypeptide

GIP is a 42-amino-acid peptide secreted from K-cells in the duodenum and proximal jejunum in response to nutrient delivery, particularly glucose and fat. GIP binds beta-cell GIPR to potentiate glucose-stimulated insulin secretion. GIP secretion is preserved in type 2 diabetes but its insulinotropic action is markedly reduced — "GIP resistance" — which contributes to postprandial hyperglycemia. GIP has additional actions on adipose tissue (enhancing lipid storage) and bone (anti-resorptive effects).

GLP-1 — glucagon-like peptide 1

GLP-1 is produced from the proglucagon gene by tissue-specific post-translational processing in L-cells, yielding GLP-1 as a 30- or 31-amino-acid active peptide (GLP-1[7-36] amide or GLP-1[7-37]). GLP-1 is secreted from ileal and colonic L-cells in response to nutrients, with a characteristic bimodal timing: early (within 15-30 min, likely neural reflex) and late (1-2 hours, direct nutrient exposure to distal L-cells). GLP-1 actions include: (1) glucose-dependent insulin secretion potentiation; (2) glucagon secretion suppression; (3) gastric emptying slowing; (4) appetite reduction via central and vagal pathways; (5) beta-cell preservation effects in animal models.

Half-life and DPP-4

Native GIP and GLP-1 have very short plasma half-lives (~7 min for GIP, ~2 min for GLP-1) due to rapid degradation by dipeptidyl peptidase 4 (DPP-4), which cleaves the N-terminal dipeptide. DPP-4 inhibitors (sitagliptin, saxagliptin, linagliptin, alogliptin) increase endogenous incretin concentrations and produce modest glucose-lowering effects. GLP-1 receptor agonists with structural modifications resistant to DPP-4 (exenatide, liraglutide, semaglutide) achieve much greater glucose-lowering and weight-reducing effects than DPP-4 inhibitors.

GLP-1 receptor agonists

The pharmacological class of GLP-1 receptor agonists has transformed type 2 diabetes and obesity treatment over the past 15 years. Key agents and milestones: exenatide (2005, first approved); liraglutide (2010); dulaglutide (2014); semaglutide (2017 for diabetes, 2021 for obesity as Wegovy); oral semaglutide (2019). The STEP trials of semaglutide 2.4 mg/week demonstrated 15-18% weight loss at 68 weeks, comparable to bariatric surgery in effect size.

Dual and triple agonists

Tirzepatide, a dual GLP-1/GIP receptor agonist approved in 2022 for diabetes and 2023 for obesity, achieves even greater weight reduction (~20-25% at the highest dose). This exceeded expectations about GIP, given GIP's apparent dispensability in type 2 diabetes — suggesting GIP agonism contributes meaningfully to the incretin effect when provided pharmacologically even when endogenous GIP is ineffective. Triple agonists (retatrutide targeting GLP-1, GIP, and glucagon receptors) are in phase 3 trials as of 2026, with preliminary data showing even greater weight reduction.

Meal composition and endogenous incretin response

Protein, fiber, and fat stimulate greater endogenous GLP-1 and GIP secretion than refined carbohydrate alone. "Protein-first" and high-fiber meal patterns therefore produce larger endogenous incretin responses, contributing to glucose homeostasis and satiety. This physiology supports dietary approaches to glycemic control that complement rather than depend on pharmacological intervention.

Safety considerations

GLP-1 receptor agonists are generally well tolerated. The most common adverse effects are gastrointestinal (nausea, vomiting, diarrhea), typically dose-limiting at initiation. Rare but significant concerns include pancreatitis (signal unclear in large databases), medullary thyroid carcinoma risk in individuals with MEN2 or family history (preclinical signal, not confirmed in human data), and gallbladder disease. Delayed gastric emptying has raised concerns about anesthetic aspiration risk with perioperative use.