GLP-1

What Is GLP-1?#

Glucagon-like peptide-1 (GLP-1) is a 30-amino-acid incretin hormone produced by intestinal L-cells in response to nutrient ingestion. It plays a central role in glucose homeostasis by stimulating insulin secretion in a glucose-dependent manner, suppressing inappropriate glucagon release, slowing gastric emptying, and promoting satiety. The discovery and therapeutic exploitation of GLP-1 biology has led to one of the most important classes of drugs developed in the 21st century: the GLP-1 receptor agonists (GLP-1 RAs), which have transformed the treatment of type 2 diabetes and obesity.

The active forms of GLP-1 are GLP-1(7-36)amide and GLP-1(7-37), both cleaved from the proglucagon precursor protein. In the circulation, native GLP-1 is rapidly inactivated by the enzyme dipeptidyl peptidase-4 (DPP-4), giving it a plasma half-life of only 1-2 minutes. This rapid degradation has driven the development of DPP-4 resistant GLP-1 analogs and long-acting formulations that maintain GLP-1 receptor activation for hours, days, or even weeks.

Mechanism of Action#

GLP-1 exerts its effects by binding to the GLP-1 receptor (GLP-1R), a class B G protein-coupled receptor expressed on pancreatic beta cells, the gastrointestinal tract, the central nervous system, the heart, and other tissues. Key actions include:

1. Insulin secretion: GLP-1 potentiates glucose-stimulated insulin secretion from pancreatic beta cells. Crucially, this effect is glucose-dependent, meaning it operates only when blood glucose is elevated, greatly reducing the risk of hypoglycemia compared to insulin or sulfonylureas.

2. Glucagon suppression: GLP-1 suppresses inappropriate postprandial glucagon secretion from pancreatic alpha cells, reducing hepatic glucose output.

3. Gastric emptying: GLP-1 slows the rate of gastric emptying, reducing the rate of glucose absorption and contributing to postprandial glucose control.

4. Satiety and appetite reduction: GLP-1 receptors in the hypothalamus and brainstem mediate appetite suppression and promote a feeling of fullness. This effect is a major contributor to the weight loss observed with GLP-1 receptor agonists.

5. Beta cell preservation: In preclinical models, GLP-1 promotes beta cell proliferation and inhibits apoptosis, though the clinical relevance of these effects in humans is still being investigated.

The Incretin Effect#

GLP-1 was identified through the study of the "incretin effect" — the observation that oral glucose produces a greater insulin response than intravenous glucose at the same blood glucose levels. This difference is attributed to gut-derived hormones, primarily GLP-1 and glucose-dependent insulinotropic polypeptide (GIP), which are released from intestinal cells during meal absorption and amplify the pancreatic insulin response. In type 2 diabetes, the incretin effect is substantially diminished, providing a rational basis for GLP-1-based therapies.

GLP-1 Receptor Agonists#

The rapid degradation of native GLP-1 by DPP-4 necessitated the development of resistant analogs for therapeutic use. Major approved GLP-1 receptor agonists include:

• Exenatide (Byetta/Bydureon): First-in-class, based on exendin-4 from Gila monster venom; twice-daily or weekly formulations
• Liraglutide (Victoza/Saxenda): Acylated GLP-1 analog with albumin binding; once-daily injection for diabetes (Victoza) or obesity (Saxenda)
• Semaglutide (Ozempic/Wegovy/Rybelsus): Long-acting GLP-1 analog; weekly injection (Ozempic/Wegovy) or oral tablet (Rybelsus)
• Dulaglutide (Trulicity): GLP-1-Fc fusion protein; weekly injection
• Tirzepatide (Mounjaro/Zepbound): Dual GIP/GLP-1 receptor agonist; weekly injection representing the next generation

These drugs have demonstrated robust efficacy in reducing HbA1c (1-2% reductions), promoting weight loss (5-20% of body weight depending on agent and dose), and in some cases reducing cardiovascular events.

Clinical Impact#

GLP-1 receptor agonists represent one of the largest therapeutic advances in endocrinology. Major clinical milestones include:

• Cardiovascular benefit: The LEADER trial (liraglutide) and SUSTAIN-6 trial (semaglutide) demonstrated reduced cardiovascular events in patients with type 2 diabetes at high cardiovascular risk
• Obesity treatment: Semaglutide 2.4 mg weekly (Wegovy) produced mean weight loss of approximately 15% in the STEP trials, transforming the obesity treatment landscape
• Oral GLP-1 agonist: Oral semaglutide (Rybelsus) achieved the previously thought impossible — oral delivery of a peptide drug with clinically meaningful bioavailability
• Dual agonism: Tirzepatide's dual GIP/GLP-1 agonism has shown even greater weight loss (up to 22.5% in clinical trials), opening new therapeutic frontiers

Beyond Diabetes and Obesity#

Research is expanding the potential applications of GLP-1 receptor agonists to include nonalcoholic steatohepatitis (NASH/MASH), heart failure with preserved ejection fraction, chronic kidney disease, and potentially neurodegenerative conditions including Alzheimer's disease. The breadth of GLP-1 receptor expression throughout the body suggests that its therapeutic potential extends well beyond metabolic disease.

Key Research Findings#

Glucagon-like peptide 1 promotes satiety and suppresses energy intake in humans, published in Journal of Clinical Investigation (Flint A et al., 1998; PMID: 9449682):

• GLP-1 infusion reduced ad libitum food intake by 12%

Liraglutide and Cardiovascular Outcomes in Type 2 Diabetes (LEADER), published in New England Journal of Medicine (Marso SP et al., 2016; PMID: 27295427):

• The study demonstrated reduction of 13% in MACE (major adverse cardiovascular events)

Once-Weekly Semaglutide in Adults with Overweight or Obesity (STEP 1), published in New England Journal of Medicine (Wilding JPH et al., 2021; PMID: 33567185):

• The study showed mean weight loss of 14.9% vs 2.4% placebo
• The study demonstrated achieved of 86.4% at least 5% weight loss

Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1), published in New England Journal of Medicine (Jastreboff AM et al., 2022; PMID: 35658024):

• The study showed mean weight loss of 20.9% at 72 weeks
• The study demonstrated of participants on 15 mg achieved of 63% at least 20% weight loss

Related Reading#

• GLP-1 research studies and evidence
• GLP-1 dosing protocols
• GLP-1 side effects profile
• Semaglutide research guide
• Liraglutide research guide