Definition

GLP-1 (glucagon-like peptide-1) is a peptide hormone secreted primarily by L-cells in the lower small intestine and colon in response to food intake. It plays a central role in glucose homeostasis and appetite regulation.

What natural GLP-1 does

In healthy physiology, GLP-1 is released after meals and produces several effects:

  • Stimulates insulin secretion from pancreatic β-cells (glucose-dependent, so it works only when glucose is elevated — important for safety)
  • Suppresses glucagon release (glucagon raises glucose; suppressing it lowers postprandial glucose)
  • Slows gastric emptying (delays carbohydrate absorption, reduces glucose spikes)
  • Increases satiety and reduces appetite via central nervous system effects
  • Has a short half-life (~2 minutes) because it’s rapidly degraded by the enzyme DPP-4

How GLP-1 medications work

The diabetes and obesity medications semaglutide (Ozempic, Wegovy, Rybelsus), liraglutide (Saxenda, Victoza), tirzepatide (Mounjaro, Zepbound — a dual GLP-1/GIP agonist), and others are GLP-1 receptor agonists. They:

  • Bind to the same receptors as natural GLP-1, but with structural modifications that resist DPP-4 degradation
  • Have half-lives of days rather than minutes (allowing weekly dosing)
  • Activate GLP-1 effects more potently and continuously than natural hormone

Clinical effects

Average weight loss outcomes in pivotal trials:

MedicationAverage weight lossDuration
Liraglutide (Saxenda)~8%56 weeks
Semaglutide (Wegovy)~15%68 weeks
Tirzepatide (Zepbound)~22%72 weeks

These are placebo-subtracted, observational averages — individual results vary substantially.

Side effects

Common (typically dose-dependent, often subside with dose titration):

  • Nausea (most common, especially at initiation)
  • Vomiting, diarrhea, constipation
  • Decreased appetite
  • Injection site reactions

Less common but clinically significant:

  • Pancreatitis (rare but real)
  • Gallbladder disease
  • Possible thyroid C-cell tumor risk (boxed warning, based on rodent data)
  • Muscle loss with rapid weight loss (a real concern if protein intake and resistance training aren’t supported)
  • “Ozempic face” — accelerated facial volume loss from rapid fat loss

Clinical considerations

GLP-1 medications are legitimate medical interventions for:

  • Type 2 diabetes (FDA-approved for years)
  • Obesity (BMI ≥30, or ≥27 with comorbidities) per current guidelines

They’re not:

  • Casual weight-loss tools without medical supervision
  • A substitute for behavioral and nutritional intervention
  • Appropriate for everyone

Discuss with a physician familiar with these medications — not telehealth services with minimal oversight. For broader context on midlife weight management, see our perimenopause weight gain guide.

Sources

  1. 1.Wilding JPH et al. Once-Weekly Semaglutide in Adults with Overweight or Obesity. NEJM, 2021. PMID: 33567185
  2. 2.Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity. NEJM, 2022. PMID: 35658024
  3. 3.Drucker DJ. Mechanisms of Action and Therapeutic Application of Glucagon-like Peptide-1. Cell Metabolism, 2018. PMID: 29320702