GLP-1 (glucagon-like peptide-1) receptor agonists represent the most transformative class of metabolic drugs developed in decades. Understanding how they work — and why the newer agents are more effective — requires a look at the underlying biology and the evolution from first-generation liraglutide to triple agonists like retatrutide.
What is GLP-1?
GLP-1 is a 30-amino acid incretin hormone secreted by L-cells in the small intestine and colon in response to food intake. Natural GLP-1 has a half-life of only 1-2 minutes because it is rapidly degraded by the enzyme DPP-4.
GLP-1's effects include: slowing gastric emptying (delayed food absorption), stimulating insulin secretion (glucose-dependent), suppressing glucagon, and most importantly for weight loss — acting on GLP-1 receptors in the brain's hypothalamus and brainstem to reduce appetite and food reward.
First Generation: Liraglutide
Liraglutide (Victoza/Saxenda) was the first long-acting GLP-1 analog, engineered to have a 13-hour half-life via fatty acid attachment enabling albumin binding.
Weight loss: ~5-8% in SCALE obesity trials
Dosing: Once daily SubQ injection (a significant drawback vs weekly)
Current role: Still used for T2D (Victoza) but largely superseded for obesity by semaglutide
Second Generation: Semaglutide
Semaglutide (Ozempic/Wegovy) improved on liraglutide with a 7-day half-life (enabling once-weekly dosing) and 94% structural homology to native GLP-1.
Weight loss: 14.9% average in STEP trials at 2.4mg dose
Oral form: Rybelsus (14mg oral daily) achieves ~10% weight loss — the first oral GLP-1
Why more effective than liraglutide: Higher receptor affinity, more sustained exposure, weekly dosing improves adherence
Third Generation: Tirzepatide (Dual Agonist)
Tirzepatide adds GIP (glucose-dependent insulinotropic polypeptide) receptor agonism to GLP-1. GIP works through a different receptor system but synergizes with GLP-1 in the hypothalamus to further suppress appetite.
Weight loss: 20-22% in SURMOUNT trials
Why more effective than semaglutide: The dual pathway amplifies central appetite suppression beyond what GLP-1 alone achieves
Fourth Generation: Triple Agonists
Retatrutide: Adds glucagon receptor agonism to GLP-1+GIP. Glucagon increases energy expenditure (thermogenesis) and fat oxidation — potentially pushing weight loss beyond 24%.
CagriSema (Cagrilintide + Semaglutide): Combines GLP-1 with amylin (a satiety hormone with completely independent central pathways). Phase 3 data expected to show results competitive with tirzepatide.
Survodutide: GLP-1 + glucagon dual agonist. Phase 2b showed superior results to semaglutide at 6 months, also being studied for NASH liver disease.
The trajectory is clear: each new generation adds more independent appetite/metabolic pathways, producing greater efficacy.
Key Takeaways
GLP-1 peptides are the most significant advance in obesity pharmacology since their introduction. The generational progression — from liraglutide to semaglutide to tirzepatide to triple agonists — shows a consistent pattern of increasing efficacy as more complementary metabolic pathways are co-activated. The next 5 years will determine the ceiling of this approach.