GLP-1 Peptide Trials: What 5,838 Subjects Tell Us
7 May 2026 · 7 min read
Three trials. Three consecutive issues of the New England Journal of Medicine. A total of 5,838 subjects. Taken together, STEP-1 (semaglutide), SURMOUNT-1 (tirzepatide), and the retatrutide Phase 2 form the clearest clinical dataset we have on what adding receptor targets does to weight-loss outcomes.
The Three Trials at a Glance
| Peptide | Trial | Year | Subjects | Duration | Mean Weight Loss (highest dose) |
|---|---|---|---|---|---|
| Semaglutide | STEP-1 | 2021 | 1,961 | 68 wk | 14.9% |
| Tirzepatide | SURMOUNT-1 | 2022 | 2,539 | 72 wk | 22.5% |
| Retatrutide | Phase 2 | 2023 | 338 | 48 wk | 24.2% |
What Each Generation Added
Semaglutide (GLP-1 only) established the GLP-1 class as viable for weight loss. 14.9% mean reduction in 68 weeks was unprecedented for a pharmacological agent at the time. The mechanism: appetite suppression via hypothalamic GLP-1 receptors + slowed gastric emptying.
Tirzepatide (GLP-1 + GIP) added GIP receptor agonism. The dual mechanism produced 22.5% in 72 weeks — a 7.6 percentage point improvement over semaglutide. Notably, 57% of tirzepatide participants lost ≥20% of body weight, a threshold previously associated only with bariatric surgery.
Retatrutide (GLP-1 + GIP + glucagon) added glucagon receptor agonism, which increases energy expenditure. The triple mechanism pushed mean weight loss to 24.2% in just 48 weeks — four weeks shorter than STEP-1 and 24 weeks shorter than SURMOUNT-1.
"Each added receptor target has produced a meaningful, not marginal, improvement in outcome."
The 5%, 15%, 20% Thresholds
Clinicians use these thresholds because they correspond to measurable health outcomes — 5% for metabolic markers, 15% for cardiovascular risk, 20% for near-surgical benefit.
| Threshold | Semaglutide | Tirzepatide | Retatrutide |
|---|---|---|---|
| ≥5% weight loss | 86% of subjects | 91% | 83% |
| ≥15% weight loss | 50% | 63% | ~70% (est.) |
| ≥20% weight loss | 30% | 57% | ~50%+ |
What the Data Does Not Tell Us
These trials are not directly comparable: different durations, different doses, different populations. The retatrutide trial enrolled fewer subjects (338 vs. 1,961 and 2,539) and was Phase 2, not Phase 3. Phase 3 retatrutide trials will produce the definitive numbers.
What the data does support clearly: each receptor added has produced compounding, not diminishing, returns across three independent trials in comparable populations.
Reference
Wilding et al. N Engl J Med 2021;384:989. · Jastreboff et al. N Engl J Med 2022;387:205. · Jastreboff et al. N Engl J Med 2023;389:514.