Background
Obesity is a chronic disease associated with serious comorbidities including type 2 diabetes, cardiovascular disease, and musculoskeletal disorders. Prior to GLP-1 receptor agonists, pharmacological options for weight management were limited and rarely produced >10% sustained body weight reduction.
Semaglutide, originally developed for type 2 diabetes at 0.5–1.0 mg weekly doses, demonstrated substantial weight loss as a secondary endpoint in diabetes trials. The STEP program was designed to evaluate higher doses (2.4 mg/week) specifically for obesity management.
Methods
The STEP-1 trial was a phase 3, double-blind, randomized, placebo-controlled trial conducted at 129 sites in 16 countries. Participants were adults with a BMI ≥30 kg/m² (or ≥27 with at least one weight-related comorbidity) without type 2 diabetes.
Participants received once-weekly subcutaneous semaglutide 2.4 mg or placebo, escalated over 16 weeks, for a total of 68 weeks. All participants received lifestyle intervention support.
Primary endpoints:
- Percentage change in body weight from baseline
- Proportion achieving ≥5% weight loss
Key Findings
| Outcome | Semaglutide 2.4 mg | Placebo | p-value |
|---|---|---|---|
| Mean weight change | −14.9% | −2.4% | <0.001 |
| ≥5% weight loss | 86.4% | 31.5% | <0.001 |
| ≥10% weight loss | 69.1% | 12.0% | <0.001 |
| ≥15% weight loss | 50.5% | 4.9% | <0.001 |
| Mean kg lost | ~15.3 kg | ~2.6 kg | — |
Clinical Significance
STEP-1 established semaglutide 2.4 mg as the most effective anti-obesity medication tested in a phase 3 trial up to that point—producing weight loss that approaches outcomes seen with bariatric surgery. The trial led to FDA approval of semaglutide (Wegovy) for chronic weight management in June 2021.
The finding that ~50% of participants lost ≥15% of body weight was particularly significant, as this threshold is associated with meaningful improvements in obesity-related comorbidities including hypertension, dyslipidemia, and sleep apnea.
Limitations
- No active comparator arm
- Predominantly white (75%) and female (74%) cohort limits generalizability
- Weight regain observed after discontinuation (not captured in this trial; addressed in STEP-4)
- Lifestyle intervention co-intervention may limit real-world applicability