The Science Behind the Weight Loss Numbers: STEP and SURMOUNT Trial Results Explained

When healthcare providers and media reports cite statistics like "15% weight loss with semaglutide" or "up to 22.5% with tirzepatide," these numbers come from specific, rigorously designed Phase 3 randomized controlled clinical trials. Understanding what these trials measured, how they were designed, who the participants were, and what the numbers actually mean in practice empowers patients to have more informed conversations with their healthcare providers and set realistic expectations for their own treatment journey.

What Is a Phase 3 Clinical Trial?

The drug approval process in the United States requires manufacturers to demonstrate a new medication’s safety and efficacy through progressively larger clinical trials. Phase 3 trials are the pivotal studies that directly support FDA approval: they involve large numbers of participants (hundreds to thousands), are typically randomized (participants are randomly assigned to drug or placebo) and double-blinded (neither participants nor investigators know which treatment was received), and measure predefined primary and secondary endpoints over a defined duration (typically 52–72+ weeks for weight management trials).

The results from Phase 3 trials represent the most rigorous available evidence for drug efficacy and form the scientific basis for FDA approval decisions and prescribing information claims.

The STEP Trial Program: Semaglutide 2.4 mg

The Semaglutide Treatment Effect in People with Obesity (STEP) trial program was a series of Phase 3 trials conducted by Novo Nordisk supporting the approval of Wegovy. The five trials differed in their populations and co-interventions:

• STEP 1 — The Pivotal Weight Loss Trial: Published in the New England Journal of Medicine (Wilding et al., 2021). Enrolled 1,961 adults with obesity (BMI ≥30) or overweight (BMI ≥27) with at least one comorbidity, without type 2 diabetes. Participants received semaglutide 2.4 mg or placebo weekly plus a lifestyle intervention program of diet and exercise counseling for 68 weeks.
  Results: Semaglutide group lost a mean of 14.9% of initial body weight (~15.3 kg) vs. 2.4% for placebo. Differences in key endpoints: 69.1% of semaglutide participants vs. 34.4% placebo achieved ≥10% weight loss; 50.5% vs. 13.8% achieved ≥15%; 32.0% vs. 5.0% achieved ≥20%.
• STEP 2 — Type 2 Diabetes: 1,210 participants with obesity and type 2 diabetes. Semaglutide 2.4 mg produced 9.6% weight loss vs. 3.4% placebo over 68 weeks, alongside significant HbA1c reductions. Weight loss was lower than STEP 1, consistent with known biology: patients with type 2 diabetes may lose less weight on GLP-1 therapy than non-diabetic individuals.
• STEP 3 — Intensive Behavioral Therapy: Combined semaglutide with an intensive 30-session behavioral therapy program. Mean weight loss was 16.0% over 68 weeks — slightly higher than STEP 1, supporting the additive benefit of combining pharmacotherapy with structured behavioral support.
• STEP 4 — Withdrawal Study: Participants who completed 20 weeks of semaglutide titration were then randomized to continue semaglutide or switch to placebo. The withdrawal group regained approximately 6.9 percentage points of body weight by week 68, while the continuation group maintained their loss. This confirmed the chronic nature of pharmacotherapy need.
• STEP 5 — Two-Year Durability: Extended follow-up to 104 weeks. The semaglutide group maintained a 15.2% weight reduction at 2 years, demonstrating sustained efficacy with continued treatment.

💡 💡 Clinical Pearl

Clinical trial results are always reported as mean (average) outcomes — which means half of trial participants lost more than the reported average, and half lost less. In STEP 1, while the mean weight loss was 14.9%, approximately 32% of semaglutide participants achieved ≥20% weight loss — extraordinary outcomes that rival bariatric surgery results. Conversely, some patients lost only 5–8% despite full adherence. Individual response variation is substantial, which is why regular monitoring and realistic, personalized expectations are essential in clinical practice.

The SURMOUNT Trial Program: Tirzepatide

The SURMOUNT clinical trial program was conducted by Eli Lilly supporting the approval of Zepbound for weight management:

• SURMOUNT-1 — The Pivotal Weight Loss Trial: Published in the New England Journal of Medicine (Jastreboff et al., 2022). Enrolled 2,539 adults with obesity or overweight with a weight-related comorbidity (but without type 2 diabetes) over 72 weeks.
  Results at 72 weeks:
  • Tirzepatide 5 mg: mean weight loss 15.0% vs. 3.1% placebo
  • Tirzepatide 10 mg: mean weight loss 19.5%
  • Tirzepatide 15 mg: mean weight loss 20.9%
  • Proportion achieving ≥20% weight loss on 15 mg: 56.8%
  These results represented the highest mean weight loss ever reported in a randomized controlled trial of pharmacotherapy at the time of publication.
• SURMOUNT-2 — Type 2 Diabetes: 938 participants with type 2 diabetes and obesity. Tirzepatide 15 mg produced 14.7% weight loss vs. 3.2% placebo over 72 weeks — greater than semaglutide 2.4 mg in the equivalent STEP 2 population.
• SURMOUNT-3 — Lifestyle Lead-In: Participants underwent 12 weeks of intensive lifestyle intervention before randomization. Subsequent addition of tirzepatide produced further weight loss, yielding a total average reduction of approximately 26.6% from original body weight by 72 weeks — approaching the magnitude of bariatric surgery outcomes.
• SURMOUNT-4 — Withdrawal: Confirmed rebound weight regain upon tirzepatide discontinuation, paralleling the STEP 4 findings for semaglutide.

Understanding the Numbers: What Does 15–22% Weight Loss Mean in Practice?

Translating clinical trial percentages into real-world context helps patients appreciate what these results mean:

• A 15% weight loss in a patient weighing 100 kg (220 lbs) represents a reduction of 15 kg (33 lbs).
• A 20% weight loss in a patient weighing 120 kg (264 lbs) represents a reduction of 24 kg (53 lbs).
• Even a 5–10% weight reduction — considered the clinical threshold for meaningful health benefit — produces measurable improvements in blood pressure, blood glucose, sleep apnea severity, joint pain, and lipid profiles.
• Weight loss of ≥15% has been associated with remission of type 2 diabetes in some patients, reduction in cardiovascular risk markers, and improvement in quality of life scores.

Trial Limitations to Consider

While Phase 3 trial data is the gold standard for efficacy evidence, several factors limit the direct applicability of trial results to all patients in clinical practice:

• Trial populations are selected: Participants in clinical trials are screened to exclude many comorbidities and medications. Real-world patients are often older, more medically complex, and more racially/ethnically diverse than trial populations.
• Adherence rates are higher in trials: Clinical trial participants receive more intensive support, monitoring, and follow-up than typical patients in community practice. Real-world adherence to GLP-1 RA therapy at 1 year is considerably lower than in trials, partially due to cost, side effects, and access.
• Lifestyle programs vary: All STEP and SURMOUNT trials included some form of dietary and exercise counseling alongside medication. The degree of lifestyle support in real-world clinical practice varies widely.

For more information about how these trials led to FDA approval, see Who Is a Candidate for GLP-1 RA Medications?. For a direct comparison of semaglutide and tirzepatide trial outcomes, see Wegovy vs. Mounjaro: A Clinical Comparison.

💡 Frequently Asked Questions (FAQ)

Q1: Were these clinical trials funded by the pharmaceutical industry?
A1: Yes — the STEP trials were funded by Novo Nordisk (manufacturer of Wegovy) and the SURMOUNT trials by Eli Lilly (manufacturer of Zepbound). Industry funding is standard for Phase 3 drug trials, but the trials were published in peer-reviewed journals (NEJM) and underwent independent academic scrutiny. Regulatory agencies independently review all trial data submitted in drug approval applications.

Q2: How do the results of these trials compare to results from bariatric surgery?
A2: Bariatric surgery produces average weight loss of approximately 20–35% depending on the procedure (gastric band: ~20%; sleeve gastrectomy: ~25%; gastric bypass: ~30–35%). The SURMOUNT-3 trial result of approximately 26.6% total weight loss approaches sleeve gastrectomy outcomes for a meaningful proportion of patients, though individual results vary widely with both approaches. Bariatric surgery still generally produces greater and more durable weight loss than current pharmacotherapy in most populations.

Q3: Will I definitely lose as much weight as in the trials?
A3: Not necessarily. Trial averages represent population means with substantial individual variation. Factors including genetics, baseline metabolic rate, dietary adherence, physical activity, the presence of type 2 diabetes (which attenuates weight loss response), and medication tolerability all influence individual outcomes. Setting realistic expectations — aiming for ≥5–10% weight loss as an initial meaningful goal — while remaining open to the possibility of greater loss is the most balanced approach.

📚 References & Sources

1. Wilding, J.P.H., et al. (2021). Once-weekly semaglutide in adults with overweight or obesity. New England Journal of Medicine, 384(11), 989–1002.
2. Jastreboff, A.M., et al. (2022). Tirzepatide once weekly for the treatment of obesity. New England Journal of Medicine, 387(3), 205–216.
3. American Diabetes Association. (2024). Obesity and Weight Management for the Prevention and Treatment of Type 2 Diabetes. Diabetes Care, 47(Suppl.1).