92.8% of weight loss from fat. 67% less muscle loss. The Phase 2b results that could change everything.
The biggest fear men have about GLP-1 medications isn't the nausea. It's the muscle loss.
And the concern is legitimate. Research consistently shows that 25-40% of weight lost on GLP-1 therapy comes from lean tissue — not fat. For men who lift, compete, or simply want to look and feel strong, that's a painful trade-off. You lose 50 pounds, but 15 of them were muscle.
The BELIEVE trial, published in Nature Medicine in March 2026, may have found the solution.
BELIEVE is a Phase 2b randomized, double-blind, placebo-controlled trial that tested bimagrumab — a first-in-class monoclonal antibody that blocks activin type II receptors — both alone and in combination with semaglutide (Wegovy) in 507 adults with obesity.
The trial was conducted at 26 sites across the United States, Australia, and New Zealand over 72 weeks. It was designed to answer a specific question: can we preserve muscle mass during GLP-1-induced weight loss by adding a muscle-sparing drug?
The answer: yes — dramatically.
| Metric | Semaglutide Alone | Bimagrumab Alone | Combination |
|---|---|---|---|
| Total weight loss | -15.7% | -10.8% | -22.1% |
| Weight loss from fat | 71.8% | 100% | 92.8% |
| Lean mass change | -7.9% | +2.5% | -2.6% |
| ≥20% weight loss achieved | 25.0% | 10.9% | 69.8% |
| ≥30% fat mass reduction | 36.4% | 50.0% | 94.0% |
Read those lean mass numbers again. Semaglutide alone: -7.9% lean mass loss. The combination: only -2.6%. That's 67% less muscle loss — while losing significantly more total weight and almost exclusively targeting fat.
Bimagrumab blocks activin type II receptors (ActRIIA and ActRIIB), which are the signaling pathways that myostatin and activin A use to limit muscle growth. When you block these signals, two things happen simultaneously:
Muscles get the green light to grow. Myostatin normally acts as a brake on muscle protein synthesis. Remove the brake, and skeletal muscle can grow even during a caloric deficit.
Fat cells get more active. Activin signaling also affects adipose tissue. Blocking it promotes lipid mobilization — essentially making it easier for your body to access and burn stored fat.
Combined with semaglutide's appetite suppression and central metabolic effects, you get a one-two punch: semaglutide reduces food intake and improves metabolic health from the top down, while bimagrumab protects and builds muscle from the bottom up.
This combination addresses the single biggest limitation of current GLP-1 therapy for men. Consider:
Muscle mass = metabolic engine. Lean tissue burns more calories at rest than fat. Losing muscle during weight loss slows your metabolism and makes weight regain more likely. Preserving muscle is how you make weight loss stick.
Functional strength matters. Losing 7-8% of your lean mass doesn't just change how you look. It affects grip strength, mobility, injury resilience, and your ability to perform daily activities as you age.
Body composition > scale weight. The combination therapy in BELIEVE didn't just produce more weight loss — it produced fundamentally different weight loss. Nearly 95% of participants achieved at least a 30% fat mass reduction. That's body recomposition at a level previously only seen with aggressive resistance training programs.
Beyond weight and body composition, the combination showed striking metabolic improvements:
While bimagrumab isn't yet available, current GLP-1 options can help you start losing fat now. Compare providers below.
Compare Providers →Here's the reality check: bimagrumab is not FDA-approved, and the BELIEVE trial was Phase 2b — not Phase 3. A Phase 3 trial is required for FDA approval, and that hasn't been announced yet.
Eli Lilly, which acquired bimagrumab, has one ongoing study evaluating the drug in combination with tirzepatide (Zepbound). Initial results from that trial are expected around April 2026. However, Lilly also terminated a separate bimagrumab trial in September 2025, which adds uncertainty.
The FDA has also signaled that muscle-preserving add-on drugs may need to demonstrate additional weight loss (not just muscle preservation) to gain approval — raising the bar for these compounds.
Realistic timeline: if Phase 3 trials proceed and succeed, bimagrumab could be available in 2028-2029 at the earliest.
You don't need bimagrumab to preserve muscle on GLP-1 therapy. The evidence-backed strategy:
The BELIEVE trial proves that the muscle loss problem in GLP-1 therapy is solvable. The combination of bimagrumab and semaglutide produced 22% weight loss with 93% of it coming from fat — a result that redefines what's possible in pharmacological weight management.
For men, this is the data point that matters most. Losing weight without losing muscle isn't a fantasy — it's a clinical reality waiting for FDA approval. In the meantime, current GLP-1 therapy combined with resistance training and adequate protein can get you most of the way there.