Myostatin Inhibitors & GLP-1 Therapy
Myostatin Inhibitors & GLP-1 Therapy: Could Muscle Be Protected During Weight Loss?
A theoretical look at an investigational idea
MedClub by Dr. Jenn | Palm Beach, FL | Research reviewed October 2026
Read This First: A Theoretical Concept
The medications discussed in this article are investigational. None is FDA-approved for weight loss, for muscle preservation, or for use together with GLP-1 medications, and none is available outside of clinical trials.
This post explores a research idea. It is not a description of an available treatment and is not a recommendation. It does not describe a service offered by MedClub.
The Muscle Question With GLP-1 Medications
GLP-1 based medications such as semaglutide and tirzepatide are FDA-approved for chronic weight management in eligible adults. When weight comes down, however, not all of it is fat. In clinical trials that measured body composition with DXA scans, roughly 28–33% of the weight lost on a GLP-1 alone was classified as lean mass.
Lean mass is not the same thing as muscle. It also includes water, organ tissue and connective tissue, so researchers are still working out how much of this change is truly muscle and whether it affects strength or function. That open question is why some investigators are asking whether a second drug could change the balance.
What Is Myostatin?
Myostatin is a protein made mainly in skeletal muscle that acts as a natural brake on muscle growth. Drugs that interfere with this signaling are being studied in several conditions. They fall into two broad groups:
- Selective myostatin antibodies (for example apitegromab and trevogrumab) target myostatin or the pathway that activates it.
- Broader activin receptor blockers (for example bimagrumab) block the receptor that several related proteins use, so their effects reach beyond myostatin alone.
The Theory in Three Steps
STEP 1
GLP-1 therapy lowers appetite and body weight, with some lean mass lost along with fat.
STEP 2
A myostatin pathway inhibitor is added in a research setting to reduce the “brake” on muscle.
THE HYPOTHESIS
Less lean mass may be lost for the same weight reduction. Unproven for long-term health or function.
This is a hypothesis under study, not an established benefit.
What the Research Shows So Far
All of the studies below are phase 2 trials, and most were sponsored by the companies developing the drugs. They are early signals, not proof.
BELIEVE: bimagrumab ± semaglutide (1)(5)
Eli Lilly • 72 weeks • 507 adults with overweight or obesity
AVERAGE WEIGHT LOSS
Lean mass changed by about −2.9% with the combination versus −7.4% with semaglutide alone, and rose about 2.5% with bimagrumab alone. Fat made up roughly 93% of the weight lost in the combination group versus about 72% with semaglutide alone.
Tolerability: muscle spasms, diarrhea and acne were reported more often in bimagrumab groups. Stopping treatment because of side effects was more common in the bimagrumab-alone groups (about 14–21%) than with semaglutide alone (about 4–9%) or placebo (about 4%). Bimagrumab was given intravenously in this trial. A study pairing bimagrumab with tirzepatide is ongoing.
EMBRAZE: apitegromab + tirzepatide (2)(6)
Scholar Rock • 24 weeks • 102 adults • published in Nature Medicine, 2026
Total weight loss was similar between groups. Lean mass made up about 14.6% of weight lost with apitegromab versus 30.2% with placebo, roughly 1.9 kg less lean mass lost. This was a short study, so durability is unknown.
COURAGE: trevogrumab ± garetosmab + semaglutide (3)(7)
Regeneron • about 600 adults • 26-week data reported; completion expected late 2026
Trevogrumab reduced the lean mass lost by about half; about 33% of weight lost on semaglutide alone was lean. The triple-drug group had more side effects, including muscle spasms in roughly 41% of participants versus about 5–9% in other arms.
Other Points Worth Knowing
- In a trial of bimagrumab in people with COPD, muscle mass increased but walking capacity did not improve (9). More muscle on a scan does not automatically mean better function.
- Apitegromab has been under FDA review for spinal muscular atrophy, a different condition (10).
- Other candidates are in early development, including taldefgrobep (Biohaven) and emugrobart (Roche). Enobosarm, a selective androgen receptor modulator, is being studied for muscle preservation through a different mechanism (4)(12).
Potential Upsides vs. Concerns
Swipe sideways on small screens to see the full table.
| Topic | Potential Upside (Theoretical) | Concerns & Unknowns |
|---|---|---|
| Lean mass | Phase 2 trials show less lean mass lost alongside GLP-1 therapy. | Lean mass is not the same as muscle; DXA cannot tell them apart. |
| Body composition | A higher share of weight lost may come from fat. | Whether this improves long-term health outcomes is not established. |
| Physical function | Preserved muscle could, in theory, support strength and mobility. | The COPD trial raised muscle mass without improving walking capacity. |
| Tolerability | Some candidates may be better tolerated than others. | Muscle spasms, diarrhea, acne and higher discontinuation rates were reported in trials. |
| Mechanism | Selective antibodies may target muscle signaling more precisely. | Broader blockers affect related pathways; long-term effects are unknown. |
| Evidence & access | Several trials are active, so more data is expected. | Phase 2, mostly industry-sponsored, short follow-up; not available outside trials. |
Regulatory & Medical Status: Why This Is Theoretical
- No myostatin or activin pathway inhibitor is FDA-approved for obesity, weight management or muscle preservation.
- No such drug is FDA-approved for use together with a GLP-1 medication.
- These agents are accessible only through clinical trials. Large phase 3 trials, long-term safety data and regulatory review would be needed before any approval.
- No medical guideline currently recommends adding a drug to protect muscle during GLP-1 therapy. Established approaches, such as adequate protein intake and resistance exercise, remain topics to discuss with your clinician.
Safety & Research Considerations
Important Safety Information
Myostatin pathway inhibitors are investigational. Their full safety profile is not known.
- Reported in trials: muscle spasms, diarrhea and acne, with more participants stopping treatment in some bimagrumab groups.
- The triple-drug COURAGE arm (including garetosmab) had a higher rate of side effects than other arms.
- Because these drugs act on pathways beyond muscle, long-term effects are not yet understood.
- GLP-1 medications such as semaglutide and tirzepatide carry a boxed warning for the risk of thyroid C-cell tumors. See our overview of weight loss medication options.
- Do not buy “research use only” products online. They are not approved for human use, and their purity and dosing are unverified.
- Do not combine medications on your own. Talk with a licensed clinician about any weight loss medication.
- Trials are listed on ClinicalTrials.gov (for example NCT05616013, NCT06445075, NCT06299098).
Considerations for Ongoing Research
- Function vs. scans: do improvements on DXA translate into better strength, mobility or quality of life?
- Longer follow-up: most data covers 24–72 weeks.
- After stopping: what happens to muscle, fat and weight when treatment ends?
- Who benefits: older adults, people with low baseline muscle, or others may respond differently.
- Compared with lifestyle: how do these drugs compare with resistance training and adequate protein?
- Broad vs. selective blockade: do narrower drugs offer a better balance of benefit and side effects?
Important Disclaimer
This article is for general educational purposes only and does not constitute medical advice. It does not establish a patient relationship and should not be used to diagnose, treat or make decisions about any condition. Discuss any medication question with a licensed healthcare provider. Research findings may change as new data is published.
Sources
1. BELIEVE trial, Nature Medicine
2. EMBRAZE trial, Nature Medicine
3. Regeneron: COURAGE phase 2 results (press release)
4. Review article, PubMed Central
8. ADA 2025 conference summary (Cipla Medical)
9. Bimagrumab COPD trial, eScholarship
10. Scholar Rock: FDA review of apitegromab (press release)
These statements have not been evaluated by the Food and Drug Administration. The drugs discussed in this article are investigational and are not FDA-approved for the uses described. This content is educational and is not intended to diagnose, treat, cure, or prevent any disease.