What body composition markers should someone track while on a GLP-1?

Researchers commonly assess DEXA scans to separate fat mass, lean mass, and bone mineral density; visceral adipose tissue; skeletal muscle index; and lab biomarkers such as IGF-1, fasting insulin, HbA1c, and total testosterone, all of which can shift during significant weight change. Discuss with your licensed provider which assessments are appropriate.

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GLP-19 min read·June 4, 2026

Bimagrumab: The Muscle-Preservation Answer to GLP-1 Lean Mass Loss

Bimagrumab blocks activin receptors to preserve lean mass during GLP-1 therapy. Learn what the BELIEVE trial found and what to track.

Bimagrumab and the Muscle-Preservation Problem on GLP-1s **Bimagrumab** is an investigational monoclonal antibody drawing serious research attention in 2025 and 2026, largely because it may address the most-tracked limitation of GLP-1 receptor agonists: the loss of skeletal muscle that accompanies rapid fat reduction. If you or someone you follow is using [semaglutide](/peptides/semaglutide), [tirzepatide](/peptides/tirzepatide), or newer agents like [retatrutide](/peptides/retatrutide), the question of what the scale does not tell you is worth understanding. This post reviews the science behind bimagrumab, what the BELIEVE trial found, how body composition is tracked in research settings, and why monitoring lean mass, not just total weight, is the lens researchers and self-trackers are now applying.

Why GLP-1 Drugs Create a Muscle-Preservation Problem

GLP-1 receptor agonists work through appetite suppression and slowing gastric emptying, creating the calorie deficit responsible for their remarkable weight-loss results. Semaglutide 2.4 mg produced roughly 15-17% mean body weight loss in the STEP trials. Tirzepatide pushed that figure even further in SURMOUNT-1.

The challenge: sustained calorie deficits reduce both fat mass and lean (muscle) mass. In the STEP-1 trial, lean mass accounted for an estimated 13-45% of total weight lost depending on how the analysis was framed. A review published in Diabetes, Obesity and Metabolism (Neeland et al., 2024) cataloged these changes across multiple GLP-1 trials and noted the need for mitigation strategies.

Muscle loss at that scale matters beyond aesthetics. Skeletal muscle is metabolically active tissue. It plays a central role in insulin-mediated glucose disposal, resting metabolic rate, and functional capacity. Researchers describe it as a "quality versus quantity" problem with weight loss: the number on the scale may look excellent while the body composition story is more complicated.

This is why body composition tracking, not just weight tracking, has become a key theme in obesity research. The [biomarkers hub](/biomarkers) on MyProtocolStack is designed exactly for logging the signals that the scale misses.

What Is Bimagrumab and How Does It Work

Bimagrumab is a fully human monoclonal antibody that blocks both activin type IIA (ActRIIA) and activin type IIB (ActRIIB) receptors. These receptors sit on the surface of muscle cells and mediate the effects of two potent muscle-growth inhibitors: myostatin and activin A.

Myostatin (also called GDF-8) is a TGF-beta family member that the body uses to cap muscle growth. Activin A has overlapping but distinct effects on muscle and fat tissue. When bimagrumab occupies ActRII receptors, neither myostatin nor activin A can send their inhibitory signal, releasing the brake on skeletal muscle protein synthesis.

Preclinical work published in Molecular Metabolism in 2024 by Nunn et al. explored this mechanism in diet-induced obese mice treated with bimagrumab alone and in combination with semaglutide. Bimagrumab alone produced roughly a 10% increase in lean mass while simultaneously reducing fat mass. When combined with semaglutide, the combination preserved lean mass despite reduced food intake, while delivering superior fat mass reduction compared to either agent alone.

Bimagrumab was originally developed by Novartis and then licensed to Versanis Bio. Eli Lilly completed its acquisition of Versanis in 2023 for approximately $1.9 billion, signaling the strategic importance Lilly places on body composition differentiation in the obesity space.

The BELIEVE Trial: Key Findings

The BELIEVE trial (NCT05616013) is the phase 2 study most closely associated with bimagrumab's body composition potential. Published in Nature Medicine in early 2026 and presented at the American Diabetes Association's 85th Scientific Sessions in June 2025, it enrolled 507 adults with obesity and randomized them across multiple arms for 48 weeks, with a 72-week extension.

|---|---|---|

*Source: Heymsfield et al., Nature Medicine, 2026. NCT05616013. Numbers represent least-squares mean changes.*

Several findings stand out in the published literature:

•The combination produced 22.1% total body weight loss, exceeding semaglutide alone by roughly six percentage points.

•Approximately 92.8% of total weight loss in the high-dose combination arm came from fat mass.

•Bimagrumab alone increased lean mass by about 2.5%; the combination limited lean mass reduction to 2.9% despite the deeper calorie deficit imposed by semaglutide.

•Visceral adipose tissue reduction was particularly pronounced in the combination arm.

•Common adverse events for bimagrumab included muscle spasms, diarrhea, and acne.

These are phase 2 results. Researchers, regulators, and clinicians will require phase 3 data before any approval pathway is established.

Regulatory Status and Development Updates

As of mid-2026, bimagrumab has no FDA approval for any indication. It is an investigational compound. Its development path has had both advances and setbacks:

•**BELIEVE trial:** Phase 2b results in non-diabetic adults with obesity, published in Nature Medicine (2026). Positive for body composition outcomes.

•**Lilly + Versanis acquisition (2023):** approximately $1.9 billion, reflecting confidence in the muscle-preservation approach.

•**Tirzepatide combination trial termination (late 2025):** Eli Lilly separately terminated a phase 2b study of bimagrumab combined with tirzepatide in people with type 2 diabetes and obesity, citing pipeline prioritization.

The picture is therefore mixed: the semaglutide combination showed compelling body composition data, but the broader development program has faced setbacks. Anyone monitoring this space should track clinical trial registries and peer-reviewed publications rather than early press coverage.

What to Track: Body Composition and Lab Markers

If you are discussing any weight-management approach with a provider, these are the measurements that appear most often in the clinical literature:

Body composition imaging:

•**DEXA (dual-energy X-ray absorptiometry):** the standard for separating fat mass, lean mass, and bone density. BELIEVE used DEXA as a primary body composition endpoint. A baseline scan before starting a GLP-1 and serial scans during therapy give a time-stamped picture of lean mass change.

•**Skeletal muscle index (SMI):** adjusts lean appendicular mass for height.

•**Visceral adipose tissue (VAT):** metabolically more harmful than subcutaneous fat.

Lab biomarkers linked to muscle and metabolic function:

•[IGF-1](/biomarkers/igf-1): reflects growth hormone activity and anabolic state. Significant calorie restriction can suppress IGF-1.

•[Total testosterone](/biomarkers/total-testosterone): both sexes depend on testosterone for muscle protein synthesis.

•[HbA1c](/biomarkers/hba1c): reflects 90-day average glucose control.

•[Fasting insulin](/biomarkers/fasting-insulin): shifts during weight loss can indicate improving insulin sensitivity.

MyProtocolStack is built to log each of these data points over time, so you can bring a complete picture, including DEXA results and lab trends, to conversations with your licensed provider. The [biomarkers hub](/biomarkers) and [calculators](/calculators) are designed for exactly this kind of longitudinal self-tracking.

Why "What the Scale Says" Is Not Enough

A central theme in bimagrumab research, and in the broader body composition conversation around GLP-1s, is that total weight is a poor proxy for health-relevant changes in tissue composition. The BELIEVE trial was designed around this insight: its headlines are not just about pounds lost but about which tissue was lost.

For self-trackers, a DEXA scan at baseline and periodically thereafter is considerably more informative than weekly weigh-ins. Seeing that 92% of weight lost came from fat versus seeing that 45% came from lean tissue tells two entirely different stories, even if the total number looks identical.

If you are ready to build that kind of longitudinal picture, [start tracking on MyProtocolStack today](/auth/login?mode=signup).

What This Research Does Not Tell Us Yet

•**Phase 3 data:** BELIEVE was a phase 2 trial. Larger, longer phase 3 trials are needed before any regulatory approval becomes plausible.

•**Durability of lean mass preservation:** it is unknown whether benefits persist beyond the treatment period or require ongoing dosing.

•**Tirzepatide combination:** Lilly terminated that study in late 2025; the implications for the broader program are not yet fully public.

•**Adverse event profile at scale:** muscle spasms and acne were the most commonly reported; long-term safety at population scale requires more data.

None of these unknowns diminish the importance of what BELIEVE demonstrated, but they are essential context for anyone reading headlines about this compound.

Frequently Asked Questions

What is bimagrumab and how does it work?

Bimagrumab is an investigational monoclonal antibody that blocks activin type II receptors (ActRIIA and ActRIIB). These receptors are activated by myostatin and activin A, two proteins that normally suppress muscle growth. By blocking those receptors, bimagrumab removes that brake, allowing skeletal muscle to grow while also reducing fat mass. It is not FDA-approved and remains in clinical investigation as of 2026.

What did the BELIEVE trial find about bimagrumab plus semaglutide?

The phase 2 BELIEVE trial (NCT05616013), published in Nature Medicine in 2026, enrolled 507 adults with obesity. The high-dose combination (bimagrumab 30 mg/kg plus semaglutide 2.4 mg) produced 22.1% total body weight loss at 72 weeks, with approximately 92.8% from fat mass. Semaglutide alone reduced lean mass by 7.4%; the combination limited lean mass loss to 2.9%.

Is bimagrumab FDA-approved for weight loss or muscle preservation?

No. Bimagrumab is an investigational drug as of 2026 with no FDA approval for any indication. Always discuss investigational therapies with a licensed healthcare provider.

Why do GLP-1 drugs cause lean mass loss?

GLP-1 receptor agonists work primarily by reducing appetite and caloric intake. Any significant calorie deficit tends to reduce both fat mass and lean mass. Researchers estimate lean mass may account for up to 25-45% of total weight lost on semaglutide, depending on study population, dose, and exercise habits.

What body composition markers should I discuss tracking with my provider while on a GLP-1?

Researchers commonly assess DEXA scans to separate fat mass, lean mass, and bone density; visceral adipose tissue; and lab biomarkers such as [IGF-1](/biomarkers/igf-1), [fasting insulin](/biomarkers/fasting-insulin), [HbA1c](/biomarkers/hba1c), and [total testosterone](/biomarkers/total-testosterone). Discuss with your licensed provider which assessments are appropriate for your situation.

Sources

1. Heymsfield S, et al. "Bimagrumab plus semaglutide alone or in combination for the treatment of obesity: a randomized phase 2 trial." Nature Medicine, 2026.

2. Nunn E, et al. "Antibody blockade of activin type II receptors preserves skeletal muscle mass during GLP-1 receptor agonism." Molecular Metabolism, 2024.

3. Melson E, et al. "What is the pipeline for future medications for obesity?" International Journal of Obesity, 2024.

4. Neeland I, et al. "Changes in lean body mass with GLP-1-based therapies and mitigation strategies." Diabetes, Obesity and Metabolism, 2024.

5. Westphal M, et al. "Effect of Bimagrumab vs Placebo on Body Fat Mass Among Adults With Type 2 Diabetes and Obesity." JAMA Network Open, 2021.

6. Eli Lilly and Company. "Lilly to Acquire Versanis." Investor Relations Press Release, 2023.

7. ClinicalTrials.gov. "Safety and Efficacy of Bimagrumab and Semaglutide in Adults Who Are Overweight or Obese." NCT05616013.

8. ADA Meeting News. "BELIEVE spotlights quality and quantity approach to weight management." June 2025.

9. BioPharma Dive. "Lilly stops trial of muscle-sparing obesity drug." 2025.

10. Circulation (AHA). "Muscle Mass and GLP-1 Receptor Agonists: Adaptive or Maladaptive Response to Weight Loss?" 2024.

*MyProtocolStack is a tracking and education tool, not medical advice, diagnosis, or treatment. Nothing in this post constitutes a recommendation to use, obtain, or adjust any medication. Bimagrumab is an investigational compound with no current FDA approval. Always consult a qualified, licensed healthcare professional before making any changes to your health plan.*

MENTIONED IN THIS POST

PEPRetatrutide PEPSemaglutide PEPTirzepatide BIOFasting Insulin BIOHbA1c BIOIGF-1 BIOTotal Testosterone

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