A new class of weight loss drugs is solving one of the biggest problems with current obesity medications: the loss of lean muscle mass along with fat. In the BELIEVE Phase 2b trial, published in Nature Medicine, researchers found that combining bimagrumab with semaglutide resulted in 92.8% of total weight loss coming from fat mass, compared to just 71.8% with semaglutide alone. Even more striking, patients who received bimagrumab by itself saw 100% of their weight loss attributed to fat, while actually gaining 2.5% in total lean mass. For anyone concerned about brain health and aging, this matters enormously — muscle loss accelerates cognitive decline, increases fall risk, and worsens outcomes for people living with dementia.
This development arrives at a critical moment. Current GLP-1 drugs like Ozempic and Wegovy can cause as much as 40% of weight lost to come from lean mass rather than fat, a ratio that poses serious risks for older adults who already face age-related muscle wasting. The drugs discussed in this article — bimagrumab, trevogrumab, and several experimental therapies — represent a fundamental shift in how we think about weight loss treatment. Rather than simply making people lighter, they aim to make people leaner and stronger. This article covers the science behind these muscle-sparing drugs, what the clinical trials have shown so far, why muscle preservation matters so much for brain health, and what patients and caregivers should realistically expect in the coming years.
Table of Contents
- How Do New Weight Loss Drugs Reduce Body Fat Without Losing Muscle?
- Why Muscle Loss During Weight Loss Is Dangerous for Brain Health
- The BELIEVE Trial — What the Numbers Actually Show
- Comparing the Leading Muscle-Sparing Drug Candidates
- Experimental Approaches Beyond Antibody Drugs
- What This Means for Dementia Caregivers Managing a Loved One’s Weight
- Where Weight Loss Drug Development Goes From Here
- Conclusion
- Frequently Asked Questions
How Do New Weight Loss Drugs Reduce Body Fat Without Losing Muscle?
The key innovation behind these next-generation weight loss drugs is targeting the biological pathways that regulate muscle growth independently from the appetite-suppression mechanisms that drive fat loss. Bimagrumab, developed by Eli Lilly, is a first-in-class monoclonal antibody that targets activin type II receptors. These receptors normally respond to signals that limit muscle growth. By blocking them, bimagrumab essentially removes the brakes on muscle maintenance and development, even while the body is in a caloric deficit from a companion drug like semaglutide. The BELIEVE trial, led by Dr. Steven Heymsfield at Pennington Biomedical Research Center, demonstrated that this two-pronged approach — suppress appetite with one drug, protect muscle with another — produced a 22.1% decrease in body weight with the overwhelming majority of that loss coming from fat. Regeneron is pursuing a similar but distinct strategy with trevogrumab, an anti-myostatin antibody.
Myostatin is a protein that acts as a natural limiter on muscle growth. In the Phase 2 COURAGE trial, adding trevogrumab to semaglutide prevented approximately half of the lean mass loss that semaglutide causes on its own, while actually increasing fat mass loss compared to semaglutide alone. These results, presented at the European Association for the Study of Diabetes meeting, suggest that there are multiple viable pathways for solving the muscle loss problem. The competition between these approaches is good news for patients, as it increases the odds that at least one effective muscle-sparing combination will reach the market. To put the comparison in plain terms: tirzepatide, sold as Zepbound and Mounjaro, produces roughly a 21.3% mean body weight reduction, but approximately 25% of that loss comes from lean mass. The bimagrumab-semaglutide combination achieved a comparable total weight reduction of 22.1%, but with only about 7% of the loss coming from lean tissue. That difference may sound small in percentage terms, but for a 200-pound person, it could mean the difference between losing 10 pounds of muscle and losing fewer than 3 pounds — a gap that has real consequences for mobility, metabolism, and long-term health.
Why Muscle Loss During Weight Loss Is Dangerous for Brain Health
The connection between muscle mass and cognitive function is more direct than most people realize. Skeletal muscle produces signaling molecules called myokines — including brain-derived neurotrophic factor, or BDNF — that cross the blood-brain barrier and support neuronal health, memory formation, and synaptic plasticity. When older adults lose significant muscle mass, whether through illness, inactivity, or aggressive weight loss, the downstream reduction in these protective molecules may accelerate neurodegenerative processes. For people already managing early-stage dementia or mild cognitive impairment, the stakes of unnecessary muscle loss are particularly high. However, the relationship between weight, muscle, and brain health is not straightforward enough to justify avoiding weight loss drugs entirely. Obesity itself is a well-established risk factor for dementia, driven by chronic inflammation, insulin resistance, and vascular damage.
The BELIEVE trial found that the bimagrumab-semaglutide combination produced greater reductions in visceral fat and markers of inflammation compared to either drug used alone. Visceral fat, the deep abdominal fat surrounding organs, is particularly associated with systemic inflammation that damages blood vessels in the brain. So the ideal outcome is not simply preserving weight, but shifting body composition — losing fat while keeping muscle — which is exactly what these new drug combinations aim to do. The practical warning here is important: if you or a loved one with cognitive concerns is currently taking a GLP-1 drug like semaglutide or tirzepatide without a muscle-sparing companion therapy, the muscle loss risk should be taken seriously. Resistance exercise can partially offset lean mass loss during GLP-1 treatment, and adequate protein intake is essential. These behavioral strategies are not a perfect substitute for pharmacological muscle preservation, but until combination therapies become widely available, they represent the best available defense against losing the muscle that the aging brain depends on.
The BELIEVE Trial — What the Numbers Actually Show
The BELIEVE trial was a randomized, double-blind, placebo-controlled, multicenter Phase 2b study — the gold standard design for evaluating drug efficacy before moving to larger Phase 3 trials. It tested three groups: bimagrumab combined with semaglutide, bimagrumab alone, and semaglutide alone (with appropriate placebo controls). The headline results have drawn significant attention, but the details are worth examining carefully. The combination group achieved 22.1% total body weight reduction, with 92.8% of that weight loss coming from fat mass. The semaglutide-alone group also lost substantial weight, but only 71.8% came from fat. Perhaps the most remarkable finding was the bimagrumab-alone group: 100% of weight loss was attributed to fat mass, and participants actually gained 2.5% in lean mass.
This means bimagrumab alone functioned not just as a fat-loss drug, but as a muscle-building one — a combination that has obvious appeal for older adults dealing with sarcopenic obesity, where excess fat and inadequate muscle coexist. There is an important caveat, though. Eli Lilly stopped a separate bimagrumab trial in patients with diabetes, raising questions about the drug’s viability in certain populations. A trial in non-diabetic people with obesity continues, with results expected around April 2026. The reasons for stopping the diabetes-focused trial have not been fully detailed in public disclosures, which means we do not yet know whether the issue was safety, efficacy, or a business decision unrelated to the drug’s clinical profile. Anyone following this space should watch for those results before drawing conclusions about bimagrumab’s future.
Comparing the Leading Muscle-Sparing Drug Candidates
For caregivers and patients trying to make sense of the emerging options, a direct comparison of the leading candidates is useful. Bimagrumab targets activin type II receptors, which influence multiple pathways related to muscle and fat metabolism. Trevogrumab targets myostatin specifically, a narrower mechanism focused squarely on removing the natural cap on muscle growth. Both approaches work when combined with semaglutide, but their profiles differ in meaningful ways. The BELIEVE trial’s bimagrumab combination showed the more dramatic composition shift, with over 92% of weight loss from fat.
The COURAGE trial’s trevogrumab combination prevented about half of semaglutide’s lean mass loss — a significant improvement, but not as complete as bimagrumab’s effect. On the other hand, trevogrumab has the advantage of being developed by Regeneron, which has a strong track record of bringing antibody drugs through regulatory approval and to market. Bimagrumab’s path is muddied by the terminated diabetes trial, even as the obesity-focused program continues. The tradeoff for patients may ultimately come down to availability and insurance coverage rather than pure efficacy differences. Both drugs are still in Phase 2 trials, meaning neither is likely to reach pharmacy shelves before 2028 at the earliest, assuming successful Phase 3 results. In the meantime, patients and caregivers should discuss the muscle-loss implications of current GLP-1 therapy with their physicians and ensure that any weight loss program includes structured resistance training and protein-rich nutrition — the non-pharmacological pillars that these drugs are designed to enhance, not replace.
Experimental Approaches Beyond Antibody Drugs
Not every research group is pursuing the antibody-plus-GLP-1 model. Swedish scientists have developed an experimental oral medication that takes a fundamentally different approach: rather than suppressing appetite, it boosts metabolism directly in the muscles. In animal studies, this pill successfully controlled blood glucose, increased fat burning, and preserved muscle mass. Early human studies have shown promising tolerability and safety profiles, though efficacy data in humans is still limited. The appeal of this approach is obvious — an oral pill with a muscle-boosting mechanism would be far simpler and likely cheaper than an injectable antibody combination. Wave Therapeutics is pursuing yet another angle, using RNA technology to lower levels of activin E, a protein produced by the liver that slows fat burning. By reducing activin E, the therapy aims to increase fat loss while preserving lean muscle mass.
This is still in early experimental stages, and the gap between RNA-based drug concepts and approved medications is historically wide and littered with failed programs. However, if successful, RNA-based approaches could offer more precise targeting with potentially fewer systemic side effects than monoclonal antibodies. The limitation that applies to all of these experimental therapies is time. Drug development is slow, expensive, and filled with attrition. The majority of Phase 2 drug candidates never reach approval. For someone caring for a parent with dementia today, or managing their own cognitive health alongside obesity, these future drugs are not a reason to wait. They are a reason to be hopeful about where treatment is headed, while acting on what is available now.
What This Means for Dementia Caregivers Managing a Loved One’s Weight
Caregivers face a difficult balancing act when a loved one with dementia is overweight. Excess weight increases cardiovascular risk and worsens mobility — both of which compound dementia’s challenges. But aggressive caloric restriction or medications that strip away muscle can leave someone weaker, more prone to falls, and less able to perform basic daily activities.
Falls are among the leading causes of hospitalization and accelerated decline in people with dementia. The emergence of muscle-sparing weight loss drugs offers a potential path through this dilemma, though it will take years before they are standard options. In the near term, the most practical takeaway is this: if a family member with cognitive impairment is prescribed a GLP-1 drug, ask the prescribing physician about monitoring lean mass through body composition measurements, not just tracking the number on the scale. A DEXA scan or bioimpedance measurement can reveal whether weight loss is coming from fat, muscle, or both — information that should guide ongoing treatment decisions.
Where Weight Loss Drug Development Goes From Here
The next eighteen months will be defining for this field. Eli Lilly’s non-diabetic obesity trial for bimagrumab is expected to report results around April 2026, and Regeneron will likely advance trevogrumab into Phase 3 studies if additional data confirms the COURAGE trial findings. The American Diabetes Association and the broader endocrinology community have signaled that “quality of weight loss” — meaning body composition, not just total pounds lost — is becoming a primary endpoint of interest, not an afterthought.
For brain health advocates, this reframing is long overdue. A drug that helps someone lose thirty pounds but takes ten of those pounds from muscle is a very different proposition than one that directs nearly all of that loss to fat. As the science advances, the most important thing patients and caregivers can do is stay informed, ask their doctors specific questions about body composition, and resist the temptation to evaluate weight loss drugs solely by how much the scale moves. The number that matters most may turn out to be the one that stays the same — lean muscle mass — because that is the number most closely tied to mobility, independence, and the brain’s long-term resilience.
Conclusion
A new generation of weight loss drugs is finally addressing the muscle loss problem that has shadowed GLP-1 medications since their rise to prominence. The BELIEVE trial’s combination of bimagrumab and semaglutide demonstrated that 92.8% of weight loss can come from fat rather than muscle, and bimagrumab alone showed that it is possible to lose fat while actually gaining lean mass. Regeneron’s trevogrumab offers a competing approach with similarly encouraging early results. For the dementia care community, where muscle preservation is directly linked to fall prevention, functional independence, and even cognitive resilience through myokine signaling, these advances carry particular significance.
None of these drugs are available yet, and the road from Phase 2 success to pharmacy availability is long and uncertain. In the meantime, patients and caregivers should focus on the tools that are proven and available: resistance exercise, adequate protein intake, regular body composition monitoring, and honest conversations with healthcare providers about the tradeoffs of current weight loss medications. The science is moving in the right direction. The goal now is to make sure that weight loss — whenever and however it is achieved — makes people stronger, not just smaller.
Frequently Asked Questions
How much muscle do current GLP-1 drugs like Ozempic cause you to lose?
Studies show that as much as 40% of weight lost during GLP-1 treatment may come from lean mass rather than fat. With tirzepatide (Zepbound/Mounjaro), approximately 25% of weight loss comes from lean mass. This means a person losing 40 pounds could lose 10 to 16 pounds of muscle, which is clinically significant, especially for older adults.
When will muscle-sparing weight loss drugs be available to patients?
The leading candidates — bimagrumab and trevogrumab — are currently in Phase 2 trials. Eli Lilly expects results from its non-diabetic obesity bimagrumab trial around April 2026. If Phase 3 trials proceed smoothly, the earliest these drugs might reach the market is likely 2028 or later. Drug development timelines frequently shift, so these estimates should be treated as approximate.
Why does losing muscle during weight loss matter for people with dementia?
Muscle tissue produces signaling molecules called myokines, including brain-derived neurotrophic factor (BDNF), that support brain health and cognitive function. Muscle loss also increases fall risk, reduces mobility and independence, and accelerates functional decline — all of which compound the challenges of living with dementia.
Can exercise prevent muscle loss while taking GLP-1 drugs?
Resistance training and adequate protein intake can partially offset the lean mass loss caused by GLP-1 medications, but they do not fully prevent it. These lifestyle measures are still strongly recommended for anyone on GLP-1 therapy, particularly older adults, as they remain the best available non-pharmacological defense against muscle wasting during weight loss.
What is the difference between bimagrumab and trevogrumab?
Bimagrumab targets activin type II receptors, influencing multiple pathways related to both muscle growth and fat metabolism. Trevogrumab specifically targets myostatin, a protein that limits muscle growth. Both are designed to be used alongside GLP-1 drugs like semaglutide. In trials, bimagrumab showed a more complete fat-to-muscle ratio improvement, but trevogrumab also demonstrated meaningful muscle preservation.
