GLP-1 Medications and Muscle: Protecting Lean Mass During Weight Loss

One of the most common — and most legitimate — questions about GLP-1 medications is what happens to your muscle. These drugs produce large amounts of weight loss, and some of that lost weight is muscle. But the story is more nuanced than the alarming headlines suggest: the muscle loss is largely proportional to weight loss, muscle quality may actually improve, and there is a clear, evidence-based way to protect your lean mass while you lose fat.

This is the muscle chapter of our GLP-1 series. Here is what the data really show and, more importantly, what to do about it.

Why Weight Loss Always Costs Some Muscle

Start with the underlying physiology. When the body is in a calorie deficit — from dieting, fasting, surgery, or medication — it does not burn only fat. It also breaks down some lean tissue, including muscle, to meet its energy needs. This is a normal, evolutionary adaptation: muscle is metabolically "expensive," so when energy is scarce the body trims it.

With traditional reduced-calorie diets, roughly 10%–30% of the weight lost is fat-free mass, of which about half is skeletal muscle — the rest being water, organ tissue, and other non-fat components (Lieberman et al., JAMA, 2026). This is not unique to GLP-1 medications. It happens with every effective weight-loss method, including bariatric surgery.

What the GLP-1 Data Actually Show

So what happens specifically with these drugs? The honest answer is that estimates vary, and it depends on how you measure.

A network meta-analysis of 22 randomized trials found that lean mass loss made up about 25% of total weight loss with GLP-1 medications — but, crucially, the relative lean mass (the percentage of your body that is lean) was unchanged (Karakasis et al., Metabolism, 2025). A separate systematic review of 36 studies reached a reassuring conclusion: these drugs produce weight loss mainly through selective fat loss, with relative preservation of lean tissue (Sawicka-Gutaj et al., International Journal of Obesity, 2026).

A more cautious picture comes from a 2026 systematic review in the Annals of Internal Medicine: across 35 trials, the median proportion of weight loss attributable to muscle-related measures was about 28%, with roughly two-thirds of studies exceeding the expected benchmark — and, importantly, no study measured actual physical function, so the real-world significance is still unknown (Batsis et al., Annals of Internal Medicine, 2026). Some estimates run as high as 40% depending on the drug and measurement method (Liu et al., Journal of Clinical Endocrinology & Metabolism, 2025).

The most potent agents — semaglutide 2.4 mg and tirzepatide 15 mg — produce the greatest total weight loss but are among the least effective at preserving lean mass in absolute terms, simply because they drive more total loss. Liraglutide, which produces less weight loss, was the only agent to achieve significant weight reduction without significantly reducing lean mass in the network meta-analysis (Karakasis et al., Metabolism, 2025).

Muscle Volume Versus Muscle Quality

Here is where the story turns more encouraging. Losing muscle volume is not the same as losing muscle quality.

In a substudy of the SURPASS-3 trial using MRI, tirzepatide caused modest reductions in thigh muscle volume — but significant improvements in muscle quality, including marked reductions in fat infiltration within the muscle (Gonzalez-Rellan and Drucker, JAMA, 2025). That intramuscular fat is strongly linked to insulin resistance and poorer muscle function, so reducing it may improve how well the muscle actually works, even if the muscle is slightly smaller. In that analysis, the reductions in muscle volume were largely proportional to the amount of weight lost — in other words, about what you'd expect for the degree of weight loss, not an outsized toxic effect on muscle.

Reviews of body composition consistently reach the same theme: fat loss predominates, visceral (abdominal) fat falls preferentially, and there is no clear evidence that these drugs accelerate sarcopenia — the clinical loss of muscle mass and function — in most patients (Bhandarkar et al., Current Opinion in Endocrinology, Diabetes and Obesity, 2025). Interestingly, laboratory and animal studies suggest GLP-1 signaling may even have direct muscle-protective, anti-inflammatory effects — a hint that the muscle loss is driven by the calorie deficit rather than the drug itself.

Who Is Most at Risk?

The concern is not equal for everyone. It is most relevant for older adults and people who already have low muscle mass or frailty, where preserving strength and function is critical to independence and to avoiding falls (Gonzalez-Rellan and Drucker, JAMA, 2025). There is also a subtler long-term concern: people who stop these drugs tend to regain weight mostly as fat, not muscle, which over repeated cycles could gradually worsen body composition (Lieberman et al., JAMA, 2026). This is one more reason these are best viewed as long-term therapies paired with lifestyle measures.

What About Bone?

Bone deserves honest treatment because the evidence is genuinely mixed. Weight loss of any kind tends to reduce bone mineral density, and some GLP-1 studies — especially in people without diabetes, where weight loss is greatest — show declines in bone density that track with the amount of weight lost (Al Refaie et al., Endocrine, 2025). At the same time, laboratory studies suggest GLP-1 signaling directly supports bone formation, and — reassuringly — meta-analyses of randomized trials have not found an increase in fractures, with some even suggesting modestly improved bone density and a possible protective signal over longer treatment (Tan et al., Acta Diabetologica, 2025; Vadini et al., Journal of Clinical Endocrinology & Metabolism, 2026). The practical takeaway: bone is a reason for attention and monitoring, particularly in older and postmenopausal patients, not a reason for alarm — and, notably, combining these drugs with exercise has been shown to protect bone density that the drug alone did not.

How to Protect Your Muscle: The Evidence-Based Plan

The good news is that muscle loss is largely preventable with the right approach. The priorities, in order:

1. Resistance training — the single most important step

Structured strength training is the most effective protection, and it should accompany every GLP-1 prescription. The joint advisory from the American College of Lifestyle Medicine and partner societies is explicit: increased protein alone is inadequate to preserve muscle without resistance training (Mozaffarian et al., 2025). The recommendation is strength training at least 2–3 times weekly targeting the major muscle groups, plus at least 150 minutes of moderate aerobic activity weekly (Mehrtash et al., JAMA Internal Medicine, 2025). Resistance training during calorie restriction can reduce lean-mass loss by 50%–95% and helps preserve bone (Lieberman et al., JAMA, 2026).

2. Adequate protein

Protein is the key nutrient, but hitting the target is harder when appetite is suppressed. The joint advisory recommends about 1.2–1.6 g of protein per kg of body weight per day during active weight loss (Mozaffarian et al., 2025). Practical tips: eat protein-rich foods first at each meal; choose lower-volume, nutrient-dense sources like fish, eggs, Greek yogurt, cottage cheese, beans, and nut butters; and use a protein shake or bar if whole-food intake falls short (Chavez et al., Current Opinion in Clinical Nutrition & Metabolic Care, 2025).

3. Monitoring

Clinicians can track more than just weight. Simple office tests — grip strength and the timed sit-to-stand test — screen for functional decline, and body-composition tools (bioelectrical impedance at point of care, or periodic DXA) can flag excessive lean-mass loss early (Mozaffarian et al., 2025; Mehrtash et al., JAMA Internal Medicine, 2025). If strength is dropping, the response is to intensify resistance training, raise protein, and consider adjusting the medication dose.

The Emerging Frontier: Muscle-Preserving Drugs

The most exciting development is a new class of medications designed to preserve muscle during GLP-1 weight loss by blocking myostatin and activin — natural signals that limit muscle growth. In the phase 2 EMBRAZE trial, adding apitegromab (a myostatin inhibitor) to tirzepatide preserved far more lean mass than tirzepatide alone (about 55% vs 30% retention) while keeping fat loss comparable. In the BELIEVE trial, bimagrumab (an activin receptor antibody) with semaglutide produced weight loss that was almost entirely fat. And in the COURAGE trial, adding trevogrumab — and a triple combination adding garetosmab — preserved 50% to as much as 80% of lean mass (Gonzalez-Rellan and Drucker, JAMA, 2025). These are still investigational (phase 2) results reported largely through interim data, and they need confirmation — but they point toward a future of "higher-quality" weight loss.

The Bottom Line

GLP-1 medications cause some muscle loss — roughly a quarter to 40% of total weight lost — but this is consistent with all effective weight-loss methods, the proportion of your body that is lean is largely preserved, and muscle quality may actually improve as fat inside the muscle falls. The risk matters most in older adults and those with existing frailty. The solution is well established and within your control: resistance training two to three times a week, adequate protein (about 1.2–1.6 g/kg/day), and monitoring of strength and function — with promising muscle-preserving drugs on the horizon. Muscle loss is not a reason to avoid these medications; it is a reason to pair them with the right habits, and it is worth building that plan with your physician.