Some of the weight that comes off on a GLP-1 isn't fat. In recent trials, lean tissue (much of it muscle) made up roughly 25-40% of the total. Strength training is the best-supported way to hold on to that muscle.
What happens to muscle during GLP-1 weight loss
GLP-1 and GLP-1/GIP medications (semaglutide, liraglutide, tirzepatide) produce large weight losses, often 15-24% of body weight in clinical trials (DOI (external link)). Not all of that is fat. Lean soft tissue made up about 26-40% of the weight lost in recent trials (DOI (external link)). Lean tissue is mostly muscle, but it also includes organs, water and other non-fat tissue, so it isn't a pure muscle count. A 2024 review in Diabetes Care put the average loss at about 10% of lean mass, or roughly 6 kg, and compared that to a decade or more of aging (DOI (external link)). Whether that share is larger than with diet-only weight loss is still debated.
Muscle matters beyond the number on the scale. It drives your resting metabolism, your strength and mobility, and your ability to keep weight off. Protecting it is one of the few things you control during treatment, and strength training is the most effective tool you have.
Is the medication itself causing muscle loss?
People often ask whether the drugs break down muscle directly, or whether muscle loss is just what happens when you lose weight fast by any method. The best current answer is mostly indirect, with one open question.
Muscle loss during rapid weight loss isn't unique to GLP-1 drugs. It happens with dieting and bariatric surgery too. A 2025 narrative review in Nutrients cites a meta-analysis of incretin trials in which lean mass made up about 25% of weight lost, while lean mass as a share of body weight didn't change significantly (DOI (external link)). Put simply, muscle came off roughly in step with the weight. Human studies haven't separated a direct drug effect on muscle from the effect of eating much less.
The open question is strength. A 2026 review found that older adults on long-term semaglutide lost grip strength and had a higher risk of sarcopenia, in ways that didn't track cleanly with the lean-mass changes seen in shorter trials. That raises the possibility of an effect on strength beyond the loss of muscle tissue. The evidence is preliminary and mostly observational, and longer controlled studies are needed (DOI (external link)).
For most people, then, muscle loss on a GLP-1 is best understood as a result of losing weight quickly. The drug isn't shown to attack muscle directly. The loss is real either way, which is why strength training and adequate protein matter.
What the research shows
Several recent reviews and studies point the same way: resistance exercise is the main way to protect muscle during GLP-1 treatment.
- The 2024 Diabetes Care review notes that in adults generally, supervised resistance training lasting more than 10 weeks can add about 3 kg of lean mass and raise strength by about 25%. Those figures come from training studies in men and women, not from people on incretin drugs. The authors propose resistance training as a standard add-on to these medications (DOI (external link)).
- A 2025 case series in SAGE Open Medical Case Reports followed three patients on semaglutide or tirzepatide who did resistance training 3-5 days a week and ate more protein. Two of the three gained lean soft tissue while losing roughly 47-62% of their fat mass (DOI (external link)). Three people is a small sample, but it shows that holding on to lean tissue is possible.
- The 2025 Nutrients review concluded that structured resistance training, combined with enough protein and other nutrients, is critical to preserving muscle and bone during incretin-based treatment (DOI (external link)).
- A 2025 clinical guidance article from the International Symposium on Diabetes and Nutrition recommends more than 1.2 g/kg/day of protein, spread evenly across meals, plus both aerobic activity and structured resistance training as the core strategy for preserving lean mass (DOI (external link)).
A caution from the 2026 British Journal of Pharmacology review: short-term studies show strength holding up despite tissue loss, but longer-term data in older adults on semaglutide showed falling grip strength and faster sarcopenia risk. Muscle mass and muscle strength are different things, and strength training needs to continue through the whole course of treatment, not only at the start (DOI (external link)).
A 2026 expert commentary adds two caveats. First, a drop in "lean mass" or fat-free mass on a scan is not the same as losing muscle. Fat-free mass also includes organ tissue and the non-fat parts of fat tissue, so some of the measured decline isn't skeletal muscle. Second, resistance training is the most potent tool for protecting muscle during an energy deficit, but direct experimental evidence specifically in people on GLP-1 medications is still very limited, and some loss of fat-free mass is expected with any large weight loss. So train for the broad, well-established benefits and to protect the muscle you can, without expecting it to prevent all lean-mass loss (Tinsley et al., Lifestyle Medicine, 2026, DOI (external link)).
When to start
No study has tested the best time to start. Starting early, alongside your first doses, is a reasonable inference: lean loss tends to track the pace of weight loss, and that pace is often fastest in the first months of treatment. Waiting means losing ground you'll have to work harder to regain.
Practical starting points:
- Start light and build. If you're new to strength training or dealing with nausea or low appetite in the early weeks, begin with 2 sessions a week of basic full-body movements (bodyweight squats, rows, presses) before adding load. Our beginner resistance training plan walks through exactly that.
- Eat enough protein. Reviews cite intakes above about 1.2 g/kg/day, spread across meals, to get the full muscle-preserving benefit of resistance training. Training without enough protein works less well.
- Check in with your prescriber before starting a new routine if you have heart problems, joint issues, or significant GI side effects.
How it compares to other exercise
Not all exercise protects muscle equally during GLP-1 treatment:
| Exercise type | Effect on lean mass | Best used for |
|---|---|---|
| Resistance/strength training | Directly stimulates muscle protein synthesis; the strongest evidence for preserving or even growing lean mass during weight loss | Preserving muscle and strength (the primary tool) |
| Aerobic exercise (walking, cycling, running) | Supports heart health and adds to weight and fat loss, but doesn't protect lean mass on its own | Cardiometabolic health, added fat loss, keeping weight off |
| Combined resistance + aerobic | Clinical guidance recommends both together | Overall body composition and health |
| No structured exercise | Nothing offsets the lean tissue lost with the weight | Best avoided during GLP-1 treatment |
Aerobic work still has a job. The 2024 review notes that after a low-calorie diet, combining aerobic exercise with liraglutide kept weight off better than either one alone (DOI (external link)). Clinical guidance recommends both: aerobic activity for heart health and weight maintenance, resistance training to protect the muscle you have.
If you're on a GLP-1 and want to protect your strength, mobility and metabolism, aim for 2-3 or more resistance sessions a week, eat enough protein, and don't count on cardio alone. The advice is the same whether the muscle loss comes from the drug or from losing weight quickly.
Study findings above are attributed to their authors and were located via PubMed.
This is general education, not medical advice. Check with your clinician before starting a new exercise program, especially if you have health conditions.