“GLP-1 muscle loss” is usually lean-mass loss during rapid weight loss, not the drug uniquely “melting muscle.” When people lose body weight—no matter how—they typically lose some mix of fat + lean mass. GLP-1 medications (like semaglutide or tirzepatide) are very effective at reducing appetite and total calories, so the weight comes off faster and more consistently than many people can achieve with diet alone. That’s where most of the “muscle loss” concern comes from.
Here are the main reasons it happens for most people:
When daily intake drops a lot (which GLP-1s commonly cause), your body must cover the gap. Fat stores cover a chunk, but lean tissue can contribute too, especially if the deficit is aggressive and protein intake is low. The body is constantly remodeling muscle protein; in a steep deficit, muscle protein breakdown can exceed muscle protein synthesis, and you slowly lose muscle tissue.
GLP-1s reduce hunger and can create food aversions or early fullness. Many people end up eating “light” foods—soups, crackers, small snacks—and unintentionally land at low protein. Protein provides the amino acids that tell your body “keep this muscle.” If protein is too low, the body has less reason (and less building material) to preserve muscle while dieting.
A huge signal that tells your body to retain muscle is mechanical tension (lifting, loading, hard resistance work). When people feel tired, nauseated, or just less motivated because they’re eating less, they often reduce workouts and daily activity. Also, carrying less body weight means you’re literally doing less “loaded” movement all day. Less loading = less stimulus to keep muscle.
Your body adapts to what it must do. If you lose 30–60 pounds, your legs and trunk no longer need as much muscle to move you around. Without resistance training to “argue back,” the body decides some muscle is “expensive” and not required, and it trims it.
A lot of headlines and social posts say “I lost muscle,” but many measurements track lean mass, which includes:
water
glycogen (stored carbs in muscle)
connective tissue
organ mass
and muscle
In early weight loss—especially with lower carbs and lower calories—glycogen and water drop quickly, and that shows up as “lean mass loss.” Some of it is true muscle, but some is simply less stored fuel and less water.
Nausea, reflux, constipation, and food aversions can shrink the diet’s quality. If someone’s diet becomes low in overall calories and low in micronutrients, it can be harder to train well, recover, sleep, and maintain muscle. Add dehydration (common if people drink less due to fullness), and performance drops further—again reducing the muscle-preserving stimulus.
Older adults already have “anabolic resistance,” meaning they need more protein and more resistance training stimulus to maintain muscle. People who start GLP-1s with low muscle mass, sedentary habits, or low protein intake are the most likely to see a bigger proportion of lean mass decline.
Most people “experience muscle loss on GLP-1s” because GLP-1s make it easier to sustain a large calorie deficit and often lead to low protein + low lifting + rapid weight loss. That combination predicts lean mass loss in any diet scenario.
Lift 2–4x/week (progressive resistance training; even simple full-body machines work)
Protein target: roughly 1.6–2.2 g/kg of your goal body weight per day (or at least 30–40g per meal if you prefer meal-based targets)
Don’t crash diet if you can avoid it—slightly slower loss is often more muscle-friendly
Walk and move daily (keeps some “baseline loading” and helps training recovery)
Manage side effects so you can eat real meals (this is often the bottleneck)
If you tell me your age, current weight, goal weight, and whether you lift now, I’ll give you a muscle-preserving target for protein + a simple weekly training template that matches your reality.