New research presented at the 2026 European Congress on Obesity found that people losing weight on Wegovy and Mounjaro shed primarily fat rather than muscle. Body scans from the STEP UP trial showed that around 84% of weight lost on Wegovy came from fat, with participants maintaining muscle strength despite modest declines in muscle mass. A separate real-world analysis of 486 people from the Medical University of Vienna confirmed similar results, with 80 to 85% of weight loss coming from fat.

What Do the New Body Scans Reveal?

In the STEP UP scan sub-study, 55 participants underwent detailed body MRI scans before and after taking Wegovy at the higher 7.2 milligram dose. The scans showed that around 84% of the weight they lost was fat. While their muscle mass fell by about 10% from the start, their muscle strength did not decline with it. On a 30-second sit-to-stand test, the group on Wegovy performed as well as the placebo group, suggesting that despite some muscle loss, functional strength remained intact.

The Vienna analysis followed 486 people for about 14 months, most of whom were on Wegovy or Mounjaro. Fat accounted for roughly 80 to 85% of the weight they lost, and relative muscle mass was maintained or increased in more than 70% of them. These findings matter because standard bathroom scales cannot distinguish between fat and muscle loss, which has fueled concerns that GLP-1 medications (glucagon-like peptide-1 receptor agonists) cause excessive muscle loss alongside weight reduction.

How Do These Results Compare to Earlier Weight Loss Studies?

The new findings align with evidence already published from major clinical trials. In a body-composition substudy of the SURMOUNT-1 trial of Mounjaro (tirzepatide), fat mass decreased by 33.9% and muscle mass by 10.9% over 72 weeks, meaning about 75% of the weight lost was fat and 25% was lean tissue. The independent SEMALEAN study followed 106 people on Wegovy for a year and found that fat mass dropped by 18% over the year, while muscle mass fell by about 3 kilograms in the first seven months and then stabilized, with handgrip strength actually improving.

Interestingly, in the SEMALEAN study, the share of people who met the definition of sarcopenic obesity, meaning low muscle combined with obesity, fell during the weight loss period, from 49% to 33% over the year. This suggests that despite some muscle loss, overall muscle health may actually improve as people shed excess weight.

Why Do People Still Lose Some Muscle?

Some muscle loss is normal with any weight loss, whether through traditional dieting, surgery, or medication. Across the published GLP-1 trials, lean tissue accounts for around a quarter of the total weight lost. However, not all lean tissue is muscle. Lean tissue is everything in the body that is not fat, including water, organs, bone, and the non-fat tissue stored within body fat. So the actual amount of muscle lost is smaller than the lean-tissue figure suggests.

Research shows that when people lose weight quickly or lose a large amount, the actual amount of muscle lost is greater, even though the proportion of fat to muscle stays similar or improves. Certain groups should pay more attention to maintaining their muscle than others. The risk of muscle loss increases on the highest doses, and when people lose 20% or more of their body weight, the amount of muscle lost is larger, particularly if the rate of weight loss exceeds 1 pound per week.

How to Maintain Muscle Mass While Taking Mounjaro or Wegovy

• Lose Weight at a Steady Pace: When you lose weight quickly, a greater share comes from muscle. A steady rate of around 1 pound per week protects more muscle as you lose weight. To achieve a slower rate on Mounjaro or Wegovy, stay on the lowest effective dose for as long as possible, as many people achieve significant weight loss without losing excess muscle at lower doses.
• Eat Enough Protein: Research on adults losing weight points to 1.2 to 1.6 grams of protein per kilogram of body weight per day. For someone weighing 198 pounds, that is roughly 110 to 145 grams per day, or a serving the size of the palm of your hand at each meal. Wegovy and Mounjaro reduce appetite, so prioritize building meals around quality protein sources like eggs, Greek yogurt, chicken, fish, tofu, beans, and lentils.
• Do Resistance Training: Resistance training is any exercise that works your muscles against resistance, from bodyweight squats and push-ups to resistance bands and weights. A 2026 American College of Sports Medicine review pooling 137 research reviews covering more than 30,000 people found that working the major muscle groups around twice a week builds and maintains muscle and strength.

Of these three habits, resistance training is the one most likely to build muscle rather than just preventing its decline during weight loss. If you are new to strength training, try at-home workouts using bodyweight exercises so you do not need to purchase equipment. You could also try "snacking on exercise" throughout the day by doing calf raises while the kettle boils or push-ups each time you have been to the toilet.

Muscle is crucial for your health beyond just strength. Most of the sugar from a meal is taken up by your muscles, which helps keep your blood sugar steady. Muscle also keeps you mobile and independent as you age, making these preservation strategies important for long-term health outcomes.