Does Semaglutide cause muscle loss?

Short answer: yes, semaglutide is associated with some lean-mass loss, but the picture is more specific than the headline suggests. In the STEP 1 trial, participants who received semaglutide 2.4 mg weekly lost a mean of 14.9% of their body weight over 68 weeks^[1]. A share of that weight came from lean tissue rather than fat. Whether that matters clinically depends on the total weight lost, the baseline body-fat percentage, and what else the person is doing during treatment.

This article explains what the trial data show about lean mass, how semaglutide compares with other weight-loss approaches, and what the evidence suggests about minimising lean-mass loss during treatment.

What STEP 1 tells us about body composition

Wilding and colleagues (2021) reported the STEP 1 results in the New England Journal of Medicine^[1]. The primary endpoint was percentage change in body weight at 68 weeks: 14.9% loss in the semaglutide group against 2.4% in the placebo group. The trial also tracked cardiometabolic markers and physical functioning, which improved significantly on semaglutide.

STEP 1 did not include DEXA imaging as a primary measure for all participants, but the trial's physical-functioning data and cardiometabolic markers pointed toward a broadly favourable body-composition shift. Fat mass fell substantially, while the lean-mass question required subsequent analysis.

The lean-mass fraction: what later analyses found

A 2026 systematic review and meta-analysis by Eisa and Barood, published in Diabetes, Obesity and Metabolism, examined 20 randomised controlled trials with 15,782 participants and calculated the proportion of total weight loss coming from lean mass for each incretin class^[2]. For semaglutide, 35.2% of total weight lost came from lean tissue. For comparison, tirzepatide sat at 25.4%, liraglutide at 26.8%, and lifestyle intervention alone at 26.2%. Adding resistance training to lifestyle lowered the lean-mass fraction to 17.5%.

The 35.2% figure for semaglutide is higher than the other agents in that analysis. The authors note that the proportion of weight lost as lean mass is broadly comparable between incretin-based pharmacotherapy and lifestyle interventions overall, but that semaglutide's stronger total weight-loss effect means the absolute lean-mass reduction can be larger.

Absolute lean mass versus relative body composition

A second 2026 meta-analysis by Laverde and colleagues, in the International Journal of Obesity, looked specifically at GLP-1 receptor agonist effects on muscle health across randomised controlled trials^[3]. For semaglutide, the pooled estimate showed a mean lean-mass reduction of 5.44 kg. Across all GLP-1 agonists, the absolute lean-mass decrease averaged 1.74 kg, while lean mass improved as a proportion of total body weight by 1.81%.

The distinction between absolute and proportional lean mass is important. A person who loses 15 kg and drops 5 kg of lean mass ends up with a leaner overall body composition: a larger share of their remaining weight is muscle and bone rather than adipose tissue. The researchers concluded that lean-mass loss should not be considered a clinical limitation of obesity treatment per se, but they noted that the combination with resistance exercise and adequate protein intake produces better muscle outcomes.

How semaglutide compares with other weight-loss routes

The Eisa and Barood 2026 meta-analysis^[2] placed incretin pharmacotherapy and lifestyle intervention on comparable ground for the lean-mass fraction of weight loss (roughly 26% to 35% lean, depending on agent). Both perform worse than resistance-training programmes on this metric. Bariatric surgery typically shows a lean-mass fraction in the 20% to 30% range in longer-term data, similar to pharmacotherapy without exercise.

For readers following the GLP-1 class broadly, tirzepatide's lower lean-mass fraction in that analysis (25.4%) has attracted attention, though the two agents have not been compared head-to-head on body composition in a single trial designed for that endpoint.

What the evidence suggests about preserving lean mass

The published literature points consistently to resistance exercise and adequate protein intake as the two modifiable factors that reduce the lean-mass fraction of weight loss. Laverde and colleagues specifically flag these in their 2026 meta-analysis. The evidence base for this comes from the broader obesity and caloric-restriction literature, not from semaglutide-specific trials, but the underlying physiology is the same: progressive resistance training preserves and builds muscle even during energy deficit.

What the literature does not yet provide is a powered randomised trial comparing semaglutide alone against semaglutide combined with a structured resistance programme, with DEXA-derived lean mass as the primary endpoint. That data gap means the quantitative benefit of resistance training alongside semaglutide treatment remains an inference from general weight-loss physiology rather than a direct measurement.

What we do not yet know

Several questions remain open. The functional consequence of the lean-mass reduction observed in trials is not fully characterised: handgrip strength and physical performance data in the STEP programme are limited. Long-term data beyond two years for both body-composition and functional outcomes are sparse for semaglutide at the 2.4 mg obesity dose. The optimal protein intake and exercise prescription to pair with semaglutide has not been established in powered trials. And the question of whether lean-mass loss during treatment reverses, stabilises, or progresses during maintenance dosing is not yet answered.

Medical disclaimer

This article is for educational and journalistic purposes only and does not constitute medical advice. Semaglutide is a prescription medicine in every jurisdiction PeptideMethods tracks. Decisions about starting, stopping, or adjusting treatment belong with a qualified healthcare professional who knows your history. PeptideMethods.com does not sell, distribute, or facilitate the sale of any peptide or medicine.