ECO 2026
GLP-1s spare muscle, but less so than diet, exercise
Clinical Takeaway: GLP-1 therapy doesn't appear to cause clinically significant muscle wasting, but patients lose a larger share of fat-free mass than they would through lifestyle change alone. Pairing pharmacotherapy with resistance exercise and adequate protein intake remains the most defensible strategy for protecting muscle and bone during weight loss, particularly in older patients and those at risk for sarcopenia.
Two new analyses presented at the European Congress on Obesity offer a clearer picture of what happens to muscle during GLP-1-driven weight loss. One asks whether GLP-1 patients lose muscle disproportionately to their fat loss; the other asks how the muscle-to-fat ratio of weight lost compares across treatment strategies. Together they suggest that whether GLP-1s "preserve muscle" depends on the comparison: muscle loss is small relative to fat loss, but larger than the muscle loss seen with diet and exercise alone with equivalent weight loss.
GLP-1 therapy delivered meaningful weight loss that came predominantly from fat rather than muscle in a Vienna real-world cohort, with relative muscle mass holding steady or improving in most patients and no signal of disproportionate muscle loss over time. Among 486 adults with obesity treated for about 14 months with liraglutide, semaglutide, or tirzepatide, mean weight loss was 9.9%. Fat mass fell 9.0 kg (19.8 lbs), or 18%, while skeletal muscle mass dropped 1.2 kg (2.6 lbs), or 5%. Fat accounted for 80 to 85% of total weight loss, and relative muscle mass (muscle as a percentage of body mass) was preserved or increased in more than 70% of patients.
When benchmarked against other weight-loss approaches, however, GLP-1s gave up a substantially larger share of fat-free mass than diet and exercise interventions did. This was roughly the same share as bariatric surgery, in a separate University of Copenhagen meta-analysis. Pooling 21 randomized trials of 1,334 participants who lost at least 10% of body weight, mean fat-free mass reductions were 1.80 kg (4.0 lbs) with diet and exercise (12 studies), 4.75 kg (10.5 lbs) with incretin drugs versus placebo (5 studies), and 9.14 kg (20.1 lbs) with bariatric surgery (4 studies). Fat-free mass accounted for roughly 14 to 21% of weight lost with diet and exercise, 31.5% with incretin drugs, and 32.9% with surgery.
Both lines of evidence point to a similar clinical implication. "These findings underscore the importance of incorporating strategies aimed at preserving fat-free mass, particularly structured exercise across all weight-loss approaches," wrote the Copenhagen authors. The Vienna investigators reached a parallel conclusion, framing GLP-1-induced weight loss as reflecting "favorable body composition changes rather than clinically relevant muscle wasting" in a cohort that received exercise counseling as standard care.
Sources: Frohner EI, Jürets A, Itariu BK. Preservation of skeletal muscle mass during GLP-1 receptor agonist and GIP/GLP-1 receptor agonist therapy in adults with obesity: A retrospective bioimpedance analysis. Poster abstract 0408, European Congress on Obesity 2026, Istanbul; Madsen LB, Busk-Cirera L, Carlsen IØ, et al. Effects of Incretin-Based Therapies, Diet and Exercise Interventions, and Bariatric Surgery on Fat-Free Mass in Adults with Overweight or Obesity: A Systematic Review and Meta-Analysis. Poster abstract 0781, European Congress on Obesity 2026, Istanbul