New research to be presented at the European Congress on Obesity (ECO2026) in Istanbul, Turkey (12–15 May) shows that following treatment with GLP-1 drugs for obesity, most weight loss is fat mass (80–85%) and relative muscle mass is preserved. The study is by Emilia Ida Frohner, Dr Alexander Jürets and Dr Bianca Karla-Itariu, Metabolism Center N°12 Antonigasse, Vienna, Austria and Medical University of Vienna, Austria.
Multiple clinical trials have shown glucagon-like peptide-1 (GLP-1)-based treatments (semaglutide/liraglutide) and GIP/GLP-1 (glucose-dependent insulinotropic polypeptide) dual agonists (tirzepatide) are safe and lead to weight reduction and decreased risk for adverse clinical outcomes. However, researchers are still trying to understand the relative effects on muscle and fat mass as a person loses weight using these drugs.
Bioelectrical Impedance Analysis (BIA) is a non-invasive, fast, and relatively inexpensive method for estimating body composition—including body fat, lean muscle mass, and water—by sending a low-level electrical current through the body. It relies on the principle that lean tissue conducts electricity better than fat.
Understanding changes in body composition, rather than weight alone, is essential to evaluate treatment quality and long-term functional outcomes using obesity drugs. In this new study, the authors analysed changes in skeletal muscle mass and fat mass during GLP-1RA or GIP/GLP-1RA therapy in adults with obesity using bioelectrical impedance analysis (BIA), and to evaluate the relationship between fat loss and muscle mass preservation over time.
This retrospective cohort study included 486 adult patients with obesity (18% male, 82% female; mean BMI 37.68 kg/m²; mean age 49.9 years) treated with GLP-1 receptor agonists or GIP/GLP-1 receptor agonists (liraglutide [8%], semaglutide [82%], or tirzepatide [8%]) at a specialised private obesity outpatient clinic in Vienna between 2022 and 2025. All patients received recommendations regarding physical exercise as per current guidelines. Body composition was assessed using bioelectrical impedance analysis (BIA), a non-invasive method for estimating fat and muscle mass.
Changes in absolute and relative skeletal muscle mass and fat mass from baseline to follow-up were analysed across different treatment durations (<6 months, 6–24 months, >24 months). If multiple measurements were available within a predefined time window, a single measurement per individual was selected based on the longest time since baseline. Computer modelling to adjust for repeated measurements was applied and adjusted for fat mass, time since treatment initiation, age, sex, and baseline BMI.
After a mean treatment duration of approximately 14 months, patients achieved a mean weight loss of 9.9%. Fat mass decreased by 9.0 kg (around 18%), while skeletal muscle mass decreased by only 1.2 kg (around 5%). Relative skeletal muscle mass was preserved or increased in more than 70% of patients. Statistical modelling showed that muscle mass remained stable over time when accounting for changes in fat mass, suggesting favourable body composition changes rather than clinically relevant muscle wasting.
In adjusted linear mixed-effects models, time since treatment initiation was not independently associated with skeletal muscle mass after accounting for fat mass and other variables. Fat mass was strongly associated with absolute skeletal muscle mass, indicating proportional changes in body composition rather than disproportionate muscle loss.
The authors say: "In this real-world cohort, GLP-1RA or GIP/GLP-1 RA therapy was associated with substantial fat mass reduction while largely preserving skeletal muscle mass in relative terms. Our analyses indicate that skeletal muscle mass remains stable over time when accounting for changes in fat mass, supporting the concept that GLP-1RA-induced weight loss reflects favourable body composition changes rather than clinically relevant muscle wasting."
The authors note that this is a retrospective real-world study with inherent limitations, including missing data, no placebo group, and a predominantly female study population. The findings apply to this specific treatment group and duration; larger prospective trials are needed to assess long-term effects.
