Both bariatric surgery and semaglutide or tirzepatide treatment were associated with substantial fat mass loss, moderate fat-free mass loss, and improved fat-free mass to fat mass ratio over 24 months.
Key Findings
Results
Bariatric surgery was associated with substantially greater fat mass reductions than GLP-1RA treatment across all time points.
Adjusted mean relative FM reductions in the surgery group were 42.4% (95% CI, 41.5%-43.2%) at 6 months, 49.7% (95% CI, 48.8%-50.6%) at 12 months, and 49.7% (95% CI, 47.8%-51.5%) at 24 months.
Adjusted mean relative FM reductions in the GLP-1RA group were 10.3% (95% CI, 9.5%-11.0%) at 6 months, 17.3% (95% CI, 16.5%-18.1%) at 12 months, and 18.0% (95% CI, 16.4%-19.7%) at 24 months.
The surgery group had a mean baseline BMI of 46.8 (SD 7.1) compared to 41.0 (SD 7.9) in the GLP-1RA group.
The study included 1257 surgical patients and 1809 GLP-1RA-treated patients from a single center (Vanderbilt University Medical Center).
Results
Bariatric surgery was associated with greater fat-free mass reductions compared to GLP-1RA treatment.
Adjusted mean relative FFM reductions in the surgery group were 7.8% (95% CI, 7.2%-8.4%) at 6 months, 10.6% (95% CI, 10.0%-11.2%) at 12 months, and 11.7% (95% CI, 10.4%-12.9%) at 24 months.
Adjusted mean relative FFM reductions in the GLP-1RA group were 1.8% (95% CI, 1.3%-2.4%) at 6 months, 3.0% (95% CI, 2.4%-3.5%) at 12 months, and 3.3% (95% CI, 2.1%-4.4%) at 24 months.
Both groups experienced moderate FFM loss alongside their fat mass reductions.
Body composition was measured using bioelectrical impedance analysis (BIA) with 2 or more measures required per patient.
Results
FFM to FM ratios increased significantly in both groups, with surgical patients maintaining a higher ratio throughout the 24-month follow-up.
FFM to FM ratios in the surgery group were 1.8 (95% CI, 1.8-1.8) at 6 months, 2.1 (95% CI, 2.1-2.1) at 12 months, and 2.0 (95% CI, 2.0-2.1) at 24 months.
FFM to FM ratios in the GLP-1RA group were 1.4 (95% CI, 1.4-1.4) at 6 months, 1.5 (95% CI, 1.4-1.5) at 12 months, and 1.5 (95% CI, 1.5-1.6) at 24 months.
An improved FFM to FM ratio indicates a more favorable shift in body composition toward relatively greater lean mass.
Surgical patients maintained a higher FFM to FM ratio than GLP-1RA-treated patients at every time point measured.
Results
Men showed better FFM preservation than women, especially after GLP-1RA treatment.
Sex-stratified analyses revealed differential FFM preservation between men and women.
The sex difference in FFM preservation was particularly pronounced in the GLP-1RA treatment group.
Similar overall trends were observed across stratified analyses by sex, race, baseline BMI, baseline diabetes status, and GLP-1RA treatment duration.
Women comprised 82.2% of the surgery group (1033 of 1257) and 80.5% of the GLP-1RA group (1457 of 1809).
Methods
The study population consisted of patients aged 18 to 65 years identified from electronic health records at a single academic medical center.
The surgery group included 1257 patients (mean age 43.4 [SD 10.3] years) who underwent first-time bariatric surgery from November 21, 2017, to July 21, 2022, without GLP-1RA treatment from 1 year before to 2 years after surgery.
The GLP-1RA group included 1809 nonsurgical patients who started semaglutide or tirzepatide between November 12, 2018, and December 6, 2023, with 2 or more prescriptions or 5% or more weight loss.
All patients had 2 or more bioelectrical impedance analysis measures and no history of end-stage kidney disease or congestive heart failure.
Analyses controlled for age, sex, race, baseline BMI, diabetes history, treatment year, time (restricted cubic splines), and time spline-by-treatment interaction.
Wang Z, Wang L, Zhang X, Lowery B, Shaffer L, Chen Y, et al.. (2026). Body Composition Changes After Bariatric Surgery or Treatment With GLP-1 Receptor Agonists.. JAMA network open. https://doi.org/10.1001/jamanetworkopen.2025.53323