Body Composition Changes in Weight Loss Treatments: GLP-1 Drugs vs. Bariatric Surgery

New research shows that GLP-1 receptor agonists and bariatric surgery primarily reduce fat mass, with only modest effects on muscle and lean tissue.

A series of recent scientific studies have analyzed changes in body composition associated with weight loss treatments, including GLP-1 receptor agonists and bariatric surgery. The findings indicate that these interventions primarily reduce fat mass while causing a modest reduction in fat-free mass, which includes muscle.

Overview of the Research

Two major analyses were published in peer-reviewed journals in early 2025:

1. Systematic Review and Meta-Analysis — Published in the International Journal of Obesity. This study examined the effects of GLP-1 receptor agonists and dual GLP-1/GIP agonists on body weight and composition in adults with overweight or obesity, with or without type 2 diabetes. The analysis included 36 studies.

2. Retrospective Cohort Study — Led by Vanderbilt Health researchers and published January 9, 2025, in JAMA Network Open. This study analyzed electronic health records to compare body composition changes in patients treated with GLP-1 medications (semaglutide or tirzepatide) or bariatric surgery.

Key Findings on Weight and Composition

GLP-1 Receptor Agonist Treatments

The meta-analysis of GLP-1 agonists, which primarily included liraglutide and semaglutide, reported the following changes over time:

• At 3 months: Body mass decreased by 9%. Fat body mass (FBM) decreased by 17%, while lean body mass (LBM) decreased by only 2%. Visceral adipose tissue (VAT) area was reduced by 29.25 cm².
• At 6 months: Body mass decreased by 5%. FBM decreased by 6%, and LBM decreased by 1%.
• At 12 months: Body mass decreased by 4%. Both FBM and LBM decreased by 4%.

The authors noted that weight loss was primarily driven by fat reduction, with relative preservation of lean mass, though some loss of lean body mass was observed.

Bariatric Surgery vs. GLP-1 Medications

The Vanderbilt Health cohort study included 1,257 bariatric surgery patients (treated between 2017 and 2022) and 1,809 patients treated with semaglutide or tirzepatide (treated between 2018 and 2023). Patients with a history of end-stage renal disease or congestive heart failure were excluded. Body composition was assessed using bioelectrical impedance analysis over a 24-month period.

• Surgery Group: Mean FM reductions were 42.4% at 6 months and 49.7% at 12 and 24 months.
• GLP-1 Group: Mean FM reductions were 10.3% at 6 months, 17.3% at 12 months, and 18% at 24 months.
• Bariatric surgery was associated with a more pronounced decrease in fat mass.

• Surgery Group: Mean FFM reductions were 7.8% at 6 months, 10.6% at 12 months, and 11.7% at 24 months.
• GLP-1 Group: Mean FFM reductions were 1.8% at 6 months, 3% at 12 months, and 3.3% at 24 months.

The ratio of fat-free mass to fat mass (FFM/FM) significantly increased in both groups, indicating an improvement in overall body composition. The surgery group showed a greater increase in this ratio.

Sex Differences: Male patients in both treatment groups demonstrated better long-term preservation of fat-free mass compared to female patients.

Clinical Context and Implications

The researchers from the International Journal of Obesity meta-analysis highlighted the need for future research on combining lifestyle interventions, nutritional support, and resistance training to preserve muscle mass, particularly in older or frail populations.

Mir Ali, MD, a bariatric surgeon at MemorialCare Surgical Weight Loss Center, stated that the findings align with his clinical observations, noting that patients primarily lose fat, though some muscle loss may occur.

The authors of the JAMA Network Open study, led by Danxia Yu, PhD, and Jason Samuels, MD, emphasized the need for further prospective studies to observe changes in fat mass and fat-free mass in real-world clinical settings following these treatments. The study was supported in part by National Institutes of Health grants.