Weight loss treatments do more than reduce weight, they subtly alter body composition, raising new scientific questions about their full impact.

New obesity treatments may be changing more than the number on the scale. Vanderbilt Health researchers found that both newer weight loss drugs and bariatric (weight loss) surgery improve body composition by sharply reducing fat mass while causing a smaller decline in fat-free mass, which includes lean muscle.

This matters because not all weight loss is biologically equal. Excess fat mass (FM) is tied to a higher risk of obesity-related death, including serious cardiovascular events. Fat-free mass (FFM), by contrast, plays a protective role and is closely linked to strength, physical function, and overall resilience.

In findings published in JAMA Network Open, the researchers said more work is needed to understand how these changes play out in real-world care after bariatric surgery or treatment with current GLP-1 receptor agonist drugs. The study was led by Danxia Yu, PhD, associate professor of Medicine in the Division of Epidemiology, and Jason Samuels, MD, assistant professor of Surgery.

Patient Data and Methodology

To explore that question, the team reviewed electronic health records from 3,066 adults treated at Vanderbilt University Medical Center. The study included 1,257 patients ages 18 to 65 who had bariatric surgery between 2017 and 2022, along with 1,809 patients who were treated with semaglutide or tirzepatide between 2018 and 2023. Patients with a history of end-stage kidney disease or congestive heart failure were excluded.

Researchers tracked changes in body composition using bioelectrical impedance analysis, a practical clinical method that estimates fat mass (FM) and fat-free mass (FFM) from factors such as height, weight, age, race, gender, diabetes history, and GLP-1 treatment duration. It is not the most precise tool available, but it allows doctors to follow body composition over time in large groups of patients.

Across 24 months, both approaches were associated with a clear pattern: fat mass dropped sharply, fat-free mass declined more modestly, and the ratio of fat-free mass to fat mass improved. Those changes were stronger in the surgery group, suggesting that bariatric procedures produced a larger shift in body composition overall.

By 24 months, adjusted fat mass reduction reached 49.7% in surgical patients, compared with 18.0% in patients treated with GLP-1 drugs. Fat-free mass also fell in both groups, but less dramatically, by 11.7% after surgery and 3.3% with medication.

Balance is Key

Weight loss is often discussed as a single number, but from a health perspective, losing fat while holding on to as much fat-free mass as possible may be the more meaningful goal. The findings suggest that both treatments move patients in that direction, even though surgery was associated with a more favorable fat-free mass to fat mass ratio over time because fat loss was greater.

The study also hints that body composition changes may not look the same for everyone. Men showed better long-term preservation of fat-free mass than women, especially with GLP-1 treatment. That raises a clinically relevant question as these drugs become more common: should some patients receive more targeted support, such as resistance training or higher protein intake, to help protect muscle during weight loss?

The researchers emphasized that the study was not designed as a direct head-to-head comparison between surgery and medication. The two groups started out with different baseline characteristics, including body mass index.

Reference: “Body Composition Changes After Bariatric Surgery or Treatment With GLP-1 Receptor Agonists” by Zicheng Wang, Lei Wang, Xinmeng Zhang, Brandon D. Lowery, Lauren Lee Shaffer, You Chen, Quinn S. Wells, Charles R. Flynn, Brandon Williams, Matthew Spann, Gitanjali Srivastava, Jason M. Samuels and Danxia Yu, 9 January 2026, JAMA Network Open.
DOI: 10.1001/jamanetworkopen.2025.53323

The study was supported in part by National Institutes of Health grants R01DK126721 and R01CA275864.

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