Bariatric surgery and GLP-1 medicines both reduce body fat – but surgery delivers greater long-term changes, study finds

A large real-world study published in JAMA Network Open has found that both bariatric surgery and newer GLP-1 receptor agonist medicines lead to significant fat loss over two years, although bariatric surgery produces greater overall changes in body composition.

The study, led by researchers at Vanderbilt University Medical Center, examined how bariatric surgery compares with treatment using the GLP-1 receptor agonists semaglutide and tirzepatide in terms of fat mass, fat-free mass and overall body composition.

Using electronic health records, researchers analysed data from 3,066 adults aged 18–65 years. Of these, 1,257 underwent first-time bariatric surgery, while 1,809 were treated with semaglutide or tirzepatide. All participants had repeated body composition measurements over a 24-month period using bioelectrical impedance analysis.

Importantly, this study reflects outcomes in routine clinical practice rather than tightly controlled trial settings, helping to address ongoing questions about how weight loss interventions affect fat and lean tissue in the real world.

Over two years, both treatment approaches were associated with substantial reductions in fat mass.

Patients who underwent bariatric surgery experienced the greatest changes, with average fat mass reductions of around 50 per cent at 12 and 24 months. In comparison, patients treated with GLP-1 receptor agonists saw more modest but still clinically meaningful reductions, averaging around 18 per cent at 12–24 months.

The study also examined fat-free mass (which includes muscle and other lean tissues). While some loss of fat-free mass was observed in both groups, this was considerably smaller than the loss of fat mass.

After 24 months, fat-free mass had decreased by approximately 12 per cent in the surgery group and just over 3 per cent in the GLP-1 treatment group. Importantly, the ratio of fat-free mass to fat mass improved in both groups, indicating a healthier overall body composition.

Men tended to preserve lean mass better than women, particularly among those receiving GLP-1 treatments – a finding that may have implications for personalised care and supportive interventions such as resistance training and nutritional optimisation.

The researchers conclude that both bariatric surgery and GLP-1 receptor agonists are effective tools for reducing fat mass and improving body composition over time. However, bariatric surgery was associated with a more favourable fat-free mass to fat mass ratio, largely due to greater overall fat loss.