As GLP-1 use for weight loss becomes more common, prior treatment with the drugs is increasingly common in patients who go on to have bariatric surgery — but research has been limited on whether that prior use changes how much weight patients ultimately lose from the operation itself.
A new retrospective study out of UCSF, published as a research letter in JAMA Surgery and first reported by Medscape Medical News Aug. 18, suggests it doesn’t. Here are six things to know.
1. Researchers led by senior author Jonathan Carter, MD, of the Department of Surgery at the University of California, San Francisco, conducted a retrospective review of all patients who underwent bariatric surgery at the center between 2022 and 2024. Of the 383 patients included, 92 (24%) had previously been treated with a GLP-1 drug — semaglutide or the dual GLP-1/GIP drug tirzepatide — before surgery. Semaglutide was the most common (63% of that group), while just four patients (4%) had used tirzepatide, at relatively low median doses of 1 mg and 3.75 mg, respectively.
2. The two groups looked similar at baseline. Mean age was 44 in both, and there were no significant differences in gender (80% female among patients with no prior GLP-1 use vs. 78% among those who had used the drugs) or mean baseline BMI (44 vs. 43, respectively). One notable difference: 51% of patients with prior GLP-1 use had diabetes, compared with 16% of those without.
3. At 12 months after surgery, there was no significant difference in weight loss between the two groups. Patients with no prior GLP-1 use lost a mean 25% of total body weight, compared with 24% for those who had used the drugs. After adjusting for age, BMI, sex, race, ethnicity, diabetes, smoking status and procedure type, prior GLP-1 use still wasn’t a predictive factor for 12-month total weight loss. A higher baseline BMI and male sex were independently tied to greater weight loss, while diabetes and sleeve gastrectomy (compared with gastric bypass) were tied to less.
4. Dr. Carter called the lack of a difference a “slight surprise,” given that GLP-1 augmentation is considered one of the mechanisms behind bariatric surgery’s own effectiveness.
“Given that one mechanism of how bariatric surgery works is via GLP-1 augmentation, I thought that weight loss in patients on GLP-1s taken preoperatively might ‘steal’ from the weight loss we see later with the surgery,” he told Medscape Medical News. “In our study we saw no such effect — those taking GLP-1s, who presumably lost weight from the medications before surgery, turned out to lose additional weight the same as the GLP-1-naive patients.”
5. Diabetes control held up regardless of GLP-1 history. Only 15 patients resumed GLP-1 therapy within a year of surgery — five for diabetes management, eight for weight control and two for unknown reasons — and their mean 12-month total weight loss, 22%, wasn’t significantly different from patients who never went back on the drugs.
“Indeed, only five out of 93 diabetic patients were put on a GLP-1 after surgery, even though 47 were on GLP-1s before surgery,” Dr. Carter said. “So the 42 people with diabetes who kept off GLP-1s achieved normal or near normal A1c levels from the surgery itself.”
6. The findings track with other recent research, including studies of 2,169 patients in Indiana and 422 patients at Rush University Medical Center in Chicago, both of which found no significant weight-loss differences at 12 months regardless of prior GLP-1 use. A separate analysis of more than 9,000 patients in the Epic Cosmos database, presented in May at the American Society for Metabolic and Bariatric Surgery’s annual meeting, found similar results looking out to three years.
“When we restricted the patients to those who had already lost weight on GLP-1s, we found that overall, their total weight was similar to patients who just came into surgery as a primary strategy,” co-author Karan R. Chhabra, MD, an assistant professor of surgery and population health at NYU Grossman School of Medicine in New York City, told Medscape Medical News.
Dr. Carter expects the picture to keep filling in. “The data on this question will evolve over time with larger studies, newer drugs, and better understanding how much weight was lost at medicine initiation and then surgery,” he said. “My guess is that we will learn that combination therapy, with medicine and surgery, will outperform either therapy alone.”
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