When people lose a significant amount of weight, not all of that loss comes from fat. Some can come from fat-free mass, a category that includes muscle, bone, organs, water and other non-fat tissue.
A new systematic review and meta-analysis presented at the 2026 European Congress on Obesity suggests that the type of weight-loss intervention may affect how much fat-free mass people lose. The review found that among adults with overweight or obesity who lost at least 10% of their body weight, diet and exercise interventions were linked to a lower proportion of fat-free mass loss than incretin-based obesity drugs or obesity surgery.
The findings should be interpreted with caution. The research was presented as a conference poster abstract and has been submitted to a scientific journal, where it is still undergoing peer review. One author reported research grants, lecture fees and advisory board involvement with Novo Nordisk, along with lecture fees or research funding from other companies. Another author reported holding shares in Novo Nordisk. Some funding support came through programs funded by the Novo Nordisk Foundation.
The review included 21 studies with 1,334 participants. Researchers looked at randomized controlled trials published between 2015 and Oct. 21, 2025, that included adults with overweight or obesity, with or without type 2 diabetes. To be included, participants had to lose at least 10% of their body weight through diet and exercise, incretin-based obesity drugs or obesity surgery.
The studies also had to include body composition measurements using dual-energy X-ray absorptiometry, known as DXA, or muscle measurements using CT or MRI scans.
Across the included studies, average weight loss differed by approach. People in diet and exercise studies lost an average of 11.6 kilograms, or about 25.6 pounds. People receiving incretin-based obesity drugs lost an average of 15.9 kilograms, or about 35.1 pounds, compared with placebo. People who had obesity surgery lost an average of 27.4 kilograms, or about 60.4 pounds.
Fat-free mass loss also varied. The average loss was 1.80 kilograms, or about 4 pounds, with diet and exercise; 4.75 kilograms, or about 10.5 pounds, with incretin-based obesity drugs; and 9.14 kilograms, or about 20.2 pounds, with obesity surgery.
When researchers looked at the proportion of total weight loss that came from fat-free mass, they found that it accounted for 14.3% of weight lost through diet and exercise, 31.5% of weight lost with incretin-based obesity drugs and 32.9% of weight lost after obesity surgery.
That does not mean one-third of the weight lost from medications or surgery was necessarily muscle. Fat-free mass is broader than muscle alone, and the researchers noted that muscle mass specifically was measured using MRI in only one study. Because of that, they could not conduct a separate analysis of muscle loss.
Still, the findings highlight an important point that can get lost in weight-loss conversations: the quality of weight loss matters. Losing fat can improve health for many people, especially those with obesity-related conditions, but preserving muscle and bone is also important for strength, mobility, metabolism and long-term function.
The authors said the results point to the importance of strategies that help protect fat-free mass during weight loss.
“Among the included studies with at least 10% weight loss in individuals with overweight or obesity, diet and exercise interventions were associated with the smallest reductions in fat-free mass, whereas incretin-based obesity drugs and obesity surgery were associated with substantially greater proportional losses of fat-free mass,” the authors said. “These findings underscore the importance of incorporating strategies aimed at preserving fat-free mass, particularly structured exercise across all weight-loss approaches.”
The review does not suggest that obesity medications or surgery should be dismissed. Both can lead to substantial weight loss and may be appropriate medical tools for some people. But the findings add to a growing conversation about how to support people during major weight loss, including through resistance training, adequate protein intake and medical guidance tailored to individual needs.
One author reported research grants, lecture fees and advisory board involvement with Novo Nordisk, lecture fees from Merck and Ferring and a research grant from Embla. Another author reported holding shares in Novo Nordisk. All other authors reported no conflicts of interest.
Funding support included a PhD scholarship from the Danish Diabetes and Endocrine Academy, which is funded by the Novo Nordisk Foundation. Other authors were supported by the Danish Cardiovascular Academy, which is funded by the Novo Nordisk Foundation and the Danish Heart Foundation.