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By Melissa Fleur Afshar
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More lean body mass may not mean better metabolic health for everyone, a new study has found.
Research involving 59,269 adults suggests that where fat is stored in the body may be an important factor in understanding the relationship between fat-free mass and metabolic health. In particular, fat around internal organs and fat within muscle tissue appeared to largely shape associations with metabolic syndrome and elevated blood sugar.
Dr. Candida Rebello, the director of the Nutrition and Chronic Disease Laboratory at Pennington Biomedical Research Center, told Newsweek that the findings show why simply measuring fat-free mass does not always tell the whole story.
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“Our findings suggest that we need to look beyond the amount of fat-free mass a person has to better understand their metabolic health,” Rebello said. “Where fat is stored in the body matters, particularly fat around internal organs and fat that accumulates within muscle tissue.
"These different patterns of fat distribution can influence the relationship between fat-free mass and metabolic risk, underscoring the importance of considering the quality and composition of body tissues, not just their quantity, when studying metabolic health.”
Why Fat-Free Mass Matters
The study, published in The Journal of Clinical Endocrinology & Metabolism, analyzed body composition and metabolic health data from participants in the U.K. Biobank.
Researchers used dual-energy X-ray absorptiometry, known as DXA, and MRI imaging to examine how different measures of body composition were associated with metabolic syndrome and elevated blood sugar.
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Fat-free mass is often used as an indirect measure of skeletal muscle because muscle makes up most fat-free mass in many adults and plays an important role in blood sugar regulation and insulin response. However, fat-free mass, sometimes used interchangeably with lean body mass, also includes bones and organs.
The results differed depending on how researchers measured fat-free mass, surprising the team.
Higher total fat-free mass and fat-free mass adjusted for height were associated with a greater prevalence of metabolic syndrome and elevated glycated hemoglobin, which reflects average blood sugar levels. By contrast, having a higher proportion of body weight made up of fat-free mass was associated with a lower prevalence of both outcomes.
Where Fat Is Stored
The researchers also assessed visceral fat, which is stored deep in the abdomen around internal organs, and fat infiltration within muscle. They investigated whether sex hormones influenced the relationships between body composition and metabolic health.
Dr. Sri Lakshmi S. Devarakonda, a postdoctoral researcher and the study’s first author, said in a statement: “The findings suggest that muscle quantity alone may not provide a complete picture of metabolic health and that where fat is stored in the body is an important part of the story.”
The findings do not mean that having more muscle is harmful. Instead, they suggest that assessments of metabolic health need to take into account both the quantity and distribution of body fat.
Dr. Shira Eytan, a board-certified endocrinologist based in New York City who was not involved in the research, told Newsweek that fat-free mass itself is a broad measurement.
“Fat-free mass includes bone and organs as well as muscle, and DXA cannot show how much fat sits inside the muscle,” Eytan said. “The authors found that visceral fat and fat within muscle shape these associations, so where fat is stored matters, not only how much muscle someone has.”
She also cautioned against interpreting the findings as evidence of cause and effect.
“It is also a cross-sectional study, so it shows association, not cause, and it cannot say whether changing body composition would change metabolic risk,” Eytan added.
By combining DXA and MRI measurements with metabolic health data, the researchers were able to examine how both the quantity and distribution of fat related to metabolic risk.
Looking ahead, the findings could help guide future research into how body composition is assessed, suggesting that data on lean or fat-free mass alone could overlook important differences in where fat is stored and what that means for each person.
Reference
Rebello, C., Devarakonda, L, S. et al. (2026). Relationship between fat-free mass and metabolic syndrome is shaped by ectopic fat in a UK Biobank cohort. The Journal of Clinical Endocrinology & Metabolism. https://doi.org/10.1210/clinem/dgag407.
Contact Newsweek editors on this story: Kara Dolman and Paul Donnelley.
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