When most people think about body fat, they think about what they can see: belly fat, body weight or the number on the scale.
But new research suggests another type of fat may matter too: fat stored inside and around your muscles.
In a large study published in Radiology, researchers in Germany used whole-body MRI scans from more than 11,000 adults to examine “intermuscular adipose tissue,” or fat hidden between muscle groups, alongside lean muscle mass. Even though participants had no known pre-existing medical conditions, higher levels of this hidden muscle fat were associated with greater odds of high blood pressure, unhealthy blood sugar and abnormal cholesterol patterns.
The study was cross-sectional, meaning it shows associations rather than proving cause and effect. But the findings add to growing evidence that metabolic health may depend on more than body weight alone.
“We found that the higher the intermuscular fat and the lower the muscle mass, the greater the cardiometabolic risk factors,” said lead author Dr. Sebastian Ziegelmayer of Technical University of Munich.
That matters because many participants considered generally healthy still showed previously undiagnosed issues, including hypertension, abnormal blood sugar and unhealthy lipid levels.
The findings also reinforce a growing shift in health science: BMI alone may not tell the full story.
Two people can have similar body weights but very different muscle quality and fat distribution, and potentially very different metabolic risk.
Researchers also found that lower physical activity was linked to both more hidden muscle fat and lower lean muscle mass, underscoring how movement may influence health even when weight does not dramatically change.
For women, the study found lean muscle mass tended to remain relatively stable until around ages 40 to 50, when it began to decline more sharply. Researchers say this pattern may overlap with menopause and hormonal changes.
While MRI is not a routine screening tool for most people, the study raises broader questions about whether future health assessments may increasingly focus on body composition, not just weight or BMI.
This study used data from the German National Cohort (NAKO), a large public research initiative funded by Germany’s Federal Ministry of Education and Research, German federal states, the Helmholtz Association, participating universities and Leibniz Association institutes.