When most people think about dangerous body fat, they picture the visible kind — the belly fat that expands waistbands, or the subcutaneous fat you can pinch beneath the skin. But a growing body of research is pointing to a less obvious threat: fat that accumulates within and between muscle tissue, largely invisible to the naked eye and undetectable by conventional health screenings.
A large-scale study published in Radiology, the journal of the Radiological Society of North America, found that this hidden muscle fat — known as intermuscular adipose tissue — is significantly associated with elevated risks of hypertension, unstable blood sugar, and unhealthy cholesterol levels, even in individuals who appeared to be in good health.
What Is Intermuscular Adipose Tissue?
Intermuscular adipose tissue (IMAT) is a type of ectopic fat — fat that accumulates in locations where it doesn’t belong metabolically. Unlike subcutaneous fat (the fat just under your skin) or visceral fat (the fat surrounding internal organs), IMAT infiltrates the spaces between muscle fibers and muscle groups.
This type of fat has long been studied in the context of aging and sarcopenia (muscle loss), but its role as an independent cardiometabolic risk factor is only now coming into clearer focus. Researchers have found that IMAT behaves differently from other fat depots: it secretes inflammatory cytokines, disrupts insulin signaling within nearby muscle cells, and may impair the muscles’ ability to metabolize glucose efficiently.
What the Research Found
The study analyzed whole-body MRI scans from more than 11,000 adults who reported no known pre-existing health conditions. Researchers used deep learning algorithms to quantify both lean muscle mass and intermuscular fat deposits, focusing particularly on spinal muscles.
The results were striking. Among participants who considered themselves healthy:
- 16.2% had undiagnosed high blood pressure
- 8.5% showed signs of unstable blood sugar levels
- 45.9% had unhealthy lipid profiles — including elevated LDL cholesterol or triglycerides
Higher intermuscular fat levels were consistently linked to increased risks across all three of these cardiometabolic markers. Participants with greater amounts of muscle fat were significantly more likely to fall into the at-risk categories for hypertension, metabolic dysregulation, and abnormal cholesterol — regardless of how “normal” they might appear by standard health metrics like body mass index (BMI).
Why Standard Metrics Miss This
One of the study’s most important implications is the inadequacy of conventional screening tools. BMI measures overall body weight relative to height, but it cannot distinguish between fat and muscle, nor can it identify where fat is stored. A person with a normal BMI could still carry significant amounts of intermuscular fat — and by extension, elevated cardiometabolic risk — without ever triggering concern during a routine physical exam.
The same applies to waist circumference measurements, which capture visceral fat accumulation but not IMAT. This means millions of people may be walking around with a hidden risk factor that current standard-of-care checkups simply don’t catch.
Dr. Cheng-Han Chen, an interventional cardiologist, noted that these findings “reiterate the importance of proper muscle composition and mass on maintaining good health” and support incorporating regular strength training into exercise regimens as a preventive health strategy.
Lean Muscle Mass as a Protective Factor — With an Important Caveat
The study also found an encouraging counterpoint: greater lean muscle mass was associated with lower cardiometabolic risk. People with higher ratios of lean muscle to fat showed better blood pressure, blood sugar control, and lipid profiles — reinforcing decades of research on the metabolic benefits of skeletal muscle.
However, researchers noted an important gender difference. The protective effect of lean muscle mass appeared to apply primarily to males. In females, particularly those in menopausal or post-menopausal age groups, this protective association was less pronounced. Scientists hypothesize that hormonal shifts — particularly the decline in estrogen — may alter how muscle composition interacts with cardiometabolic risk in women, though further research is needed to fully understand these mechanisms.
Can Intermuscular Fat Be Detected?
At present, MRI is the gold standard for quantifying intermuscular adipose tissue — and the researchers noted that MRI scans, already widely used clinically for orthopedic, neurological, and oncologic diagnoses, could potentially be repurposed to assess muscle composition without requiring additional procedures.
Deep learning-assisted image analysis — like the algorithms used in this study — could make this kind of assessment increasingly practical and affordable. As AI-assisted radiology tools advance, routine MRI scans may one day provide cardiometabolic risk profiling alongside their primary diagnostic purpose.
Other imaging modalities, such as computed tomography (CT) and dual-energy X-ray absorptiometry (DEXA), can also provide some muscle composition data, though MRI generally offers the highest resolution for soft-tissue differentiation.
What You Can Do to Reduce Muscle Fat
Research suggests that lifestyle interventions — particularly regular physical activity — are the most effective way to reduce IMAT and improve overall muscle composition. In the study, lower physical activity levels were strongly correlated with higher intermuscular fat deposits and lower lean muscle mass.
The American Heart Association recommends:
- 150 minutes per week of moderate-intensity aerobic activity (such as brisk walking, cycling, or swimming), or
- 75 minutes per week of vigorous aerobic activity (such as running or high-intensity interval training)
- Muscle-strengthening exercises on at least two days per week
Resistance training, in particular, appears to be highly effective at reducing intramuscular fat while simultaneously building lean muscle mass. Studies indicate that progressive resistance exercise can shift muscle composition favorably over time, even in older adults and individuals who are not classified as overweight by standard metrics.
Dietary strategies may also play a role. Research suggests that diets rich in lean protein, omega-3 fatty acids, and anti-inflammatory foods — such as those found in the Mediterranean diet — may support healthy muscle composition. Conversely, ultra-processed food consumption has been linked to greater muscle fat accumulation in some studies.
The Bigger Picture: Rethinking How We Assess Health
This research joins a growing body of evidence challenging the simplistic notion that a “normal” weight or BMI equals good health. Cardiometabolic risk is multidimensional, and hidden fat depots — whether visceral, pericardial, or now intermuscular — are emerging as important pieces of the puzzle.
As diagnostic tools like MRI and AI-assisted imaging become more accessible, clinicians may be better positioned to identify patients who appear healthy by conventional measures but carry significant metabolic risk beneath the surface. For now, the most actionable takeaway remains consistent with longstanding public health guidance: regular physical activity, including strength training, is one of the most powerful tools available for maintaining not just visible fitness, but deep metabolic health.
Consult your healthcare provider to discuss the most appropriate screening tools and exercise plan for your individual health profile and risk factors.
Disclosure: This content is for informational purposes only and is not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen.