Fat stored deep inside the abdomen and liver can silently damage arteries—even in individuals who appear healthy or maintain a normal weight, says a groundbreaking study by McMaster University scientists.
Published in Communications Medicine, the research challenges the long-standing reliance on body mass index (BMI) as the main indicator of obesity and heart disease risk. The findings show that hidden fat, often called visceral and hepatic fat, carries dangerous cardiovascular implications invisible through traditional weight measurements.
Going Beyond BMI to Measure True Risk
For decades, BMI has been used to determine whether someone is underweight, healthy, overweight, or obese. However, BMI merely divides weight by height and does not distinguish between muscle, fat, or fat distribution across the body.
Lead author Dr. Russell de Souza, associate professor at McMaster’s Department of Health Research Methods, explained: “Our data show that hidden fat reserves are stealthily contributing to artery injury, undetected by standard obesity measures. Many people who seem slim by BMI still carry dangerous visceral or liver fat.”
Using advanced MRI scans and data from 33,000 adults in Canada and the United Kingdom, the research team demonstrated that greater levels of visceral and liver fat correlate with significant thickening and plaque buildup in the carotid arteries—the vessels that deliver blood to the brain. These arterial changes heighten the risk of stroke and heart attack, regardless of cholesterol, blood pressure, or overall weight.
Inside the Study: Two Major Cohorts, One Global Message
The scientists combined data from two landmark studies:
The Canadian Alliance for Healthy Hearts and Minds (CAHHM)
The UK Biobank
Participants underwent high-resolution MRI imaging to assess internal fat distribution and carotid artery health. Researchers then adjusted their findings for lifestyle and metabolic factors such as diet, physical activity, smoking, cholesterol levels, and blood glucose.
The results remained clear—visceral fat (fat packed around internal organs) showed the strongest connection to artery thickening, while liver fat exhibited a modest but still independent effect. Collectively, both types of fat form the foundation of a hidden yet potent risk for cardiovascular disease.
The Metabolic Danger of Hidden Fat
Visceral and liver fat differ from the fat found beneath the skin. These internal fat deposits are metabolically active, releasing inflammatory molecules, hormones, and lipids that disrupt vascular function and accelerate artery damage.
Dr. Sonia Anand, corresponding author and vascular medicine specialist at McMaster, emphasized the silent threat: “This kind of fat is metabolically active and dangerous. It fuels inflammation and artery damage even in people who aren’t visibly overweight. You can’t always tell by looking whether someone is at risk.”
This redefines how doctors and the public should think about obesity. Even people categorized as “normal weight” can have high levels of internal fat—sometimes called “skinny fat”—that increase cardiovascular risk.
Hidden Risk in Plain Sight
The concept of “metabolically obese, normal-weight” individuals is gaining scientific recognition. These are people who may appear fit externally but have internal fat surrounding their organs, driving metabolic and vascular diseases.
Professor Marie Pigeyre, co-lead author of the study, noted: “Traditional measures like BMI and even waist circumference often fail to capture where fat is stored. MRI scans provide a much clearer picture of who may be at risk, even among those with a healthy weight.”
The research showed that visceral fat’s impact persisted across BMI categories: thin individuals with high internal fat were statistically as likely to have arterial thickening as visibly obese individuals with similar internal deposits.
Why BMI Alone Doesn’t Tell the Whole Story
Under current guidelines, BMI between 18.5 and 24.9 is considered healthy. Yet, emerging evidence suggests that relying on BMI alone underestimates health risks. This underestimation affects nearly 60% of adults with hidden fat accumulation.
A recent international analysis involving the University of Arizona, Harvard, andMassachusetts General Hospital reached similar conclusions—adding waist circumference and body fat imaging could redefine obesity metrics across populations. This shift could double obesity classification rates in older adults. It provides earlier opportunities for preventive healthcare. These opportunities arise before major cardiovascular events occur.
Striking Results After Lifestyle and Risk Adjustments
Adjusting for diet, exercise, and cholesterol made little difference. The fat deep inside the abdomen remained an independent predictor of artery injury. “These findings suggest that visceral fat behaves like an organ of its own,” said de Souza. “It actively produces inflammatory molecules that accelerate artery thickening.”
Even when accounting for blood pressure, glucose control, and body weight, the effect size of internal fat remained strong. This reinforces the need for advanced screening methods beyond visual or scale-based assessments.
What the Results Mean for Public Health
The study’s implications extend far beyond clinical practice. Millions of “healthy-weight” individuals could unknowingly face heightened risks of stroke, heart attack, or metabolic diseases.
Clinicians may need to adopt imaging tools like MRI scans. They might also use specialized ultrasound to assess fat distribution. These assessments are part of preventative heart care. However, researchers acknowledge that MRI scans are expensive, and broader screening may depend on future cost-reduction in imaging technology.
“If imaging isn’t available,” said Anand, “risk assessments should integrate lifestyle factors and metabolic indicators. Critically, waist-to-height ratios should also be included to gauge potential internal fat burden.”
Changing How We Think About Obesity and Heart Health
The research represents a paradigm shift in understanding cardiovascular disease risk. Instead of focusing solely on body size or visible obesity, science now recognizes where fat is stored as equally important.
Future treatment and prevention should address the underlying metabolic environment. This environment encourages visceral and liver fat buildup. Nutrition, physical activity, and stress management are essential. For example, diets rich in whole foods are effective in reducing visceral fat. Regular aerobic exercise helps improve arterial function. Adequate sleep further supports these outcomes.
Funding and Institutional Collaboration
The project was supported by the Canadian Partnership Against Cancer, Heart and Stroke Foundation of Canada, and the Canadian Institutes of Health Research. Collaborating institutions included the Population Health Research Institute, Montreal Heart Institute, Sunnybrook Health Sciences Centre, and Bayer AG, which contributed imaging contrast material.
This landmark effort brought together researchers across multiple disciplines, setting a new benchmark for assessing internal fat and cardiovascular health in over 33,000 participants.
The Bottom Line: Hidden Fat, Visible Consequences
The McMaster study serves as a wake-up call — metabolic health can’t be judged by weight alone. Hidden visceral and liver fat are silent dangers. They can damage arteries. They increase heart disease risk, regardless of outward appearance.
As de Souza concluded, “The scale can deceive you, but your arteries don’t lie. What matters most is what’s beneath the surface.”
