Hidden Fat, Not Body Weight, May Be the Real Culprit Behind Heart Disease

For decades, body weight, and particularly body-mass index (BMI), has been the cornerstone of how clinicians assess obesity and cardiovascular risk. But emerging research suggests that what we can’t see may matter far more than what we can.

A large new study led by scientists at McMaster University has found that fat stored deep inside the abdomen and liver can quietly damage arteries, even in people who appear healthy and fall within a “normal” weight range. The research, published on October 17, 2025, challenges the long-standing reliance on BMI and adds compelling evidence that hidden fat, not scale weight, may be a major driver of heart disease.

Going Beyond BMI to Understand True Cardiovascular Risk

Visceral fat, the fat that surrounds internal organs, and hepatic fat, which accumulates inside the liver, have long been associated with conditions such as Type 2 diabetes, hypertension, and metabolic syndrome. Until now, however, their direct impact on artery health had not been well established in large population studies.

Using advanced MRI imaging and data from more than 33,000 adults in Canada and the United Kingdom, researchers examined fat distribution alongside measures of artery health. The results were striking: higher levels of visceral and liver fat were strongly associated with thickening and plaque buildup in the carotid arteries, the major blood vessels that supply the brain.

When these arteries become narrowed or stiffened, the risk of stroke and heart attack rises substantially. Importantly, the association between hidden fat and artery damage remained significant even after accounting for traditional cardiovascular risk factors such as cholesterol levels, blood pressure, physical activity, and smoking.

Hidden Fat Adds Risk—Even When Everything Else Looks “Normal”

The findings suggest that visceral and liver fat exert harmful effects on blood vessels independent of other known risk factors. In other words, a person can have acceptable cholesterol, normal blood pressure, and a “healthy” BMI, yet still carry a silent burden of cardiovascular risk due to fat stored around internal organs.

The analysis drew on two major population cohorts: the Canadian Alliance for Healthy Hearts and Minds and the UK Biobank. MRI scans were used to precisely measure both fat distribution and carotid artery structure.

Visceral fat showed the strongest and most consistent relationship with artery wall thickening and plaque formation. Liver fat had a smaller but still meaningful association. These relationships persisted even after adjusting for diet, exercise habits, and metabolic health markers, underscoring that hidden fat itself appears to be biologically active and harmful.

Rethinking How We Measure Obesity and Heart Risk

This research highlights a critical limitation of BMI: it cannot distinguish between fat and muscle, nor can it reveal where fat is stored. Two people with the same BMI may have vastly different cardiovascular risk profiles depending on how much visceral or liver fat they carry.

For clinicians, these findings point to the need for more nuanced risk assessment strategies. Imaging tools that identify fat stored around internal organs may provide a more accurate picture of cardiovascular health than weight or waist circumference alone, particularly for individuals in midlife who appear outwardly healthy.

For patients, the message is both sobering and empowering. You can’t always tell by looking at someone whether they carry dangerous visceral or liver fat. This “hidden” fat is metabolically active, promotes inflammation, and contributes to artery damage, even in people who are not visibly overweight.

The Bigger Takeaway

Heart disease does not always announce itself through excess body weight. Sometimes, it develops quietly beneath the surface. This study reinforces the idea that true cardiovascular health is about more than what the scale says, it’s about what’s happening inside the body.

As science continues to refine our understanding of obesity and heart disease, one thing is becoming clear: to prevent cardiovascular events, we must look beyond BMI and start paying closer attention to the fat we can’t see.