You might look healthy, but hidden fat could be silently damaging your heart

The research, published on October 17, 2025, in Communications Medicine, questions the long-standing use of body-mass index (BMI) as a reliable indicator of obesity and heart risk. It offers new evidence that the fat people cannot see may be just as dangerous as the weight they can.

Going Beyond BMI to Understand True Health Risks

Visceral fat (which surrounds internal organs) and hepatic fat (fat stored in the liver) have long been associated with Type 2 diabetes, high blood pressure, and heart disease. However, their direct impact on artery health had not been well established until now.

Using advanced MRI scans and data from more than 33,000 adults in Canada and the United Kingdom, the researchers discovered that higher levels of visceral and liver fat were closely tied to thickening and clogging of the carotid arteries in the neck. These arteries carry blood to the brain, and when they narrow, they increase the risk of stroke and heart attack.

"This study shows that even after accounting for traditional cardiovascular risk factors like cholesterol and blood pressure, visceral and liver fat still contribute to artery damage," says Russell de Souza, co-lead author of the study and associate professor in the Department of Health Research Methods, Evidence, and Impact at McMaster.

Hidden Fat Adds Risk Even in the Absence of Other Factors

"The findings are a wake-up call for clinicians and the public alike," says de Souza, a faculty member in the Mary Heersink School of Global Health and Social Medicine, and member of the Centre for Metabolism, Obesity and Diabetes Research (MODR) and at McMaster, the results should prompt both doctors and patients to pay attention to hidden fat, not just visible weight. He led the research with co-author Marie Pigeyre, associate professor in McMaster's Department of Medicine.

The analysis drew from two major population studies: the Canadian Alliance for Healthy Hearts and Minds (CAHHM) and the UK Biobank. MRI scans were used to measure fat distribution and artery condition. The team found that visceral fat was consistently associated with plaque buildup and artery wall thickening, while liver fat had a smaller but still important effect. These relationships remained significant even after adjusting for lifestyle habits and metabolic risk factors such as diet, exercise, and cholesterol.

Rethinking How We Measure Obesity

The findings highlight the need for clinicians to look beyond BMI or waist measurements when assessing heart risk. Imaging tests that reveal fat stored around internal organs may offer a more accurate picture of cardiovascular health.

For people in midlife, the study is a reminder that even a normal weight does not guarantee a healthy heart. Hidden fat can quietly increase the risk of serious disease without obvious physical signs.

"You can't always tell by looking at someone whether they have visceral or liver fat," says Sonia Anand, corresponding author of the study, a vascular medicine specialist at Hamilton Health Sciences and professor in the Department of Medicine at McMaster. "This kind of fat is metabolically active and dangerous; it's linked to inflammation and artery damage even in people who aren't visibly overweight. That's why it's so important to rethink how we assess obesity and cardiovascular risk."

This research was supported by the Canadian Partnership Against Cancer, Heart and Stroke Foundation of Canada, and the Canadian Institutes of Health Research, with additional contributions from the Population Health Research Institute, Montreal Heart Institute, Sunnybrook Health Sciences Centre, and others. MRI reading costs were supported in-kind by Sunnybrook Hospital, and Bayer AG provided IV contrast. The study also drew on data from the Canadian Partnership for Tomorrow's Health and the PURE Study.