‘Skinny fat’ can quietly damage arteries even in people who appear healthy

A new global analysis has found that hidden fat—particularly the kind that builds up deep inside and around your organs and liver—may be quietly harming your arteries. The damage can begin years before you notice any signs. The research sheds light on how these hidden stores of fat contribute to cardiovascular disease, even in people who appear healthy.

For decades, the standard measure of obesity has been body mass index (BMI). But researchers at McMaster University and collaborating centres say that number on the scale might not be the entire story.

Their study shows that the fat that covers internal organs, visceral fat, and fat in the liver, hepatic fat, both are associated with the early hardening and narrowing of neck arteries, the carotid arteries. These arteries carry blood to the brain, and when they become constricted, the danger of stroke and heart attack increases.

"This research indicates that even after adjusting for established cardiovascular risk factors such as blood pressure and cholesterol, both liver and visceral fat still play a role in damaging arteries," explains Russell de Souza, co-lead author and associate professor in McMaster's Department of Health Research Methods, Evidence, and Impact.

The scientists compared more than 33,000 adults from the two large cohorts: the Canadian Alliance for Healthy Hearts and Minds (CAHHM) and the UK Biobank. The participants had advanced MRI scans to measure fat distribution and artery health.

In Canada, almost 6,800 volunteers were scanned with MRI to measure their visceral fat, liver fat, and carotid artery wall thickness. In the UK, over 26,000 adults were scanned with MRI for fat and their artery walls with ultrasound. When the two large datasets were combined, a clear trend emerged. People with more visceral fat had thicker artery walls. This link held true even after scientists accounted for age, blood pressure, cholesterol, smoking, and diabetes.

The link between liver fat and artery expansion was weaker but still robust. On average, every standard increase in level of visceral fat was associated with a measurable increase in thickness of artery walls—enough to suggest a substantial rise in cardiovascular risk.

Visceral fat does not resemble the fat right under your skin. It grows around vital organs such as the heart, liver, and intestines and is closer to a chemical factory than a storehouse that stays dormant. Visceral fat releases inflammatory chemicals and hormones that disrupt metabolism, raise blood pressure, and undermine artery walls.

"You can't necessarily see by looking at a person if they have liver or visceral fat," says Sonia Anand, lead author of the study and vascular medicine expert at Hamilton Health Sciences. "This type of fat is active and toxic; it's associated with inflammation and damage to the arteries even in individuals who are not overtly overweight."

Scientists believe that when the body's subcutaneous fat, or fat underneath the skin, is at capacity, excess energy spills over into visceral stores and into the liver. This excess stimulates inflammation and insulin resistance, leading to atherosclerosis, the gradual accumulation and stiffening of arteries.

Among the Canadian sample, men had about 82 mL of visceral fat compared to women at 61 mL and greater liver fat and wider artery walls. With every standard increase in visceral fat, artery wall volume was increased by about 6 mm³ after adjustment for blood pressure, cholesterol, and diabetes.

For each UK group standard rise in visceral fat, the thickness of arteries rose by 0.014 mm. That may not sound much, but other research has shown such differences to contribute to as large a change in relative cardiovascular risk as 30 percent.

These are nuanced results on imaging, but they are of practical importance," de Souza says. "They suggest that where you deposit fat in your body might be as important—if not more so—than how much fat you have."

The findings suggest that BMI and waist size may overlook precisely the patients most at risk. Imaging studies may enable doctors to detect early heart changes in seemingly healthy patients.

In the general population, reduction of liver and visceral fat is often possible by means of lifestyle such as exercise, healthy diets with low amounts of processed sugars and saturated fats, and a normal weight. Moderate weight loss has proven to reduce visceral fat stores and enhance cardiac health.

"People are likely to view weight loss in terms of a number on a scale," says Anand. "But targeting the sort of fat that causes inflammation and damage to arteries may be a healthier way to safeguard the heart."

This study could redefine the way cardiologists screen and treat cardiovascular disease. With visceral and liver fat identified as predictors of artery damage independent of other factors, clinicians will be able to tailor prevention even more precisely.

Imaging technology such as MRI or ultrasound could become part of expanded health testing for individuals at risk, particularly those with a normal BMI but signs of metabolic stress.

For the public, the message is this: even if you are thin, your arteries can be threatened by extra fat that wraps around organs. Exercise every day, eat sensibly, and have a little alcohol and sugars in moderation to reduce your unseen fat and heart attack and stroke risk.

Research findings are available online in the journal Communications Medicine.

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