An international study has found that hidden fat stored deep inside the body — including around vital organs and in the liver — may silently damage arteries even in people who appear slim.
Researchers at McMaster University in Canada analysed MRI scans and medical data from more than 33,000 adults in Canada and the UK and their findings show that visceral fat, the hard-to-detect type that accumulates around internal organs, is strongly linked to the thickening and clogging of the carotid arteries, which supply blood to the brain. Narrowing of these arteries is a key risk factor for stroke and may signal blockages forming in other major blood vessels, including those feeding the heart.
The study, published in Communications Medicine, challenges the long-standing dependence on body-mass index (BMI) as the primary measure of obesity.
“Even after accounting for traditional cardiovascular risk factors like cholesterol and blood pressure, visceral and liver fat still contribute to artery damage,” said co-lead author Professor Russell de Souza of McMaster’s Department of Health Research Methods, Evidence and Impact. He called the findings “a wake-up call for clinicians and the public alike”.
Co-lead author Professor Marie Pigeyre, from McMaster’s Department of Medicine, said the results underscore the need for more advanced ways of assessing fat distribution rather than relying on overall weight or waist measurements alone. Corresponding author, Professor Sonia Anand, a vascular medicine specialist at Hamilton Health Sciences, added: “You can’t always tell by looking at someone whether they have visceral or liver fat. 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.”
Experts say the study highlights the value of imaging techniques to detect “hidden” fat deposits and could pave the way for more personalised strategies to prevent heart disease.
The findings emerge as global experts propose a radical shake-up of how obesity is diagnosed — a change that could classify up to 60 per cent more adults as obese.
Currently, a BMI of 18.5–25 is considered healthy, 25–29 overweight, and 30 or above obese. But 58 international specialists have proposed incorporating waist size and waist-to-height ratio to create a more accurate picture of harmful body fat.
A separate study by Harvard University and Massachusetts General Hospital, involving more than 300,000 American adults, found that using the new definition would increase obesity prevalence by almost 60 per cent. In Britain, where around 13 million adults are currently obese, the same shift could push that number to nearly 21 million.
Researchers warn that the surge in obesity prevalence under the new criteria could bring “profound financial and public-health implications”. Under the proposed rules, someone could be classed as obese even with a BMI below 30 if they had at least two raised measures — for instance, a waist over 37 inches for men or 31.5 inches for women, or an elevated waist-to-height ratio.
Writing in JAMA Network Open, the Harvard team reported that older adults were particularly affected: nearly 80 per cent of participants aged 70 or over met the new obesity criteria — twice the current rate. Many of these individuals had a healthy BMI but were found to have “anthropometric obesity”, associated with a significantly higher risk of organ dysfunction, diabetes and more than triple the risk of organ damage.
The renewed focus on harmful fat comes as the UK rolls out a series of long-awaited anti-obesity laws. “Buy one, get one free” promotions on sugary drinks, snacks and junk food have been banned in England, as have free fizzy-drink refills in restaurants. From January, online junk-food advertising and TV adverts before the 9pm watershed will also be restricted.
