For decades, BMI (body mass index) has been the go-to number doctors use to decide whether someone is underweight, “normal,” overweight or obese.
But new global research suggests it’s possible to have a normal BMI, look thin in clothes and still meet the medical definition of obesity.
A large study published in JAMA Network Open based on World Health Organization (WHO) survey data examined health data from more than 471,000 adults across 91 countries.
The researchers looked at people whose BMI fell in the normal range (18.5–24.9) but who carried extra fat around their midsection, measured by waist circumference.
The results showed that roughly 1 in 5 people with a normal BMI had abdominal obesity, a large waistline linked to serious health risks.
These so-called “skinny fat” individuals weren’t necessarily healthy just because the scale said they were.
Compared to people of normal BMI and smaller waists, those with belly fat had 29% higher odds of high blood pressure and 81% higher odds of diabetes. They were also significantly more likely to have unhealthy cholesterol and triglyceride levels.
In short, their metabolism looked more like that of people with obesity, even if their weight didn’t.
The findings seem to expose a blind spot in how obesity is measured. BMI only considers weight and height but says nothing about where fat is stored.
Fat that sits deep in the abdomen and surrounds vital organs is particularly dangerous, fueling inflammation, insulin resistance and cardiovascular problems, according to experts. Someone can be “thin outside, fat inside,” a concept the researchers call TOFI.
Lifestyle also plays a role, the researchers noted. People with normal BMI who had belly fat were more likely to be inactive, eat fewer fruits and vegetables and have desk jobs or unemployment.
That combination of little muscle and lack of movement makes it easier to accumulate visceral fat even without gaining much overall weight.
The latest research suggests that waist size may be just as important to track as weight.
For most adults, a waist above roughly 32 inches for women or 37 inches for men signals excess abdominal fat and higher health risks.
The researchers noted several limitations. Because the study was cross-sectional, it can’t prove cause and effect, only associations.
Data also came from many countries and time periods, and high-income nations were underrepresented, which may affect how widely the results apply.
The study didn’t include actual body fat scans and instead depended on people’s own reports about their habits, which can make the results less reliable. However, the WHO’s survey methods are generally well-validated.
