Though it’s not a perfect tool for measuring fat, body-mass index (BMI), calculated using weight and height, is still the first line of analysis for determining if someone is overweight or obese. A new study published today in Annals of Internal Medicine complicates that calculation, finding that carrying weight around the midsection puts people at higher risk of dying than being obese. This was true even if someone was “normal weight” according to their BMI, but had “central obesity,” as the study terms it.

The researchers, who were primarily from the Mayo Clinic, looked at more than 15,000 adults aged 18 to 90 years old and followed them for 14 years. People with central obesity (as measured by waist-to-hip ratio) had the worst chances of long-term survival, even after controlling for age and BMI, and the effects were similar for both men and women.

It’s not entirely clear why packing a spare tire would be more dangerous than carrying fat throughout the body, but the researchers speculate that it may be because central obesity is associated with the build-up of visceral fat. Visceral fat lives in the abdominal cavity, can surround internal organs, and puts people at greater risk for diabetes and other health complications. BMI doesn’t account for this—it just calculates a body’s mass, without differentiating between fat mass and lean mass.

“Waist-to-hip ratio is a simple and reliable measure for central obesity, but it is infrequently used in daily clinical practice,” Paul Poirier, a cardiology professor at Laval University, wrote in an editorial that accompanied the study. “To better target persons at greatest risk … these new data provide evidence that clinicians should look beyond BMI.”