A new study from UPMC and the University of Pittsburgh found that a person’s waist size compared to their height is better than body mass index (BMI) at predicting heart disease.

The research, published in The Lancet Regional Health—Americas, could change how doctors and the public measure heart risk, especially for people who are not considered officially “obese.”

Scientists looked at data from 2,721 adults in the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil). None of the people had heart disease at the start, and they were followed for more than five years.

At first, higher BMI, bigger waistlines, and a higher waist-to-height ratio were all linked to a greater chance of future heart disease. But when the researchers adjusted for other major risks—like age, sex, smoking, exercise, diabetes, high blood pressure, and cholesterol—only the waist-to-height ratio still predicted future heart problems.

A lot of this effect was seen in people with a BMI under 30. These people do not meet the usual definition of obesity, so they might not know they are at risk.

BMI does not show where fat is stored or tell the difference between dangerous belly fat and safer fat under the skin. But waist-to-height ratio (WHtR), which is waist size divided by height, shows how much fat is around the middle. Belly fat is more strongly tied to heart disease. So a person with a BMI under 30 but a WHtR over 0.5 could still be at higher risk for future coronary artery calcification, an important sign of heart disease, even if they have no other risk factors.

Using WHtR as a screening tool could help doctors find at-risk patients sooner. It may catch problems in people whose weight, cholesterol, and blood pressure all look normal.