Belly fat, even in people who are not overall overweight, is a sign of an unhealthy heart, a new study reports.

“See your doctor if your waist is bigger than your hips,” said study author Dr. Jose Medina-Inojosa

A fat belly is a bad sign for your health.

The most common way to measure how overweight or underweight someone is the Body Mass Index (BMI), a simple measure defined as the body mass divided by the square of the body height. BMI is a decent index, but it’s far from perfect and can be misleading. BMI completely disregards the distribution of muscle and fat. Muscles are much denser and compact than fatty tissue, so someone with a lot of muscle might seem overweight and similarly, a person might have a normal BMI but be high on fat and low on muscle. Storing fat around the belly is a very bad sign for your health, even if your overall weight seems fine.

The study tracked 1,692 residents of Olmsted County, Minnesota, aged 45 years or older, a sample that was representative of the county population for age and sex. Participants underwent clinical evaluation and also underwent measurements of their weight, height, hip, and waist circumference. Researchers defined central obesity, in a ratio which divides the waist circumference by the hip circumference, as 0.90 or above for men and 0.85 or above for women.

The initial study was carried out from 1997 to 2000, and follow-up evaluations were carried up until 2016. Researchers found that people with a normal BMI and central obesity were twice as likely to suffer from heart problems compared to participants without central obesity, regardless of their BMI. Dr. Medina-Inojosa said:

“People with a normal weight but a fat belly have more chance of heart problems than people without a fat belly, even if they are obese according to BMI. This body shape indicates a sedentary lifestyle, low muscle mass, and eating too many refined carbohydrates.”

The opposite is also true: people with a higher BMI but no belly fat probably have more muscle tissue, which is a sign of better health.

“The belly is usually the first place we deposit fat, so people classified as overweight BMI but without a fat belly probably have more muscle which is good for health,” he continued. “Muscle is like a metabolic storehouse and helps decrease lipid and sugar levels in the blood.”

The main takeaway, Medina-Inojosa says, is for doctors not to assume that patients are at low risk of heart conditions just because they have a normal BMI. As useful as the BMI can be, it can also be misleading, tricking patients into a sense of false security.

“Our study provides evidence that doctors should also measure central obesity to get a better picture of whether a patient is at risk,” Medina-Inojosa concludes.

The results have not been peer-reviewed yet.