Having a big belly in middle age appears to greatly increase one’s risk of developing Alzheimer’s disease or another form of dementia decades later, researchers have said.

According to Reuters, the study tracked 6,583 people in northern California for an average of 36 years starting when they were ages 40 to 45. Their abdominal size was measured at the outset of the study.

A total of 1,049 of the subjects, about 16 percent, went on to develop Alzheimer’s disease or some other form of dementia by the time they reached their 70s. The researchers found that those in the upper 20 percent in terms of belly size in middle age were almost three times more likely to develop dementia than those in the bottom 20 percent of belly size, Reuters said.

Alzheimer’s disease is the most common form of dementia among older people, and researchers have been working to understand the causes and risk factors for the brain disease.

Belly size in middle age was revealed as a much better predictor of later development of dementia than looking merely at obesity as shown by a person’s body mass index, a measure of body fat based on height and weight, the researchers said.

“It’s not just weight, it’s where you carry your weight that is a very important risk factor,” said Rachel Whitmer, a research scientist at the Kaiser Permanente Division of Research in Oakland, California, who led the study.

“If you have two people who are both 10 pounds overweight, one carries it around the middle and one carries it around the hips, that person who carries it around the middle needs to know they are at greater risk,” Whitmer told Reuters.

Previous research has shown that having a large abdomen in middle age elevates one’s risk for diabetes, stroke and heart disease, but the researchers said this was the first study linking belly fat in middle age to increased risk of dementia, Reuters reported.

The study was published in the journal Neurology.