FRESNO, Calif. – Going into middle age with a big belly could mean dementia in later life, a new study says.

Scientists have known for some time that a large belly is associated with an increased risk for diabetes, stroke and heart disease, but this is the first study to show a connection between mid-life abdominal fat and dementia.

“People need to be concerned not only about their weight, but where they carry their weight in mid-life,” said Rachel Whitmer, a research scientist at the Kaiser Permanente Division of Research in Oakland, Calif., and lead researcher of the study.

“The person who carries weight around the belly is at greater risk than the person carrying it around the hips,” she said.

Kaiser researchers studied 6,583 men and women in Northern California who had had their belly-fat measured when they were ages 40 to 45. Some 36 years later, 16 percent had been diagnosed with dementia.

Dementia can be caused by a number of disorders, including Alzheimer’s disease, that affect the brain. On average, some type of dementia affects 13.9 percent of the American population age 70 and older, according to a 2007 National Institutes of Health study.

The Kaiser study found the risk for developing dementia was 2.3 times greater for men and women who were overweight and who had a large belly than for those with a normal weight and waist size.

The chance of developing dementia was 3.6 times greater for people who were both obese and had large bellies than people with normal weight and bellies.

To determine belly fat, researchers used an instrument called a caliper to measure the distance from the back to the upper abdomen, midway between the top of the pelvis and the bottom of the ribs.

People who want a ballpark idea of what constitutes an unhealthy girth can use the World Health Organization numbers for waist sizes as a guide. For men, a waist size of 40 inches or greater indicates unhealthy levels of belly fat. For women, a waist of 35 inches or greater puts one at higher risk for health problems.

The Kaiser study was published in Wednesday’s online edition of “Neurology,” the medical journal of the American Academy of Neurology.

More research also needs to be done to determine if reducing waist size can lower the risk factor for dementia, Whitmer said. Researchers don’t know if the study participants who had large bellies in their 40s lost the fat before developing dementia in their 70s, she said.

But other studies have found a positive effect on high cholesterol and glucose levels with a smaller belly, Whitmer said.

Belly fat is easier to lose than other fat, Whitmer said. “You can get rid of it with moderate exercise and diet,” she said. “This is a modifiable risk factor for people in mid-life.”