Sporting a significant belly and over 40? A study finds that by the time you reach your 70s, you could be at an increased risk of dementia.

When researchers at the Kaiser Permanente Division of Research in Oakland, Calif., measured the bellies of 6,583 people between the ages of 40 and 45 and then followed up 36 years later, they found that 16 per cent or 1,049 participants had been diagnosed with dementia.

Those with a large belly and who were overweight were 2.3 times more likely to develop dementia than those with a normal weight and belly size. Obese participants who had a large belly were 3.6 times more likely to develop dementia, the study found.

Overall, a large belly increased the risk of dementia in all participants — of normal weight or obese — and regardless of existing health conditions such as diabetes, heart disease and stroke.

Belly size was measured with a caliper and measured the distance from the back to the upper abdomen and midway between the top of the pelvis and the bottom of the ribs.

Weight was calculated using Body Mass Index, a calculation based on a person's weight-to-height ratio. A BMI of 18.5 to 24.9 was considered normal while a BMI of over 30 was considered obese.

The initial measurements were taken by researchers in 1964 to 1973. The medical records of the participants were assessed for a diagnosis of dementia between January 1994 and June 2006.

Researchers aren't sure why belly fat may play a part in the onset of dementia. "It is possible that insulin resistance could be a confounder in the association between midlife central obesity and dementia," reads the study. However, the authors point out that they did not measure insulin resistance, the way the body processes insulin, and cannot verify this.

They also theorize that sporting a large belly could result in a "lifetime exposure to an altered metabolic and inflammatory state" that is associated with a cognitive decline.

The study was published Wednesday in the online issue of the journal Neurology.