The findings, they say, suggest that a heavier weight in middle age may mean a higher risk of dementia later in life.

Reporting in the journal Neurology, the researchers speculate that higher rates of cardiovascular disease or diabetes might help explain the link. But it's also possible that substances produced by fat cells, such as the hormone leptin, have direct effects on the brain.

Both obesity and dementia, including Alzheimer's disease, are becoming increasingly common, noted lead study author Dr. Maxime Cournot, of Toulouse University Hospital in France.

"Our results, along with other previous studies, strongly suggest a greater risk of dementia in these (overweight) persons at middle-age," Cournot told Reuters Heath.

The study included 2,223 healthy French adults who were between the ages of 32 and 62 in 1996. At that time, they took a battery of standard cognitive tests, assessing abilities like memory, attention and speed of learning. Five years later, they took the tests again.

In general, the researchers found, people with a high body mass index (BMI) garnered lower test scores than those with a lower BMI. They also tended to show greater cognitive decline between the two test periods.

Factors such as age, education and general health did not seem to explain the link.

According to Cournot, the tests used in the study are sensitive enough to detect "small variations" in cognition, and the weight-related differences seen among these healthy middle-aged adults would probably not be obvious in daily life.

But over time, the researcher explained, there could be more apparent effects on the rate of age-related mental decline.

It's possible, according to Cournot's team, that excess fat cells have some direct effect on brain function. For example, some studies suggest the "hunger" hormone leptin, which is produced by fat cells, plays a role in learning and memory.

And although these study participants were in generally good health, disorders like elevated blood pressure and diabetes could act as a bridge between high BMI and poorer cognitive function.

Thickening and hardening of the blood vessels supplying the brain can contribute to dementia, Cournot noted. Similarly, diabetes may harm cognition by either leading to artery disease or via direct effects of the hormone insulin on brain cells.

Regardless of what the impact of weight on dementia risk turns out to be, Cournot said, there are already many reasons to maintain a healthy weight. The potential effects on mental function, the researcher added, may give people added motivation to change their lifestyle habits.