Extra pounds and a wider waistline won't do your brain any favors as you get older, a new study suggests.

In fact, obesity appears to accelerate brain aging by a decade or more...

People with a wide waist circumference and higher body mass index (BMI) were more likely to have a thinner cerebral cortex, a condition that has previously been linked to a decline in brain function.

"This association was stronger in those aged less than 65 years," "Overall, this suggests that weight status, especially before older age, is related to less gray matter later in life."

The cerebral cortex is the outer layer of the brain, and it plays a critical role in memory and language.

For this study, Caunca and  colleagues measured the BMI and waistlines of nearly 1,300 people with an average age of 64. BMI is a measurement of body fat based on height and weight.

About six years later, on average, the participants underwent MRI brain scans to measure the thickness of their cortex, overall brain volume and other factors.

About a quarter of the participants had a BMI of 30 or higher, which is considered obese, and about half were overweight with a BMI between 25 and 30. The remaining quarter had a BMI less than 25, which is considered normal weight.

High BMI was associated with a thinner cortex, even after researchers adjusted for high blood pressure, alcohol use, smoking and other factors that can affect the cortex.

Overweight people had a 0.1-millimeter (mm) thinner cortex for every unit increase in their BMI, while obese people lost 0.2 mm of cortex thickness for every BMI unit increase, researchers found.

The findings were published online July 24 in the journal Neurology.

"In normal aging adults, the overall thinning rate of the cortical mantle is between 0.01 and 0.10 mm per decade," senior researcher Dr. Tatjana Rundek said in a journal news release.

"Our results would indicate that being overweight or obese may accelerate aging in the brain by at least a decade

Having a wider waist also was associated with a thinner cortex and, to a lesser extent, both BMI and waist circumference were related to increased shrinking of overall brain volume.

Excess weight has been implicated in poor brain health in previous studies, which have linked obesity to greater risk of dementia and a decline in brain function, Caunca said.

It's not clear exactly why being heavy might harm your cortex

"Obesity is thought to be a chronic inflammatory state, so this chronic exposure to inflammation may impact brain health," Caunca said. "There are also other studies that suggest metabolic changes, like insulin resistance, affect cortical hypometabolism [slow metabolism]. These metabolic changes often coexist with obesity."

Experts also can't say whether seniors could protect their brain by dropping some pounds later in life.

"It's just difficult to know whether the length of time you've had these risk factors would reduce the benefits you get when you lose the weight..

this to be associated with poorer cognitive performance, and that performance is important,. "Consider the kinds of intricate thinking you need to be able to do to perform day-to-day activities. Whether it leads directly to dementia or simple cognitive impairment, they're both important."