Older adults with high body mass index, or BMI, and big bellies are more likely to have lower cognitive function than those with a lower BMI.

The study, published in the journal Age and Aging, included 250 people older than 59 who underwent a variety of weight measurement, scans and cognitive performance tests. People between 60 and 70 with the highest BMIs were linked to the lowest cognitive function.

The Korean study showed a particular association between visceral fat, or fat around the torso, and poor mental performance.

"Aging is characterized by lean body mass loss and adipose tissue increase without weight gain, which may not be captured by BMI, and traditional adiposity measures like BMI are less useful in elderly persons," said Dr. Dae Hyun Yoon, associate professor of psychiatry at Seoul National University Hospital.

Study results changed in adults older than 70, and the high BMI and large weight circumferences were not associated with cognitive decline.

"A higher BMI is related to lower dementia risk in the oldest old. It is possible that persons with low BMI lost their weight because of premorbid dementia," Yoon said. "It is also possible that a low BMI is the consequence of hyperinsulinemia (high insulin levels), which precedes weight loss and is related to higher dementia risk."

Dr. Ken Fujioka, director of the Center for Weight Management at Scripps Clinic in San Diego, said the results make sense and are on par with what he sees clinically.

"As patients gain central obesity - that is the key - they increase their level of inflammatory agents and atherosclerotic agents that will wreck havoc on the brain," Fujioka said.

While it is unclear whether the participants in the study went on to develop dementia or Alzheimer's disease, past research has shown that excess fat might play a role in a person's cognitive decline.

"The prevention of obesity, particularly central obesity, might be important for the prevention of cognitive decline or dementia," Yoon said. "In participants aged 70 years and older, high BMI, waist circumference, and visceral and subcutaneous adipose tissue area, were not associated with poor cognitive performance."