People who are overweight or obese in their 40s have a greater risk of developing Alzheimer's disease later in life, a new study suggests.

"The implication is that keeping a healthy weight in middle age is linked to good outcomes in old age," said study author Rachel A. Whitmer, a research scientist at Kaiser Permanente of Northern California.

Whitmer and other researchers are finding increasing evidence that Alzheimer's and other forms of dementia are often linked to physical, and sometimes preventable, causes.

"There has been a whole bunch of recent evidence that being overweight is bad for the brain," she said.

Even when compensating for common health problems that can contribute to Alzheimer's, such as diabetes and cardiovascular disease, fat itself seemed to play a surprisingly important role in the development of the brain disease, Whitmer said.

The new findings were to be presented Wednesday at the American Academy of Neurology annual meeting, in San Diego.

Whitmer's research is part of a larger study, started in 1964, that followed nearly 9,000 Kaiser Permanente patients for up to 30 years. The participants were evaluated by measuring the thickness of skin folds both below the shoulder and at the back of the upper arm.

Those with the thickest shoulder measurements were nearly three times as likely to develop Alzheimer's disease as those in the lowest group. For arm measurements, those in the highest group were two and a half times as likely to develop the disease.

Whitmer said future studies are needed to examine the molecular mechanisms that seem to link obesity and Alzheimer's disease.

Maria Carrillo, director of medical and scientific relations for the Alzheimer's Association, said the findings aren't surprising.

She suggested that overweight people aren't as physically fit, so arteries can get clogged and restrict blood flow. And, she added, overweight people are probably not eating right. "Studies have shown that healthy eating can contribute to brain health," Carrillo said.

Alzheimer's disease is at epidemic proportions, Carrillo said, with 4.5 million Americans diagnosed now. As the population ages and people live longer, that number is expected to increase to 16 million by 2050.