WEDNESDAY, Oct. 13 (HealthDay News) -- Walking about six miles a week appears to protect against brain shrinkage in old age, which in turn helps stem the onset of memory problems and cognitive decline, new research reveals.

"We have always been in search of the drug or the magic pill to help treat brain disorders," noted Kirk I. Erickson, an assistant professor of psychology at the University of Pittsburgh and the study's lead author. "But really what we are after may be, at least partially, even simpler than that. Just by walking regularly, and so maintaining a little bit of moderate physical activity, you can reduce your likelihood of developing Alzheimer's disease and [can] spare brain tissue."

A report on the research, which was supported by the U.S. National Institute on Aging, is published online Oct. 13 in Neurology.

Erickson and his colleagues began tracking the physical activity and cognitive (or thinking) patterns of nearly 300 adults in 1989. At the start, all participants were in good cognitive health, they averaged 78 years old and about two-thirds were women. The researchers charted how many blocks each person walked in a week.

Nine years later, they were given a high-resolution MRI scan to measure brain size. All were deemed to be "cognitively normal."

But four years after that, testing showed that a little more than one-third of the participants had developed mild cognitive impairment or dementia.

By correlating cognitive health, brain scans and walking patterns, the research team found that being more physically active appeared to marginally lower the risk for developing cognitive impairment.

But more specifically, they concluded that the more someone walks, the more gray matter tissue the person will have a decade or more down the road in regions of the brain -- namely the hippocampus, the inferior frontal gyrus and the supplementary motor area -- that are central to cognition.

And among the more physically active participants who had retained more gray matter a decade out, the chances of developing cognitive impairment were cut in half, the study found.

However, the researchers stressed that the relationship between walking and gray matter volume appears to apply only to people who regularly walk relatively long distances that equal about six to nine miles a week.

Walking more than the six- to nine-mile range, however, did not have cognitive benefit, the study found.

"That's because the size of our brain regions can only be so large," Erickson said, adding that the opposite isn't true. "So with no exercise, there can be significant deterioration and decay with age."

However, he added, "what we often tend to think of as an inevitable component or characteristic of aging -- memory decline and brain decay -- is clearly not inevitable. There's plenty of evidence now, and this study is part of that, that shows that we can retain our brain tissue and retain our memories well into late adulthood by maintaining an active and engaged lifestyle."

Dr. Steven V. Pacia, chief of neurology at Lenox Hill Hospital in New York City, described the study's finding as both "intriguing" and an "undoubtedly positive message to send to the public."

"My first reaction to studies like this is that only in America do we have to prove to people that it's good to walk," he said with a chuckle.

"But it stands to reason that being active as we age is going to have a beneficial effect on the brain, just as being inactive is going to have a negative impact," Pacia noted. "Because the brain lives in the environment of the body."

But there may be a catch. "This is just an observational study," Pacia noted. "And while we may assume that the relationship between the brain and activity is a prevention-of-atrophy issue -- just like it is with muscle and bone -- this study doesn't actually prove that. We don't yet know enough about the use-it-or-lose-it notion with respect to brain and exercise. So we do need more research to look at that."

More information

The U.S. National Institute on Aging has more on healthy aging.