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Want to have a big, healthy brain in your old age? Then get your feet moving now.

A new study suggests that people who regularly take long walks experience less brain shrinkage and less cognitive impairment as they grow older than sedentary folks.

"The findings really continue to amaze me," said lead researcher Kirk Erickson of the University of Pittsburgh. "What we have been able to show is that the effect of physical activity on the brain is much greater than what we had thought previously."

For the study, the researchers recruited 299 seniors and recorded the distances they walked in a typical week. Nine years later, the volunteers underwent MRI scans, which measured their brain size. After four more years, they were tested for signs of cognitive impairment or dementia.

Dr. Erickson noted that the brain tends to shrink with age. And this shrinkage is often associated with memory problems and dementia. But his study, published in the journal Neurology, suggests the simple act of walking can help prevent or slow the loss of grey matter. The people who walked between six miles (9.7 kilometres) and nine miles (14.5 km) a week had less brain shrinkage compared with those who walked less. Even more important, the regular walkers cut their risks of developing cognitive problems in half.

This new study adds to a growing body of evidence that shows exercise is good for the mind as well as the body.

But how does an energetic stroll protect the brain? The researchers are uncertain. But Dr. Erickson said animal studies indicate that exercise stimulates the development of blood vessels in brain, which provides additional oxygen and nutrients to the organ. What's more, exercise seems to support the growth and survival of new brain cells.

Dr. Erickson speculates that any type of moderately intense physical activity would likely produce similar results in humans. But the nice thing about walking is that nearly everyone can do it.

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