New research suggests that walking at least six miles per week may protect brain size and in turn, preserve memory in old age. 

For the study, 299 dementia-free people recorded the number of blocks they walked in one week. Then nine years later, scientists took brain scans of the participants to measure their brain size. After four more years, the participants were tested to see if they had developed cognitive impairment or dementia.

The study found that people who walked at least 72 blocks per week, or roughly six to nine miles, had greater gray matter volume than people who didn't walk as much, when measured at the nine-year time point after their recorded activity. Walking more than 72 blocks did not appear to increase gray matter volume any further.

"Brain size shrinks in late adulthood, which can cause memory problems. Our results should encourage well-designed trials of physical exercise in older adults as a promising approach for preventing dementia and Alzheimer's disease," said study author Kirk I. Erickson, PhD, with the University of Pittsburgh in Pittsburgh.

By four years later, 116 of the participants, or 40 percent, had developed cognitive impairment or dementia. The researchers found that those who walked the most cut their risk of developing memory problems in half.

"If regular exercise in midlife could improve brain health and improve thinking and memory in later life, it would be one more reason to make regular exercise in people of all ages a public health imperative," said Erickson.

Citation: K.I. Erickson, C.A. Raji, O.L. Lopez, J.T. Becker, C. Rosano, A.B. Newman, H.M. Gach, P.M. Thompson, A.J. Ho, and L.H. Kuller, 'Physical activity predicts gray matter volume in late adulthood: The Cardiovascular Health Study', Neurology, first published on October 13, 2010 doi:10.1212/WNL.0b013e3181f88359