How many blocks of walking per week did it take to see improvement? The magic number was 72, or about six to nine miles. Walking more than that didn't further improve gray-matter volume.

Although all participants were deemed cognitively normal at the beginning of the study, 40% developed cognitive impairment or dementia four years in. However, those who walked the most reduced their risk of acquiring memory loss by half.

Lead author Kirk Erickson of the University of Pittsburgh said in a news release, "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."

-- Jeannine Stein / Los Angeles Times