According to this BusinessWeek article:
"We found that walking five miles per week protects the brain structure over 10 years in people with Alzheimer's and MCI, especially in areas of the brain's key memory and learning centers," he said. "We also found that these people had a slower decline in memory loss over five years."Further in the article, the author notes that another study showed a similar positive impact on adults' cognitive abilities but at a slightly higher exercise threshold of six miles per week.
"Alzheimer's is a devastating illness and, unfortunately, walking is not a cure," Dr. Raji said. "But walking can improve your brain's resistance to the disease and reduce memory loss over time."Article: Regular Walking May Slow Dementia