Study at the Rush Alzheimer's Disease Center (716 dementia-free volunteers, average age of 82): one's level of activity can impact the risk for developing Alzheimer's--even in people in their 80s. Individuals in the bottom 10% of intensity of physical activity were 2.8 times as likely to develop Alzheimer's disease compared to those in the top 10%. An actigraph, a watch-sized device worn on the wrist, was used to measure physical activity. Interestingly, much of the recorded movements came from regular activities of daily living rather than formal exercise. The bottom line? Stay active! (Source)
Regular physical exercise and an increase in intellectual activities seem to offer protection against developing Alzheimer’s disease. Offers specific suggestions for building up leg muscular strength as important for brain health. (Restak, Richard, MD. Mozart’s Brain and the Fighter Pilot. NY: Harmony Books, 2001, pp 185-186)
Exercise gives the brain many of the things it needs to function at full power. It is known to raise baseline serotonin levels, decrease the amount of hydrocortisone (e.g., in excess it can shrink brain mass through cell death), improve blood flow, and bring the brain into an awake state. (Giuffre, Kenneth, MD., with Teresa Foy DiGeronimo. The Care and Feeding of Your Brain. NJ: Career Press, 1999, pp 235-236)
See Exercise and the Brain for additional information.