In a recent study published in the journal Neurology, American researchers looked at 299 adults with a mean age of 78 years old. They evaluated how active each person was by measuring how many blocks they walked over the period of one week (this ranged between zero and 300!). The researchers then waited nine years (!), and then took brain images for all the participants and evaluated their level of cognitive impairment.
Not surprisingly, the more someone walked, the greater their brain volume after nine years. Greater amounts of physical activity predicted bigger volumes for several brain regions associated with thinking and memory, such as the hippocampus and the frontal cortex. What's more, the bigger brains associated with physical activity cut the risk for cognitive impairment in half.
The magic number in this study is 72. Walking a minimum of 72 blocks per week was necessary to see the bigger brain effect. Walking more than 72 blocks didn't lead to an even bigger brain. While I wouldn't necessarily shoot for walking only and exactly 72 blocks per week (as physical activity is also associated with a decreased risk for some illnesses), it's nice to have a baseline number, and to know that an exercise as simple as walking can make a difference.
So what are you waiting for?
Reference: Physical activity predicts gray matter volume in late adulthood: The Cardiovascular Health Study. Erickson KI et al. Neurology (2010) [Epub ahead of print].