It’s all downhill after 20. Not only do workouts leave you feeling more sore than they used to, but memory, creative thinking, and mental speed all start to decrease around the time most people finish college. Not surprisingly, the brain also suffers: it starts to shrink. Infomercials would love to convince you that simple video games can fight aging, although they don’t really help. Does anything work?
If anything might help, it’s exercise. Beyond the usual health benefits, exercise is known to slow the brain shrinkage that comes with aging. Sadly, people don’t exercise as much as they should; some surveys say that as many as 60% of people don’t get the minimum recommended amount of weekly exercise. Inactivity increases with age, so 
if anyone would benefit from a workout it would be older, sedentary adults. Participants that fit this description were brought into the lab in a recent study and began a program where half of them performed aerobic exercise while the rest stretched and toned. They received an MRI and took a memory test at the beginning of the program and again a year later.
One reason aerobic exercise slows brain shrinkage is because it increases production of a chemical that causes cell growth. The hippocampus, which is a brain region important for memory, actually grew in the aerobic exercise group. Also, the more a person’s fitness increased, the more the hippocampus grew. Aerobic exercise seems to be key because the stretch and tone group had the usual hippocampus shrinkage during the program. But there was good news for them too: the more fit they were at the beginning of the program, the less shrinkage occurred. People who had a larger hippocampus or who were more fit also tended to score better on the memory test.
This is exciting news for everyone, not just older folks. You don’t need to run marathons or work out like crazy to reverse some of the effects of aging; the people in this study only walked for 40 minutes three times a week. If all the warnings about obesity and heart disease weren’t enough, hopefully this will be enough to get you off the couch.
P.S.: Some important caveats with this paper have been mentioned, but don’t let it dissuade you from getting outside. Connections between fitness and memory have been found both in this particular paper and in others.
Erickson, K., Voss, M., Prakash, R., Basak, C., Szabo, A., Chaddock, L., Kim, J., Heo, S., Alves, H., White, S., Wojcicki, T., Mailey, E., Vieira, V., Martin, S., Pence, B., Woods, J., McAuley, E., & Kramer, A. (2011). Exercise training increases size of hippocampus and improves memory Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1015950108