Exercise improves memory and increases levels of BDNF (brain growth hormone)
A study published in Proceedings of the National Academy of Sciences in 2011 showed that we can actively modify the gene for the production of brain-derived neurotrophic factor (BDNF) with simple exercise. The investigators looked at 120 elderly nondemented individuals over a 1-year period who either stretched or did aerobics. They measured 3 variables: serum BDNF levels, memory function, and morphometric analysis of hippocampal size on MRI before and after the intervention period. After 1 year, the group that did the aerobic exercise had an increase in hippocampus size by about 2%, improvement of memory function, and higher levels of serum BDNF. Why makes BDNF levels so important? This protein growth factor is essential to keeping neurons healthy, and to the growth of new ones - most active in areas of the brain vital to executive function, learning and long term memory.
Per the study "In sum, we found that the hippocampus remains plastic in late adulthood and that 1 y of aerobic exercise was sufficient for enhancing volume. Increased hippocampal volume translates to improved memory function and higher serum BDNF. We also demonstrate that higher fitness levels are protective against loss of hippocampal volume. These results clearly indicate that aerobic exercise is neuroprotective and that starting an exercise regimen later in life is not futile for either enhancing cognition or augmenting brain volume."
Here's the study: http://www.pnas.org/content/early/2011/01/25/1015950108.abstract