Many studies have linked exercise with improved brain health later in life. Now a new controlled trial reveals more about how aerobic activity might be helping to bolster the brain by beefing up the hippocampus.

As we age, parts of the brain tend to shrink—even in the absence of neurocognitive diseases, such as dementia or Alzheimer's. The new research shows that at least some parts of the brain can be saved from atrophy—and even built up—by relatively modest amounts of activity late in life. The findings were published online January 31 in Proceedings of the National Academy of Sciences and could have implications for preventing memory decline in the growing segment of older adults in the U.S. 

The research team found that adults aged 55 to 80 years who walked around a track for 40 minutes on three days a week for a year increased the volume of their hippocampus, the brain region that is implicated in memory and spatial reasoning. Older adults assigned to a stretching routine showed no hippocampal growth.

The 120 previously sedentary older adults recruited for the study did not yet have diagnosable dementia but were experiencing typical age-related reduction of the hippocampus, according to pre-study MRIs. "We think of the atrophy of the hippocampus in later life as almost inevitable," Kirk Erickson, a psychology professor at the University of Pittsburgh and co-author of the new study, said in a prepared statement. "But now we've shown that even moderate exercise for one year can increase the size of that structure."

The growth of the hippocampus was modest, increasing 2.12 percent in the left hippocampus and 1.97 percent in the right hippocampus, which effectively turns back the clock one to two years in terms of volume. The stretching group, on the other hand, experienced continued reduction in pace with expected age-related losses, losing on average 1.40 percent and 1.43 percent in the volume of their left and right hippocampus, respectively.

When tested on a computerized spatial memory test, subjects in both the walking and stretching groups improved in accuracy. But those who were in better shape—and thus also tended to have a larger hippocampus—at the start of the study did the best on the memory tests, suggesting that "increased hippocampal volume after the exercise intervention should translate to improved memory function," noted the researchers behind the new paper. And they did find that for those in the walking group, growth in the hippocampus was correlated with improved memory test scores.

Despite almost daily discoveries about the brain's impressive plasticity, particularly in its ability to change to accommodate for damaged areas, the new findings show that even at a relatively advanced age, "the brain at that stage remains modifiable"—even in key structural areas, Erickson said.

In addition to the increased size of the hippocampus, the aerobic exercise group also tended to have a higher level of brain-derived neurotrophic factor (BDNF), a compound that is associated with having a larger hippocampus and better memory. The researchers did not see any changes in the thalamus or caudate nucleus, two other parts of the brain that are involved with spatial sense and memory, respectively. Because only the hippocampus seemed to be affected by the aerobic exercise regime, the researchers reasoned that the activity might be acting specifically on certain molecular pathways to prompt "cell proliferation or dendritic branching," they noted.

The results of the study should help develop a deeper understanding of the precise biological mechanisms at work. The findings also support the notion that although being in better shape to begin with is linked to better memory, "starting an exercise regimen later in life is not futile for either enhancing cognition or augmenting brain volume," the researchers concluded. And even though stretching might be good for physical flexibility and peace of mind, for mental agility, aerobic exercise seems to be best.

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