Exercise and the Brain

If you've ever been told exercise is "good for your brain," here's some of the actual evidence behind that. Researchers put 120 older adults (ages 55–80) through a yearlong, randomized trial, the gold-standard study design where a coin-flip, not personal preference, decided who got aerobic exercise (walking, 3 days a week) and who got a stretching-and-toning routine as the comparison group. This study from 2011, has inspired the most relevant research on BDNF to date.

The result: the walking group's hippocampus, the brain region central to memory grew by about 2% over the year. The stretching group's hippocampus shrank by about 1.4%, which is the typical rate of age-related decline. So, exercise didn't just slow the loss, it reversed a year or two of typical shrinkage. That's a meaningful, measurable finding, and this is a well-controlled study and 120 people is a solid sample for this kind of trial, and it was randomized, which rules out a lot of the "maybe healthier people just choose to exercise" bias you'd worry about in a survey study.

A few things worth knowing before assuming the whole story is settled, though. The growth wasn't spread evenly, it only showed up in the front part of the hippocampus, not the back. That's a real result, but it means "exercise grows your hippocampus" is a bit more specific than the headline version.

The BDNF story, the brain-growth protein this research is famous for is weaker than it's often given credit for. The exercisers' blood BDNF levels didn't change significantly more than the stretching groups did. The researchers found that within the exercise group, people whose BDNF rose more also tended to have more hippocampal growth, but that's a correlation inside one group, not a group-level, cause-and-effect finding. It's suggestive, not proof that exercise works by raising BDNF.

Something similar applies to memory. Both groups got better at the memory task over the year  walkers didn't out-improve stretchers on the test itself. What the researchers found instead is that people with bigger hippocampi (in either group) tended to do better on the memory task, and within the exercise group, people whose hippocampus grew more also improved more on memory. Again: a real pattern, but a correlation, not a clean "exercise improved memory" result on its own.

None of this weakens the core finding, a randomized trial showing measurable brain volume change from walking is genuinely strong evidence, and it's held up well in the 15 years since. But the mechanism (BDNF) and the memory benefit are better described as promising leads than settled facts.

This is educational information, not a treatment plan — if you're thinking about starting an exercise routine for brain health, it's worth a quick conversation with your provider, especially if you have other health conditions.

Erickson KI, Voss MW, Prakash RS, et al. Exercise training increases size of hippocampus and improves memory. Proc Natl Acad Sci U S A. 2011;108(7):3017-3022. doi:10.1073/pnas.1015950108