One Year of Walking Grew Older Adults' Hippocampus by 2%, One Trial Found

In 2010, 120 sedentary older adults signed up for a study without knowing which group they'd land in. Half were assigned to walk briskly three times a week. The other half did stretching and toning — real exercise, just nothing that raised their heart rate for long. A year later, researchers scanned everyone's brains again.

The walking group's hippocampus had grown by about 2%. The stretching group's had shrunk by 1.4%.

Reference: NoRotZone Exercise & Hippocampus overview — Erickson et al. 2011 PNAS trial on aerobic exercise, BDNF, and hippocampal neurogenesis

Why that number matters more than it sounds

The hippocampus is the brain structure most associated with forming new memories, and it doesn't hold steady with age. Past 50 or so, it typically shrinks by 1 to 2% a year — a slow, cumulative loss that tracks with declining memory and rising dementia risk. Kirk Erickson's team at the University of Pittsburgh, publishing in PNAS in 2011, didn't just slow that decline in their walking group. They reversed roughly a year or two of it, in twelve months, with nothing more exotic than a brisk 40-minute walk three days a week.

Spatial memory improved along with the volume change, and the two were linked: people whose hippocampus grew more also tended to perform better on memory tasks.

What's likely driving it

The leading mechanism involves a protein called brain-derived neurotrophic factor, or BDNF, which supports the growth and survival of neurons. Erickson's team found that hippocampal volume gains correlated with higher serum BDNF levels after training. The hippocampus is one of the few brain regions that continues producing new neurons throughout adulthood, concentrated in a subregion called the dentate gyrus, and aerobic exercise appears to be one of the more reliable ways to support that process.

What this actually asks of you

Nobody in Erickson's walking group trained like an athlete. Three sessions a week, 40 minutes each, at a pace brisk enough to raise your heart rate — that's the entire protocol that produced a measurable structural change in the brain. Swimming, cycling, and dancing weren't tested in this specific trial, but the underlying mechanism (sustained aerobic effort, not intensity for its own sake) suggests they'd plausibly work through the same pathway.

It's worth noting this cuts both ways. The hippocampus that grows under aerobic exercise is the same structure that measurably shrinks under chronic stress — see our piece on cortisol for the other half of that picture.

A year is a long time to wait for a brain scan to confirm something worked. The upside is that the thing being tested here wasn't a supplement, a device, or a paywalled app. It was walking, three times a week, and a hippocampus that grew instead of shrinking.