A Year of Walking Made the Hippocampus Grow

It is well known that the brain shrinks a little as we age. The hippocampus in particular, which is essential for forming new memories, tends to lose a bit of volume every year from middle age onward. Is this simply something we have to accept? A 2011 study published in the Proceedings of the National Academy of Sciences (PNAS) offered a fairly encouraging answer.

The experiment: a walking group and a stretching group

The researchers randomly divided 120 older adults who rarely exercised into two groups (Erickson et al., 2011). For a year, one group did aerobic walking three times a week at a pace that left them slightly out of breath, while the other did stretching and toning exercises just as often. Before and after, the researchers measured the volume of the hippocampus with MRI and tested memory.

After a year, the results were clear. In the aerobic walking group, the volume of the front of the hippocampus grew by about 2% on average, while in the stretching group it shrank by about 1.4%. The researchers interpreted this increase as roughly equivalent to reversing one to two years of age-related decline. People whose fitness improved more also showed larger gains in hippocampal volume, and the growth of the hippocampus was linked to better scores on spatial memory tests.

How exercise changes the brain

The researchers also looked closely at brain-derived neurotrophic factor (BDNF), a protein that supports the survival and growth of neurons and changes at synapses. In the walking group, the increase in hippocampal volume was associated with higher BDNF levels in the blood. Animal studies have repeatedly reported that aerobic exercise promotes the birth of new neurons in the hippocampus, although how much this happens in the human brain is still being researched.

Things to keep in mind about this study

• The participants were older adults aged 55 to 80. We can’t assume that young adults would see an effect of the same size.
• A strength of the study is that the comparison group did stretching rather than nothing at all. This helps filter out the effects of simply meeting people and following a schedule.
• Later analyses combining many studies have found mixed results on the size of exercise’s effect on hippocampal volume. It is best read not as a single study, but as part of a steadily growing body of evidence.

Today’s practice

1. Walk one stop on your way home. The exercise in the study was walking, which needs no special equipment. Aim for a pace where you can talk but would find it hard to sing.
2. Build up slowly toward 40 minutes, three times a week. Participants in the study also started with about 10 minutes and increased the time over several weeks.
3. Tie it to the same moment each day. Attaching a walk to something that happens every day, like “a lap around the neighborhood before I change shoes after work,” makes it easier to become a habit.
4. Track numbers, not moods. Writing down how long and how far you walked lets you see your own change in fitness after two or three months.

If you have a medical condition or haven’t exercised in a long time, talk to your doctor before increasing the intensity.

References

• Erickson, K. I. et al. (2011). Exercise training increases size of hippocampus and improves memory. Proceedings of the National Academy of Sciences, 108(7), 3017–3022.
• Hillman, C. H., Erickson, K. I., & Kramer, A. F. (2008). Be smart, exercise your heart: exercise effects on brain and cognition. Nature Reviews Neuroscience, 9(1), 58–65.