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Walking grew the hippocampus
The study
The hippocampus is very involved in declarative memory. In a randomized study, one group was assigned to walk several times a week, and a comparison group did stretching, which is still exercise but not cardiovascular exercise.
After a year, the walking group's hippocampus had increased in volume by 1 percent, and their memory was much better a year later. Both the brain and the behavior changed.
Why it works
The mechanism isn't known exactly, but it probably trickles down to improvements in sleep as well. Exercise earlier in the day may bring a better mood and better sleep, a lot of memory formation happens during sleep, and sleep quality tends to decline with age even as the amount of sleep increases.
Nothing is special about walking beyond the fact that it is easy to randomly assign people to do it. Biking, swimming, and dancing can be beneficial, too.
Why would exercise grow a brain region?
Aerobic exercise raises levels of brain-derived neurotrophic factor, a protein that supports the survival and growth of neurons. In mice, running increases the number of new neurons born in the hippocampus, as shown in work by Henriette van Praag and colleagues in 1999. Exercise also improves blood flow, which helps feed brain tissue. The hippocampus is one of the few places in the adult brain where new neurons are known to form.
Why does sleep matter so much for memory?
During deep sleep, the hippocampus replays the day's experiences and passes them to the cortex for longer-term storage. Matthew Wilson and Bruce McNaughton showed in 1994 that rats replayed the firing patterns of the cells they had used to explore a track while they slept. This transfer is a major reason sleep after learning improves recall. Deep slow-wave sleep also tends to shrink with age.
Why does the hippocampus shrink with age?
Long-term brain scans show the hippocampus losing volume gradually in healthy adults, with the loss speeding up in later decades. Chronic stress hormones, reduced blood flow, and fewer new neurons all appear to contribute. The hippocampus is also among the first regions hit by Alzheimer's disease, which is why doctors measure its volume on brain scans.
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