Regular walking for a year grew the memory center of older adults' brains by 2 percent, undoing one to two years of age-related shrinkage. A study of 120 older adults showed the brain can still grow late in life. Memories form in the hippocampus, a seahorse-shaped part of the brain that shrinks with age and raises the risk of dementia as it does. Kirk Erickson and colleagues at the University of Pittsburgh and the University of Illinois randomly split 120 older adults into two groups: one did walking workouts, and the other did stretching and toning. The study appeared in PNAS in 2011. In the walking group, the front part of the hippocampus grew by 2 percent. In the stretching group, it shrank, as normally happens with age. The walkers also got better at remembering where things are. Their bigger hippocampus came with higher levels of BDNF, a protein that works like fertilizer for brain cells. People who were fitter at the start lost less brain volume, even in the stretching group. Other brain areas the team checked stayed the same. Brain scans measure size and can't count new brain cells, so the study can't say how many were added. Even so, the memory center grew late in life, and a year of regular walking was enough to reverse its shrinkage.