The study was published online on 13 October in Neurology, the medical journal of the American Academy of Neurology.
The lead and corresponding author was Dr Kirk I. Erickson, from the University of Pittsburgh in Pennsylvania; other authors were also from the University of Pittsburgh, the University of Nevada in Las Vegas, and the University of California, Los Angeles.
Erickson told the press that our brains shrink in late adulthood, and this can lead to memory problems.
Researchers have promoted the theory that physical activity helps to preserve grey matter volume in late adulthood, which in turns protects memory function, but there have not been enough studies following a sizeable group of elderly people for a good number of years to back this with evidence.
Grey matter comprises mostly the cell bodies of neurons, as opposed to the axons or fibres that link them together and transmit signals.
Erickson said he and his colleagues hope their results will now prompt some "well-designed trials of physical exercise in older adults as a promising approach for preventing dementia and Alzheimer's disease".
For their study, the team looked at data on 299 adults of average age 78 years who took part in the Cardiovascular Health Cognition Study, where researchers had taken measures of grey matter volume as well as physical activity and cognitive impairment.
Physical activity was assessed at the start of the study (baseline), in terms of number of blocks walked per week. None of the participants had dementia at baseline.
The participants underwent high-resolution MRI brain scans to assess white matter volume and other brain health characteristics 9 years after baseline, and they underwent cognitive impairment tests, to assess memory and thinking skills and signs of dementia, 13 years after baseline.
The results showed that:
- At baseline, walking amounts ranged from 0 to 300 blocks, with an average of 56.
- At the 9-year point, participants who had reported walking at least 72 blocks a week (about 6 to 9 miles) at the start of the study, had more grey matter volume than those who walked less (they had "greater volumes of frontal, occipital, entorhinal, and hippocampal regions").
- Walking more than 72 blocks did not appear to increase grey matter further.
- At the 13-year point, 116 (40 per cent) of the participants had developed cognitive impairment or dementia.
- The participants who had walked the most appeared to have halved the risk of developing memory problems.
"Greater amounts of walking are associated with greater grey matter volume, which is in turn associated with a reduced risk of cognitive impairment."
"If regular exercise in midlife could improve brain health and improve thinking and memory in later life, it would be one more reason to make regular exercise in people of all ages a public health imperative."
It is interesting that the researchers did not measure grey matter volume at baseline: presumably this measure was not available to them in the Cardiovascular Health Cognition Study data set.
So strictly speaking, they have not assessed whether walking helped to preserve each person's brain volume, only that people who walked the most tended to have larger grey matter volumes than those who walked less.
There is no proof of cause and effect here (this was a longitudinal study and not a controlled trial), only the suggestion of a strong link, and the reasonable, but still arguable, assumption that it is walking that preserved brain volume rather than having more brain volume encouraged people to do more walking.
The reasonableness of assuming it is the former is bolstered by the numerous studies that show our brains shrink as we age.
Funds from the National Institute on Aging in the US helped pay for the research.
"Physical activity predicts grey matter volume in late adulthood: The Cardiovascular Health Study."
K.I. Erickson, C.A. Raji, O.L. Lopez, J.T. Becker, C. Rosano, A.B. Newman, H.M. Gach, P.M. Thompson, A.J. Ho, and L.H. Kuller.
Neurology, published online 13 October 2010.
Additional source: American Academy of Neurology.