Elderly people who get about by walking are less likely to suffer mental decline or even dementia, a study says.
Brain scans revealed that older people walking between six and nine miles a week appeared to have more brain tissue in key areas.
The Pittsburgh University study of 299 people suggested they had less "brain shrinkage", which is linked to memory problems.
The research was reported in the journal Neurology.
The volunteers, who had an average age of 78, were checked for signs of "cognitive impairment" or even dementia.
The Pittsburgh team also had access to brain scan results from four years previously which measured the amount of "grey matter" in their brains.
This is found at various parts of the brain and is known to diminish in many people as they get older.
Each of them had been quizzed in their 60s about the number of city blocks they walked each week as part of their normal routine.
The results showed that those who walked at least 72 blocks - six to nine miles - a week had a greater volume of grey matter.
Four years after the scans, 40% of the group had measurable cognitive impairment or even dementia.
Those who walked the most were half as likely to have these problems compared with those who walked the least.
Dr Kirk Erickson, who led the study, said: "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."
Susanne Sorensen, from the Alzheimer's Society, said that the study was further evidence that a healthy heart could lead to a healthy brain.
She added: "One of the benefits of this research is that it eliminates the impact other socio-economic factors may play and focuses specifically on walking rather than exercise more generally.
"Although a link has been found between lack of exercise and brain shrinkage, we need more research to find out why physical activity may affect the brain.
"The best way to reduce your risk is to take regular exercise, eat healthily, don't smoke and get your blood pressure and cholesterol checked."