Scientists have claimed that walking six miles a week could improve people's memories and prevent brains from shrinking.
Brain scans were given to 299 healthy older people nine years after they volunteered to record the distances they walked in a week. Four years later they were tested to see if they were suffering from mental decline or dementia.
The brain scans revealed that those who walked between six to nine miles per week had more grey matter - cell bodies of neurons rather than the fibres that transmit signals - in their brains than those who did not. Walking greater distances than this did not appear to have any further benefits.
Results of the mental tests showed that 116 of the participants (40%) had developed some degree of dementia or cognitive impairment. The findings, which were reported online in the journal Neurology, found that those who had walked greater distances were half as likely to be experiencing memory problems.
Study leader Dr Kirk Erickson, from the University of Pittsburgh, said: "If regular exercise in mid-life 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."