A study of 300 volunteers over a 13 year period has found that those who walk most often could actually be defending themselves against a host of memory problems, including dementia and Alzheimer‘s.

Walking may actually protect the brain from shrinking and preserve memory in elderly people, said neurologists who were involved in the research.

The US study performed neurological tests on dementia-free people in Pittsburgh who agreed to log their walks and receive brain monitoring in 1995. The participants received more testing nine years later, and then again in 2008.

The follow-up tests showed that those who walked the most cut their risk of developing memory problems by half.

The neurologists suggest that the optimum distance for a good “neurological exercise” is about 9 miles per week. They reported in the journal Neurology that going the extra mile after that showed no discernible bonus.

The first round of scans showed that nine-mile walkers had larger brains than those who walked less.

After four years, about 40 percent of the participants -- 116 people -- had developed some dementia or cognitive impairment. The effects were 50 percent greater on those who only walked short distances or not at all.

Leader of the study, Dr Kirk Erickson, of the University of Pittsburgh, said: “Our results should encourage well-designed trials of physical exercise in older adults as a promising approach for preventing dementia and Alzheimer's disease.”

Brain size normally shrinks in late adulthood, and can lead to memory problems. “But if regular exercise in midlife could improve brain health, 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,” said Erickson.