Healthy people who walk at least six miles (9.7km) a week have bigger brains, better memories and improved mental function compared with couch potatoes, a study has found.
Among older people showing signs of forgetfulness, walking even five miles a week - a daily 20-minute trip to the shops - slowed the progression of their condition.
The findings suggest that walking is the best way of preserving both mental and physical health in old age. Research has shown that it is an ideal form of exercise for maintaining physical fitness and warding off heart disease.
Now scientists have found that it can maintain mental fitness and ward off Alzheimer's disease as well.
A 10-year study of more than 400 elderly people found that greater amounts of physical activity were associated with greater brain volume.
Dr Cyrus Raji, of the department of radiology at Pittsburgh University, who led the study, said: "Volume is a vital sign for the brain. When it decreases that means brain cells are dying. But when it remains higher brain health is being maintained."
Dr Raji added: "We found that walking five miles a week protects the brain structure over 10 years in people with Alzheimer's and mild cognitive impairment."
Healthy adults needed to walk at least six miles a week to maintain brain volume and reduce their risk of mental decline.
The results are due to be presented to the Radiological Society of North America today.