Regular exercise reduces the risk of dementia and Alzheimer's disease by up to 40%, US research suggests.
Exercise improves blood flow to the brain
The University of Washington study claims to be the most definitive investigation into the effect of exercise on dementia.
The Annals of Internal Medicine study found the more frail a person was, the more exercise was likely to help them.
A regular gentle work-out was enough to produce a positive effect - even for people aged over 65.
Lead researcher Dr Eric Larson said walking for 15 minutes three times a week was enough to cut the risk.
Not only did regular exercise cut the risk of dementia, the results suggested it might also help to delay progression of the condition in people who begin to develop symptoms.
Previous studies into the effect of exercise on dementia have produced mixed results.
The study followed 1,740 people aged 65 and older over a six-year period. At the start of the study none showed signs of dementia.
After six years, 158 participants had developed dementia, of which 107 had been diagnosed with Alzheimer's disease.
People who exercised three or more times a week had a 30% to 40% lower risk of developing dementia compared with those who exercised fewer than three times per week.
Dr Larson believes exercise may improve brain function by boosting blood flow to areas of the brain used for memory.
He said: "Earlier research has shown that poor blood flow can damage these parts of the brain.
"So one theory is that exercise may prevent damage and might even help repair these areas by increasing blood flow."
"Even if you're 75 and have never exercised before, you can still benefit by starting to exercise now."
Rebecca Wood, of the Alzheimer's Research Trust, said previous research had suggested exercise could reduce the risk of Alzheimer's.
In particular, a large study from Sweden published last year drew similar conclusions.
Mrs Wood said: "Most previous studies have found that exercise improves brain function.
"Many researchers believe that what is good for the heart is good for the head."
But she added: "The study is particularly important since it shows that exercise is beneficial even after the age of 65 and even among frailer people."
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "The study's findings are consistent with our message that leading a healthy life may reduce your risk of developing dementia.
"As we do not yet know the cause of Alzheimer's disease and other forms of dementia, and there is no cure, it is vital that people are aware of anything they can do to reduce their risk."