Regular exercise can decrease the chances of developing dementia by as much as 40%, according to new research.
Exercise improves blood flow to the brain
The US study adds to evidence that a healthy lifestyle can delay the onset of diseases like Alzheimer's.
An estimated 24 million people suffer from dementia worldwide, and Alzheimer's is the most common cause.
As populations age the numbers are set to rise. By 2040, more than 81 million people will be affected, of whom 70% will be in developing countries.
There are currently only four drugs for Alzheimer's. These only delay the development of the disease and do not always work, so prevention is key, says BBC heath reporter Ania Lichtarowicz .
The University of Maryland study, published by the Annals of Internal Medicine, looked at 1,700 people over the age of 65.
The scientists found that even modest exercise - for instance walking for 15 minutes three times a week - reduced the risk of developing dementia by between 30% to 40%.
Lead researcher Dr Eric 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.