Regular exercise reduces the risk of dementia and Alzheimer's disease by up to 40%, US research suggests.

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.