Review lead author Dorothy Forbes of the University of Western Ontario said researchers have shown that exercise can improve cognition and mental health in older adults and some studies suggest that it could delay dementia from three to six years or reduce the risk that patients will develop cognitive problems.

"It is less clear if physical activity manages or improves other symptoms among persons with a diagnosis of dementia," Forbes said in a statement.

However, the review authors found only four studies that examined the effects of exercise in humans with dementia and two of the studies weren't included because the reviewers could not get details from the original study authors.

Both of the remaining studies were small and only included Alzheimer disease patients. One looked at 11 patients; the other examined 134 but many of those didn't complete the exercise regimens.

However, the researchers in the latter study found that those who did exercise seemed to do better at handling the tasks of daily living.

The review is published in The Cochrane Library.