While physical exercise may stave off dementia, it does not delay mental decline in people after they've been diagnosed, a study in nearly 500 people with the condition reported Thursday.
While a fitness regime improved physical fitness in people with mild to moderate dementia, it "does not slow cognitive impairment," researchers reported in The BMJ medical journal.
It is generally accepted that exercise can delay the onset of Alzheimer's and other forms of dementia.
But whether or not it can slow symptoms after the onset of mental decline, has been the subject of much debate.
For the latest study, researchers took 494 people in England who had been diagnosed with dementia, and assigned 329 of them to an exercise programme.
They took part in 60-90-minute group sessions in a gym twice a week for four months, and home exercises for an additional hour per week.
The average age of the group was 77.
Participants were assessed at six and 12 months after starting the programme.
The researchers noted that cognition had declined in both the exercise and non-exercise groups.
In the exercise group, the decline was steeper, "however, the average difference was small and clinical relevance was uncertain," said a press statement.
Commenting on the study, Brendon Stubbs of King's College London's Institute of Psychiatry, said its findings was "enormously important" for the care of people with Alzheimer's disease.
"Whilst previous smaller studies have suggested that exercise can prevent or improve cognitive decline in people with Alzheimer's disease, this robust and very large study provides the most definitive answer we have on the role of exercise in mild-moderate Alzheimer's disease," he said via the Science Media Centre.
"The search for effective lifestyle interventions that can delay cognitive decline in dementia must continue."
A separate study, published in the journal Jama Psychiatry, among English people aged 65 and older, said people with fewer financial resources appeared to be at higher risk of dementia.
According to the UN's World Health Organization, about 50 million people worldwide suffer from dementia, including Alzheimer's Disease -- the most common form with about two-thirds of cases.
There are about 10 million new dementia cases each year.
It is generally accepted that exercise can delay the onset of Alzheimer's and other forms of dementia