The onset of Alzheimer's could be masked, rather than delayed, by doing mentally stimulating activities like crossword puzzles, new research suggests.
In the past, reading and listening to the radio had also been touted as ways to keep the brain active and stave off the disease.
However, US researchers now think that doing these things may simply serve to prevent the symptoms showing and risk the disease reaching a later stage before being discovered.
As a result, brain exercises can be associated with more rapidly progressing disease.
Dr Robert Wilson, from Rush University Medical Centre in Chicago, said: "Our results suggest that the benefit of delaying the initial signs of cognitive decline may come at the cost of more rapid dementia progression later on, but the question is – why does this happen?
Mentally stimulating activities may help the brain "rewire" itself to circumvent the effects of dementia, said Dr Wilson.
However, once the disease is diagnosed, damage to the brain is likely to be greater than it would be in someone who was not mentally stimulated.
Mental activity appeared to delay the start of Alzheimer's and then speed up its progress, while reducing the overall amount of time a person suffers from the disease.
The 12-year-study, published online in the journal Neurology, involved evaluating the mental activity of 1,157 people aged 65 and over, none of whom had dementia at the start.