Crossword puzzles and other mentally stimulating pursuits may hide rather than prevent the progress of Alzheimer's disease, research has shown.
Evidence suggests that keeping the brain active by reading, listening to the radio or doing puzzles, can delay the onset of dementia.
But the reality could be that, even without symptoms, the brain is suffering progressive damage behind the scenes.
When Alzheimer's symptoms do appear, the disease may be further advanced than expected. As a result, brain exercises can be associated with more rapidly progressing disease.
Dr Robert Wilson, from 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.
Mental decline was measured for each point on a "cognitive activity scale" which reflected how much brain stimulation participants had.
Over a period of six years, the rate of decline was reduced by 52pc for each scale point in those without cognitive impairment. For individuals diagnosed with Alzheimer's, the average rate of decline per year increased by 42pc for each point on the cognitive activity scale.
Dr Susanne Sorensen, of the Alzheimer's Society, said: "The jury is still out on whether pouring over a crossword or enjoying a good book could keep your brain ticking over for longer. This study adds considerable weight to the argument that, at least in later life, it could and it may even delay the symptoms. However although the symptoms are delayed, there is no evidence changes in the brain associated with dementia have been reduced."