People who do puzzles and crosswords may stave off dementia longer but experience a more rapid decline once the disease sets in, a study suggests.
While there has long been speculation that "exercising" your brain could protect against Alzheimer's, there has been little evidence to back this up.
Now US researchers who followed more than 1,000 people suggest the more mentally active may delay the disease.
But once symptoms appeared, decline was quicker, the research suggested.
The team from the Rush University Medical Center in Chicago recruited 1,157 people aged over 65 in the early 1990s.
They were given a maximum of five points based on how often they engaged in a variety of activities which involved processing information, including listening to the radio or watching TV, reading a book, carrying out a crossword puzzle or jigsaw, or going to a museum.
Slowing down, speeding up
They were then followed for an average of 12 years, with assessments every three years.
For each additional point those without a diagnosed cognitive impairment saw a 50% slower decline in their brain function, which was examined through a variety of tests.
But the 148 people who had a diagnosis of Alzheimer's saw a 42% faster decline for each point they had accumulated for mental activity.
Writing in the journal Neurology, the authors suggest that cognitive activity enhances the brain's ability to maintain normal function as disease develops, allowing the mind to tolerate significant pathological changes without compromising its performance.
But when Alzheimer's is finally diagnosed, the disease appears to be at a more advanced stage.
"In effect, these results suggest that the benefit of delaying the initial appearance of cognitive impairment comes at the cost of more rapid dementia progression," says study author Dr Robert Wilson.
It does however, he added "reduce the overall amount of time that a person may suffer from dementia".
The Alzheimer's Society said this was a "robust study" which added considerable weight to the argument that, at least in later life, activities like puzzles could keep the brain ticking over for longer.
"However although the symptoms are delayed, there is no evidence changes in the brain associated with dementia have been reduced," a spokesperson said.
"That the brain is allowed to deteriorate to a larger degree before symptoms like memory loss become apparent could explain why the condition seems to progress more quickly after diagnosis.
"More research is now needed to establish why this happens and what role mental stimulation may have in keeping people functioning for longer.'"