Adults who indulge in leisure activities that exercise the brain can delay the onset of Alzheimer's disease, while television watching may increase the risk of Alzheimer's, an American study suggests.

The survey of people in their 70s showed that those who had participated regularly in intellectually challenging hobbies in their younger years tended to be protected from Alzheimer's. The finding supports other studies showing that brain power unused is brain power lost. The study, at the Albert Einstein College of Medicine in New York City, surveyed 551 people in their 70s. Information about the 193 participants who had Alzheimer's was gathered from family and friends, while members of the control group (those who did not have Alzheimer's) were interviewed directly. They were asked about their leisure activities between the ages of 20 and 60. The survey centred on three types of activities: * passive, such as watching television, talking on the phone or listening to music * intellectual, such as reading, jigsaw or crossword puzzles, playing musical instruments, chess or other board games, knitting or woodwork * physical, such as sports, bike riding, swimming or walking.

 Dr Robert Friedland, an associate professor at Case Western Reserve University School of Medicine and a member of the research team, said the Alzheimer's patients had been less active in all activities - apart from television watching.

"Television watching is not protective and may even be a risk factor for Alzheimer's disease," he said. Intellectual activities seemed particularly protective, he said, noting that those whose leisure centred on mind-challenging hobbies were about two and a half times less likely to develop Alzheimer's. Intellectual stimulation in early and middle adulthood did not provide absolute protection against Alzheimer's in late adulthood, but the activities could delay the disease for years, he said.

Other researchers have discovered evidence that mental activity may guard against dementia. But it is hard to prove, since early dementia may cause people to drop their former hobbies: dementia affecting the hobbies rather than the other way around. Dr Friedland and his co-authors tried to minimise that possibility by considering only those who were dementia-free for seven years after joining the study. They also tried to eliminate the potential role of education and intelligence in guarding against dementia by adjusting the results for these variables.

 No matter what the profession or the amount of education, Dr Friedland said, there was still a beneficial effect. Physical activity does not appear to offer protection against dementia. The only exception was frequent dancing. The researchers postulated that music might engage the dancer's mind. It is not clear what physiological mechanism might confer the protection. But researchers say evidence is accumulating that the brain is much more easily molded than scientists once thought.

Mentally engaging hobbies might lay down new neural pathways, one theory holds. "The cerebral cortex and hippocampus, which are critical to these activities, are remarkably plastic, and they rewire themselves based upon their use," said Dr Joseph Coyle, a Harvard Medical School psychiatrist who wrote the research report. Brain-challenging activities seemed to 'build up a reserve' of neuron (nerve cell) connections. Because of this reserve, he said, it took longer for the Alzheimer's process to destroy enough neurons for there to be identifiable symptoms. "Intellectual stimulation may delay the onset," he said. "There is no evidence, however, that it will actually alter the disease course."