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Sometimes, medical science makes breakthroughs that almost no-one sees coming. Other times, it just seems to catch up with what ordinary people have known intuitively for generations. Though the latest finding from the University of New South Wales falls into the second category, that doesn't diminish its significance. Having pored over thousands of pages of data, researchers are now all but convinced that by exercising their brains people can substantially reduce their risk of dementia. Ideally, regular intellectual stimulation would begin early in life and continue through the twilight years. But there's mounting evidence that a late-life surge in mental activity - taking up board games, crosswords, arts and craft - can help stave off the ravages of this dreadful illness.

Scientists have conducted several hundred studies of the theory that brain reserve - the effect of formal education and mentally challenging work and leisure pursuits - may, through some mechanism not fully understood, protect people against dementia. Aware that the studies had tossed up contradictory results, University of N.S.W. neuroscientist Michael Valenzuela and colleague Perminder Sachdev last year conducted the first systematic review of research on brain reserve. Having integrated data from 22 studies of possible links between people's behavior and their subsequent brain health, the pair bring down their verdict in a paper about to be published in British journal Psychological Medicine. In short, they say, people with high brain reserve have almost half as much risk of developing dementia as those with low brain reserve. In one sense the brain appears to be no different from the muscles of the body, says Valenzuela: "It's a case of use it or lose it."

Prevention is crucial with dementia, as medicines do no more than alleviate the symptoms for the 200,000 sufferers in Australia and New Zealand. The most common type of dementia, Alzheimer's Disease, is characterized by the spread of sticky plaques and clumps of tangled fiber that disrupt communication between brain cells. Gradually robbing people of their memory, personality and eventually all cognitive function, it typically kills within 5 to 10 years. Apart from the distress it causes sufferers and their loved ones, Alzheimer's is extremely costly: in Australia it drains an estimated $A3 billion a year from the public purse.

While most experts presume that aerobic exercise protects people from dementia by maintaining good blood flow to the brain, how mental exercise could help is still a puzzle. "There are a lot of theories," says Valenzuela, "but it's very difficult to pinpoint a single neurobiological characteristic that distinguishes people with high brain reserve from those with low brain reserve. I think that's been part of the problem: we've been looking for a magic bullet." Instead, Valenzuela postulates that mental activity alters the central nervous system in different ways at various levels. Research on mice, he says, shows that a highly stimulating environment increases both the production of new brain and nerve cells and the density of blood vessels around them. A few years ago, Valenzuela headed a project in which a group of elderly Sydney residents had their brains analyzed before and after five weeks of memory training. Investigators found that the exercises induced biochemical changes that were the opposite of what occurs when Alzheimer's takes hold.

That finding still excites Valenzuela because it suggests that even those people who've had their minds in low gear for most of their lives can compensate with a late burst of effort. "It seems you can make up for whatever education or job history you may have," he says. "You're not locked into some dementia destiny."

But there's much we still don't know about the relationship between brain reserve and dementia. No one can yet say for sure whether an elderly person's disinclination to mental exercise is a cause or a symptom of the disease. There's also uncertainty about whether high brain reserve helps prevent Alzheimer's telltale plaques and tangles from forming, or whether it minimizes their impact - or both. It's possible that high brain reserve fosters unusually sturdy neurons that allow the brain to carry on as usual despite the presence of plaques, much as some people can maintain their focus when jackhammers start up outside their window. Autopsies of Alzheimer's sufferers confirm no neat correlation between the extent of plaques and tangling and the severity of symptoms. "After almost 100 years of research," says Valenzuela, "we still don't understand the fundamental link between the neurobiological changes and the expression of disease."

Valenzuela would like to obtain funds for a trial in which half the 500 participants would engage in a program of intensive mental stimulation while the rest would carry on as normal. In the meantime, though, why wait? A nightly game of chess may do nothing to thwart dementia. But unlike an experimental drug, there's no risk of its doing any harm, either.

• Daniel Williams
• Mental exercise can indeed protect the brain against dementia, according to a survey of global research