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updated 9/2/2010 8:21:04 AM ET 2010-09-02T12:21:04

Exercising your brain can keep you sharp longer into old age, confirmed a new study. But here's the twist: When symptoms of dementia finally settle in, the decline happens faster in those whose brains have been busiest.

The research suggests that mind-challenging activities don't actually ward off dementia-inducing diseases. Instead, these exercises prolong the brain's ability to function well despite the degeneration happening within.

When symptoms finally do show up, the disease has already progressed to the point of no return. From there, the downhill slide happens fast.

"I think the findings are heartening," said lead researcher Robert Wilson, a neuropsychologist at the Rush University Medical Center in Chicago. "If we're correct, a cognitively active lifestyle reduces the proportion of your lifespan that you spend in a cognitively disabled and demented state. That's what most people want."

In 1993, Wilson and colleagues began interviewing adults, ages 65 and older. The seniors answered questions about how frequently they did brain-challenging activities, such as reading books, going to museums, and playing thinking games like chess. Every three years since the study began, the scientists have continued to check in with the same group, asking similar questions and assessing brain health.

As time has marched on, some of the 1,100-plus participants have developed Alzheimer's disease and other types of dementia. That has allowed the researchers to compare the timing and progression of brain degeneration with each person's level of mental activity.

Among other findings, the study found that dementia takes twice as long to set in among people who challenge their brains the most, the team reports in the journal Neurology. Once symptoms begin, though, the disease progresses much more quickly in that group.

Brain-imaging studies have shown that strenuous mental activities (such as studying for medical school exams) increase the size of the hippocampus, a part of the brain involved in learning and memory, Wilson said. He suspects that the same kind of changes somehow protect older people from the symptoms of dementia, even as other parts of the brain show physical signs of disease.

"The benefit of delaying the initial appearance of symptoms appears to come at the cost of a more rapid progression later on," Wilson said. "You aren't really stopping the pathology from building up. You're simply buying some protection at the front end."

The findings aren't that surprising, said Yaakov Stern, a neuropsychologist at the Columbia University Medical Center in New York. Other studies have used different methods to come to the same conclusions.

Still, he said, the findings emphasize the importance of using your brain as you age. Putting off the clinical onset of Alzheimer's by five years, he said, would halve the number of people who have the disease, though there are still no guidelines about which types of activities are best and how much thinking is enough. Scientists are still working on those experiments.

Stern also warned people against blaming themselves for getting the disease by, say, not doing enough puzzles. Plenty of individual cases defy the trend.

But when dementia does come later and progress more quickly, experts say, that might be a good thing.

"It's not bad to have more good years of life and fewer years of bad life," Stern said. "I think it's a good deal."

© 2012 Discovery Channel

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