Prior studies have suggested that people who are well educated and stay mentally active build up brain reserves that allow them to stay sharp even if deposits of the destructive protein called beta amyloid form in the brain.

But the latest study, based on brain-imaging research, suggests that people who stay mentally engaged beginning in childhood and remain so throughout their lives actually develop fewer amyloid plaques.

"We're not talking about the brain's response to amyloid. We're talking about the actual accumulation of amyloid," Dr. William Jagust of the University of California, Berkeley, whose study appears in the Archives of Neurology, said in an interview. "It's a brand new finding."

While small, the study also shows that starting brain-stimulating activities early enough might offer a way to prevent Alzheimer's-related plaques from building up in the brain.

Currently, there are no drugs that can prevent Alzheimer's disease, which scientists now think begins 10 to 15 years before memory problems set in.

Alzheimer's Disease International estimates there are now 36 million people with the disease worldwide. As the population ages, that number will increase to 66 million by 2030, and to 115 million by 2050.

Last week, the U.S. government released draft recommendations for a national Alzheimer's plan that calls for finding effective treatments or prevention strategies by 2025.

The new study involved the use of an imaging agent known as Pittsburgh Compound B or PiB, which works with positron emission tomography, or PET scanners. This chemical sticks to and highlights deposits of beta amyloid.

"Beta amyloid is the protein that many people feel may be the initiating factor in Alzheimer's disease. It is the protein that is in the plaques of the brains of people with Alzheimer's," Jagust said.

STARTING CROSSWORD PUZZLES LATE WON'T HELP

The researchers studied 65 healthy, cognitively normal people aged 60 and older. Study participants were asked a battery of questions about how mentally active they had been during different periods of their lives starting at age 6. The questions included whether they had read newspapers, went to the library, wrote letters or e-mails and played games.

They also underwent extensive testing to assess their memory and thinking skills and their brains were scanned using the new tracer to look for amyloid deposits in the brain.

The team compared the brain scans with those of 10 Alzheimer's patients and 11 healthy people in their 20s.

They found that people who had been the most mentally active had lower levels of beta amyloid than others who had been less mentally active.

People in the study who had recently taken up crosswords and other mental exercises did not appear to see much benefit.

"What our data suggests is that a whole lifetime of engaging in these activities has a bigger effect than being cognitively active just in older age," said Susan Landau, another Berkeley researcher who worked on the study.

She said amyloid probably starts accumulating many years before symptoms appear, so by the time memory problems start, there is little that can be done. "The time for intervention may be much sooner," she said in a statement.

One weakness is that the study relies on people's memory of their mental activities, Jagust said.

He said staying mentally engaged may make the brain more efficient, which could have a protective effect, but that is still not clear.

(Reporting by Julie Steenhuysen; Editing by Michele Gershberg and Sandra Maler)

• A Permanent Economic Emergency