The finding is part of a wave of research aimed at early detection of the deadly dementia -- and one day perhaps even preventing it.
Researchers scanned the brains of middle-aged and older people while they were still healthy. They discovered that lower energy usage in a part of the brain called the hippocampus correctly signaled who would get Alzheimer's or a related memory impairment 85 percent of the time.
"We found the earliest predictor," said the lead researcher, Lisa Mosconi of New York University School of Medicine. "The hippocampus seems to be the very first region to be affected."
But it is too soon to offer Alzheimer's-predicting PET scans. The discovery must be confirmed. Also, there are serious ethical questions about how soon people should know that Alzheimer's is approaching when nothing yet can be done to forestall the disease.
Still, the discovery may provide leads to scientists searching for therapies to at least delay the onset of the degenerative brain disease. It already affects 4.5 million people in the U.S. and is predicted to strike 14 million by 2050 as the population ages.
Moreover, researchers are honing in on lifestyle choices that may help protect the brain in the first place.
"It's exciting that we can even talk about prevention," said William Thies, scientific director of the Alzheimer's Association. He noted that just 10 years ago there was hardly any research into that possibility.
Among the findings presented Sunday at the association's first Alzheimer's prevention conference:
A brain-healthy lifestyle aside, a big quest is to develop ways to identify Alzheimer's disease before symptoms emerge -- finding biomarkers that could be targets for preventive therapies.
Think of it as hunting the equivalent of the cholesterol test for Alzheimer's, Dr. Neill Graff-Radford of the Mayo Clinic said.
He measured blood levels of different types of beta amyloid, the sticky protein that makes up Alzheimer's hallmark brain plaques, in 565 people. Those with lowest ratios of a particular amyloid type were three times more likely to develop dementia within five years. The reason: Probably less amyloid was floating in the blood because it was sticking in the brain instead.
PET scans already can show Alzheimer's plaques in advanced disease. Mosconi's study is the first to so rigorously examine people's brains before symptoms appear.
PET, or positron emission tomography, scans show images of how brains use glucose, or sugar, which is the brain's main fuel.
Mosconi scanned 53 healthy people. She tracked them for up to 24 years. Six so far have developed Alzheimer's and 19 developed an Alzheimer's precursor called "mild cognitive impairment," or MCI. Those people showed less glucose metabolism in the hippocampus than the still healthy.
Other research supports the hippocampus' early role.
University of Wisconsin researchers gave a different brain scan, called a functional MRI, to healthy adult children of Alzheimer's patients. The researchers found that the hippocampus was not as active as in people without that familial risk.
To prove if these early indicators are real, the National Institute on Aging, with financial help from the pharmaceutical industry and Alzheimer's Association, is beginning a $60 million study to scan the brains of 800 older Americans and try to pin down Alzheimer's earliest biological changes.
That Alzheimer's begins developing so early means even young people should adopt a brain-healthy lifestyle, said Dr. Mark Sager of the Wisconsin Registry for Alzheimer's Prevention. "What we're hoping is that 55 is not too late," he said.
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