At age 65, seemingly healthy women have about a 1 in 6 chance of developing Alzheimer's during the rest of their lives, compared with a 1 in 11 chance for men.
Scientists once thought the disparity was just because women tend to live longer but there's increasing agreement that something else makes women more vulnerable.
"Women are really at the epicenter of the Alzheimer's disease crisis," said Dr Kristine Yaffe of the University of California, San Francisco. "We don't really understand what this is all about."
First, Duke University researchers compared nearly 400 men and women with mild cognitive impairment, early memory changes that don't interfere with everyday activities but that mark an increased risk for developing Alzheimer's.
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They measured these people's cognitive abilities over an average of four years and as long as eight years for some participants.
The men's scores on an in-depth test of memory and thinking skills declined a point a year while the women's scores dropped by two points a year.
Nor could it explain why the women declined faster, but the researchers said larger Alzheimer's prevention studies should start analyzing gender differences for more clues. And two other studies presented today offered additional hints of differences in women's brains.
Amyloid plaques are a hallmark of Alzheimer's, and growing levels can help indicate who's at risk before symptoms ever appear.