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Women with memory problems that may signal early Alzheimer’s descend into dementia twice as fast as men, researchers said Tuesday.

It helps explain why so many more women than men have Alzheimer’s disease, they said. Two-thirds of Americans suffering from Alzheimer’s are women.

Women in their 60s are twice as likely to develop Alzheimer’s as they are to get breast cancer, the Alzheimer’s Association says.

It may have something to do with the biology of the brain, researchers told the annual Alzheimer’s Association International Conference in Washington, D.C.

“We haven’t done enough work parsing out some of the gender differences,” said Kristine Yaffe of the University of California San Francisco. Women are more likely to have depression, a risk factor for Alzheimer’s, and women are more vulnerable to stress, another risk factor.

“Probably what this is going to be about is a complicated interaction between genetics, hormones and the way the brain develops,” Yaffe said.

Some studies are shedding just a little light on what may be happening.

“Our findings suggest that men and women at risk of Alzheimer’s may be having two very different experiences."

Katherine Lin of Duke University and colleagues studied 400 people with mild cognitive impairment — a loss of memory and thinking skills that doesn’t yet strongly affect someone’s life but that can become Alzheimer’s.

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They used an 11-part test that’s routine for diagnosing memory loss and Alzheimer’s. Women, they found, declined at a rate of more than two points a year on the test, compared to men who declined at a rate of just over one point a year.

“Our findings suggest that men and women at risk of Alzheimer’s may be having two very different experiences,” said Lin, a student at Duke who said she become interested in Alzheimer’s research after a female relative was diagnosed with the disease.

Some other researchers say they have hints about what may be going on.

Dr. Katie Schenning of Oregon Health & Science University and colleagues found that the combination of surgery and anesthesia could affect brain volume and thinking – and that women are more affected by this than men.

It didn’t matter what type of anesthesia it was, Schenning told a news conference. “Women exposed to surgery and anesthesia had a more rapid rate of decline…than men exposed to anesthesia,” she said.

Her team studied 527 people, 182 of whom had surgery with anesthesia. They had comparable years of education and were equally likely to have the APOE4 gene, which puts people at a higher risk of Alzheimer’s.

People often come out of anesthesia with confusion, an effect that usually wears off fairly quickly. But some people never quite recover. This happened to both men and women, but it measurable declines came faster in women and women had more evidence of brain shrinkage after surgery than men did, Schenning’s team found.

“It was not just anesthesia but the combination of surgery and anesthesia,” Schenning said. Her team couldn’t see a different between different types of anesthetics but she said that would be an important thing to look at. In the meantime, people considering elective surgery should discuss this risk with their doctors, she said.

Dr. Michael Weiner of the University of California San Francisco has another piece of the puzzle. His team did PET scans of people’s brains, and found women in general have more of the brain-clogging protein called amyloid that is a hallmark of Alzheimer’s.

“Women have more amyloid in the brain than men even when you adjust for other factors."

His team looked at 1,000 people: 273 normal people, 557 with mild cognitive impairment and 145 who had diagnosed Alzheimer’s.

“Women have more amyloid in the brain than men even when you adjust for other factors,” Weiner told a news conference.

Having the APOE4 gene did not seem to matter for women – they still had more amyloid in their brains, and this effect became much greater once they had Alzheimer’s.

Dr. Roberta Brinton of the University of Southern California says much more work needs to be done on sex differences in the brain. She led a three-day workshop of experts who agreed that hormones such as estrogen and testosterone must play a role, although it’s not clear at all what, precisely, that role is. Women also have metabolic differences, she said, and it may be that women are affected differently by diet and exercise than men are.

It’s clear that exercise and a healthy diet do affect people’s risk of Alzheimer’s.

More than 5 million Americans have Alzheimer’s disease now. The Alzheimer’s Association says more than 28 million baby boomers will develop the disease between now and 2050, and the cost of caring for them will consume nearly 25 percent of Medicare spending in 2040.