By Theresa Tamkins

NEW YORK, Mar 23 (Reuters Health) - One of the first studies to look at how naturally-occurring estrogen affects the aging memory has found that a longer reproductive period seems to offer no protection to women when it comes to Alzheimer's disease.

If estrogen protects against the memory-robbing illness, women who menstruate early and go through menopause later--and thus are exposed to the maximum amount of estrogen--should have a lower risk of the illness.

However, the results of a new study show no protective link between the reproductive phase of a woman's life and Alzheimer's disease. Indeed, some women with a longer reproductive period actually had a higher risk of the disorder than women with shorter reproductive periods.

The findings were unexpected, given that many animal and laboratory studies had suggested that estrogen might protect the brain from memory loss, said lead study author Dr. Lenore Launer, chief of neuroepidemiology at the National Institute on Aging in Bethesda, Maryland.

"So in fact when we started, we expected the relationship to go the other way, so we were quite surprised," she told Reuters Health.

While the study only looked at naturally-occurring estrogen--and not hormone replacement therapy--the findings suggest that it may be too early to assume that estrogen replacement protects against memory loss. Studies that specifically looked at the effect of estrogen replacement on memory have had mixed results.

"The story is going to be a little more complicated and people should really be looking out for the results of trials that are going to be completed," Launer said. "It's not going to be a magic bullet for Alzheimer's disease."

Although taking estrogen and other hormones after menopause can dampen the symptoms of menopause and reduce the chances of bone thinning, it may be premature to conclude that the drugs prevent memory loss, she said.

The findings are published in the March 21st issue of The Journal of the American Medical Association.

In the study, Launer and colleagues at Erasmus University in the Netherlands, looked at about 3,600 healthy women aged 55 and older. During the approximately 6-year study, 199 women developed dementia, including 159 cases of Alzheimer's disease.

The investigators found that those with the longest reproductive period--over 39 years--were actually at a higher risk of dementia or Alzheimer's disease than those with the shortest period--less than 34 years.

For every extra year that women retained their reproductive capacity, their risk of Alzheimer's disease increased 3%, according to the report.

The risk seemed to be limited to women who had a genetic susceptibility to Alzheimer's disease because they carried a gene known as ApoE-e4, which is known to increase the risk of dementia.

• More news on Alzheimer's Disease

Related MEDLINEplus Pages:

• Alzheimer's Disease