A new study has found that diagnoses of Alzheimer's disease were less likely in women who used hormone replacement therapy (HRT) in the past than in women who had never used it.
The risk was nearly three times lower among current users who reported taking HRT for more than a decade than in women who never used HRT, reaching a level similar to that seen in elderly men. However, the women in the study were not randomly assigned to take the treatment .
Dr Peter Zandi from Johns Hopkins University in Baltimore, Maryland, and colleagues collected information from nearly 2000 women (average age 74), and over 1300 men (average age 73).
The researchers interviewed the participants about their medical history, diets, and lifestyle habits, including use of hormone treatments. Three years later nearly 3% of men and nearly 5% of women had Alzheimer's disease.
But women who had used HRT in the past had a 41% lower risk of the disease than their peers who had not used HRT.
Among current users, only women who reported that they had taken HRT for more than 10 years were protected—they were 2.5 times less likely to have Alzheimer's disease than women who never used HRT.
While it is unclear why hormones would have any effect in relation to Alzheimer's disease, a leading hypothesis is that oestrogen protects the brain from Alzheimer's disease long before dementia sets in. Women may benefit most by taking the treatment at the time of menopause, when women's production of oestrogen falls dramatically.
The new research is likely to complicate the decision whether to take HRT. In the study the protective effect against Alzheimer's disease seemed to be stronger the longer a woman took the hormones. That finding, however, runs counter to recent recommendations to minimise exposure to HRT to cut the risk of breast cancer.
Also, in July this year, JAMA published a landmark study showing an increased risk of breast cancer and cardiovascular disease in women taking combined oestrogen and progestogen, for many years.
Doctors say, however, that the current results aren't persuasive enough to argue for HRT as a preventive measure against Alzheimer's disease. Klim McPherson, professor of public health epidemiology at Bristol University and a main critic of earlier studies that wrongly linked HRT to reduced risk of heart disease, voiced his concerns about the study.
“This is exactly how we all started believing HRT was protective for heart disease,” he said. “We are either going to begin a long, 15 year story in which the discussion about Alzheimer's disease just goes on and on, or we start being a little bit more sensible and saying how can this possibly work? What is the biological evidence?”
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Jim Broadbent and Dame Judy Dench in the film Iris about Iris Murdoch, who had Alzheimer's disease