In women, an association between higher risk of dementia and fewer total reproductive years has been discovered. The findings of the study are published in the online issue of Neurology, the medical journal of the American Academy of Neurology.
"Since women are 50 percent more likely to develop dementia over their lifetimes than men, it's important to study any risk factors that are specific to women that could eventually lead us to potential points of intervention," said study author Paola Gilsanz, ScD, of Kaiser Permanente Division of Research in Oakland, Calif.
The study involved 6,137 women who were members of Kaiser Permanente Northern California. Participants completed health surveys and had medical exams. They were asked when they had their first menstrual cycle, when they went through menopause and if they had a hysterectomy. Researchers then calculated the number of reproductive years for each participant. Researchers also used medical records to determine which participants received a diagnosis of dementia later in life.
Of all participants, 42 percent later developed dementia.
Of the 258 women who had their first menstrual cycle at age 16 or older, 120 later developed dementia, compared to 511 of the 1,188 women who had their first menstrual cycle at age 13. They also found that women who went through natural menopause before age 47 had a 19 percent greater risk of dementia than women who went through menopause at age 47 or older. Of the 1,645 women who entered menopause at 47 or younger, 700 later developed dementia, compared to 1052 of the 2,402 women who entered menopause at age 47 or older.
When looking at total reproductive years, from the age of first period to the age of menopause, women who had fewer than 34 years had a 20 percent greater risk of dementia than women who had 34 or more reproductive years. Of the 1,702 women who had fewer than 34 total reproductive years, 728 later developed dementia, compared to 1024 of the 2,345 women who entered menopause at age 47 or older.
Women who had hysterectomies had an 8 percent greater risk of dementia than those who did not. The results were all after researchers adjusted for other factors that could affect the women's risk of dementia, such as smoking, diabetes and high blood pressure.
"Estrogen levels can go up and down throughout a woman's lifetime," said Gilsanz. "Our results show that less exposure to estrogen over the course of a lifetime is linked to an increased risk of dementia. However, while our study was large, we did not have enough data to account for other factors that could affect estrogen levels, like pregnancies, hormone replacement therapy or birth control, so more research is needed."