Certain reproductive factors are associated with greater or lower risk of dementia, according to researchers who conducted a large population-based study with UK Biobank data.
Jessica Gong, a PhD candidate at the George Institute for Global Health at University of New South Wales in Australia, and coauthors found a greater dementia risk in women with early and late menarche, women who were younger when they first gave birth, and those who had had a hysterectomy, especially those who had a hysterectomy without concomitant oophorectomy or with a previous oophorectomy.
After controlling for key confounders, the researchers found lower risk of all-cause dementia if women had ever been pregnant, ever had an abortion, had a longer reproductive span, or had later menopause.
Use of oral contraceptive pills was associated with a lower dementia risk, they found.
In this study, there was no evidence that hormone therapy (HT) was associated with dementia risk (hazard ratio, 0.99, 95% confidence interval [0.90-1.09], P =.0828).
The analysis, published online April 5 in PLOS Medicine, comprised 273,240 women and 228,957 men without prevalent dementia.
The authors noted that dementia rates are increasing. Globally, 50 million people live with dementia, and the number is expected to triple by 2050, according to Alzheimer’s Disease International.
“Our study identified certain reproductive factors related to shorter exposure to endogenous estrogen were associated with increased risk of dementia, highlighting the susceptibility in dementia risk pertaining to women,” Ms. Gong told this publication.
Risk comparison of men and women
Men were included in this study to compare the association between number of children fathered and the risk of all-cause dementia, with the association in their female counterparts.
The U-shaped associations between the number of children and dementia risk were similar for both sexes, suggesting that the risk difference in women may not be associated with factors associated with childbearing
“It may be more related to social and behavioral factors in parenthood, rather than biological factors involved in childbearing,” Ms. Gong said.
Compared with those with two children, for those without children, the multiple adjusted HR (95% CI) was 1.18 (1.04, 1.33) (P = .027) for women and 1.10 (0.98-1.23) P = .164) for men.
For those with four or more children, the HR was 1.14 (0.98, 1.33) (P = .132) for women and 1.26 (1.10-1.45) (P = .003) for men.
Rachel Buckley, PhD, assistant professor of neurology with a dual appointment at Brigham and Women’s and Massachusetts General hospitals in Boston, told this publication she found the comparison of dementia risk with number of children in men and women “fascinating.”
She said the argument usually is that if women have had more births, then they have had more estrogen through their body because women get a huge injection of hormones in pregnancy.
“The idea is that the more pregnancies you have the more protected you are. But this study put that on its head, because if men and women are showing increased [dementia] risk in the number of children they have, it suggests there must be something about having the children – not necessarily the circulating hormones – that might be having an impact,” Dr. Buckley said.
“I had never thought to compare the number of children in men. I do find that very interesting,” she said.
As for the lack of a link between HT and dementia risk, in this study she said, she wouldn’t shut the door on that discussion just yet.
She noted the long history of controversy in the field about whether there is a protective factor against dementia for estrogen or whether exposure to estrogen leads to increased risk.
Before the landmark Women’s Health Initiative (WHI) study in the 1990s, she pointed out, there was evidence in many observational studies that women who had longer exposure to estrogen – whether that was earlier age at first period and later age at menopause combined or women had taken hormone therapy at some point, had less risk for dementia.
Dr. Buckley said that in a secondary outcome of WHI, however, “there was increased risk for progression to dementia in women who were taking hormone therapy which essentially flipped the field on its ahead because until that point everybody thought that estrogen was a protective factor.”
She said although this study found no association with dementia, she still thinks HT has a role to play and that it may just need to be better tailored to individuals.
“If you think about it, we have our tailored cocktail of hormones in our body and who’s to say that my hormones are going to be the same as yours? Why should you and I be put on the same hormone therapy and assume that will give us the same outcome? I think we could do a lot better with customization and calibration of hormones to aid in women’s health.”
Lifetime approach to dementia
Ms. Gong says future dementia risk-reduction strategies should consider sex-specific risk, and consider the reproductive events that took place in women’s lifespans as well as their entire hormone history when assessing dementia risk, to ensure that the strategies are sex sensitive.
Dr. Buckley agrees: “I don’t think we should ever think about dementia in terms of 65 onwards. We know this disease is insidious and it starts very, very early.”
Regarding limitations, the authors noted that it was a retrospective study that included self-reported measures of reproductive factors, which may be inherently subject to recall bias.
A coauthor does consultant work for Amgen, Freeline, and Kirin outside the submitted work. There were no other relevant financial disclosures.