Women with longer periods of fertility at higher risk of developing dementia when they get older, finds study
Women who have a longer reproductive period have an increased risk of developing dementia when they get older as compared with those who are fertile for a shorter period, reveals a study that followed women for over 40 years. Of the women studied with a shorter reproductive period (32.6 years or less), 16% (53 of 333 individuals) developed dementia. In the group of women who were fertile for a longer period (38 years or more), 24% (88 of 364) developed dementia. The difference was thus 8 percentage points, write experts in the study published in the journal Alzheimer’s & Dementia.
While most women whose menopause is delayed do not develop dementia because of this factor alone, the study may provide a clue as to why women are at higher risk than men for dementia after age 85, the most common age of onset, the authors argue. Alzheimer’s disease, on the other hand, starts developing some 20 years before symptoms of the disorder become apparent, they add. “Our results may explain why women have a higher risk of developing dementia and Alzheimer's disease than men after age 85 and provide further support for the hypothesis that estrogen affects the risk of dementia among women,” explains study author Jenna Najar, a medical doctor and doctoral student at AgeCap, the Centre for Ageing and Health at the University of Gothenburg.
The analysis covers 1,364 women who were followed between 1968 and 2012 in the population studies collectively known as the “Prospective Population-based Study of Women in Gothenburg (PPSW)” and the “Gothenburg H70 Birth Cohort Studies in Sweden" (the H70 studies).” The reproduction period spans the years between menarche (onset of menstruation) and menopause when menstruation stops.
From 1968 to 2012 (mean follow-up 26.8 years), 291 women (21.3%) developed dementia during 36,579 person-years of follow-up. Longer reproductive period and later menopause were associated with the onset of dementia and Alzheimer’s disease after 75 years. The researchers found that the risk for dementia and Alzheimer’s disease increases successively for every additional year that the woman remains fertile. The association was strongest for those with dementia onset after age 85, and the effect was most strongly associated with age at menopause. These results held even after adjustment for other factors with influence, such as educational attainment, physical activity, BMI, smoking, and cardiovascular disease. “However, no association was found between dementia risk and age at menarche, the number of pregnancies, duration of breastfeeding, or exogenous estrogen taken in the form of hormonal replacement therapy (HRT) or oral contraceptives,” the findings state.
According to the authors, several studies have investigated how estrogen in the form of HRT affects dementia risk. Some studies show that dementia risk falls and others that it rises, especially in women who take estrogen late in life. But the current analysis explores the long-term association between factors related to endogenous estrogen and dementia.
“What’s novel about this study is that we have had access to information about several events in a woman’s life that can affect her estrogen levels. Examples are pregnancies, births, and breastfeeding. Being pregnant boosts estrogen levels tremendously, then they decline once the baby is born, and if women breastfeed, the levels fall to extremely low levels. The more indicators we capture, the more reliable our results are,” writes Najar.
Ingmar Skoog, professor of psychiatry at Sahlgrenska Academy, University of Gothenburg and head of AgeCap, explains that the varying results for estrogen may be due to it having a protective effect early in life but being potentially harmful once the disease has begun. At the same time, the duration of women’s fertile periods is one risk factor for dementia among many, says Skoog, who led the study.
The team says that these results could contribute with additional knowledge to identify people at increased risk of dementia, which is important in the implementation of preventative strategies. “As a result of global aging, the number of people affected by dementia will increase. To be able to implement preventive strategies, we need to identify people with an elevated risk of dementia,” suggests Najar.