The researchers were led by Dr. Kathleen Szegda — who, at the time of the study, was a postdoctoral research associate at the University of Massachusetts in Amherst — and the findings were published in the journal Human Reproduction.
Dr. Szegda and her team started from the observation that high and low levels of fat have been linked with reproductive function in previous studies. They therefore hypothesized that how much a woman weighs may also affect when she experiences menopause.
To test this theory, the researchers analyzed data on 78,759 women from the Nurses' Health Study II, and they clinically followed them between 1989 and 2011. The women were aged 25–42 years, and the researchers gathered information on their height, weight, and menopausal status using a questionnaire.
This information, along with data on hormone therapy use, was gathered and updated every 2 years. Additionally, information was gathered on the participants' teenage years.
For the study, "early menopause" was defined as occurring before a woman reaches 45. The team used multiple regression models to assess early menopause risk, and overall, 2,804 women in the study reported having experienced it.
Early menopause risk raised by nearly a third
The National Institutes of Health (NIH) consider a weight between 18.5 and 24.9 kilograms per square meter to be normal, while women who weigh less than that are considered to be underweight.
In the study, women with a body mass index (BMI) below 18.5 kilograms per square meter — at any age — were 30 percent more likely to experience early menopause compared with women whose BMI was between 18.5 and 22.4 kilograms per square meter.
The researchers also found that women whose BMI was between 25 and 29.9 kilgrams per sqaure meter were up to 30 percent less likely to experience early menopause.
The highest likelihood of having early menopause seemed to be among women who, when they were 18 years old, had a BMI below 18.5 kilgrams per sqaure meter and reported having experienced "severe weight cycling."
Finally, women who had a BMI lower than 17.5 kilgrams per sqaure meter at the age of 18 were 50 percent more likely to have an early menopause compared with normal-weight women.
Clinical implications of the findings
Dr. Szegda sums up the findings, saying, "[W]omen who are underweight in early or mid-adulthood may be at increased risk for early menopause."
This is "associated with a higher risk of cardiovascular disease, and other health conditions such as cognitive decline, osteoporosis, and premature death, so these findings have important implications for women and their doctors," she explains.
Dr. Szegda adds that 10 percent of women experience early menopause, and that "[u]nderweight women may want to consider discussing the potential implications of these findings with their doctors."
"Causes of early menopause are not clearly understood," she continues. However:
"Our findings suggest that being underweight may have an impact on the timing of menopause. More research is needed to understand how it increases the risk of early menopause."
Dr. Kathleen Szegda
The researchers also admit some limitations in their study. "It is possible that underweight women may have been misclassified with an earlier age at menopause if being underweight led to amenorrhea [i.e., the absence of menstruation]," they write.