A recent study published in npj Women’s Health sheds light on the significant relationship between body mass index (BMI) and menstrual cycle irregularities. Researchers examined how extreme BMIs—both low and high—could contribute to menstrual abnormalities, affecting women’s reproductive health.
Understanding the Menstrual Cycle and BMI
The menstrual cycle is a key indicator of female health, reflecting the functionality of the hypothalamic-pituitary-ovarian (HPO) axis. Disruptions to this axis can lead to conditions such as anovulation (lack of ovulation), amenorrhea (absence of menstruation), and irregular cycles. BMI extremes—either too low or too high—have been identified as factors that influence these disruptions.
High BMI and obesity are well-documented contributors to infertility, often due to hormonal and metabolic disruptions that interfere with ovulation. On the other hand, underweight individuals, particularly those with conditions like anorexia nervosa or in female athletes, are also at a higher risk of ovulatory infertility. However, past studies have often yielded inconsistent results regarding the relationship between BMI and menstrual irregularities.
About the Study
In this study, researchers analyzed data from over 8,700 participants, examining 191,000 menstrual cycles. The data was gathered using the LunaLuna period-tracking mobile app in Japan between January 2019 and March 2021. Participants provided information about their health, lifestyle, education, and employment via bimonthly questionnaires.
The study aimed to investigate the relationship between BMI and menstrual cycle irregularities, particularly cycle length (CL), frequency of menstrual irregularities, and the proportion of biphasic cycles (a sign of ovulatory function). Participants with a BMI under 15 kg/m² or over 35 kg/m² were excluded from the analysis, as well as those who were pregnant, on hormonal contraceptives, or undergoing infertility treatments.
Key Findings
The study revealed significant findings about how extreme BMI levels affect menstrual cycle regularity:
Absence of Menstrual Bleeding (AMB): Women with a BMI below 19 or above 26 were more likely to experience absent menstrual bleeding. This indicates that both low and high BMI can disrupt the regularity of menstrual cycles.
Cycle Length and Irregularities: On average, participants had a cycle length of 31.5 days. Those with a BMI below 16 kg/m² or above 30 kg/m² had significantly longer cycles. Additionally, overweight and obese individuals had a higher risk of infrequent menstrual bleeding (IMB) compared to those with a normal BMI. Underweight and obese individuals also faced a greater risk of absent menstrual bleeding (AMB).
The J-Shaped Relationship: The study identified a J-shaped relationship between BMI and menstrual irregularities. Both underweight and obese individuals had higher risks of experiencing IMB and AMB. Specifically, women with a BMI of 20 kg/m² exhibited the lowest incidence of irregular cycles.
Biphasic Cycles and Ovulatory Function: The study also observed an inverted J-shaped relationship between BMI and biphasic cycles. The proportion of biphasic cycles, which are indicative of ovulatory function, was highest at a BMI of 20 kg/m², with deviations at both higher and lower BMI values leading to a decreased likelihood of ovulation.
Conclusion
The study suggests that even small changes in BMI outside the normal range can significantly increase the likelihood of longer, irregular menstrual cycles. Both low and high BMI can disrupt ovulatory cycles, raising the risk of anovulation and potentially leading to infertility.
The findings highlight the importance of maintaining a healthy BMI for reproductive health, with the optimal BMI for regular and ovulatory menstrual cycles appearing to be around 20 kg/m². Individuals with a BMI in this range had the least risk of menstrual irregularities and non-ovulatory cycles.
However, the study was conducted in Japan, and the authors caution that BMI-adiposity relationships can vary across different ethnic groups. These findings may not be fully applicable to non-Asian populations. The reliance on self-reported BMI data and potential selection bias from app users should also be considered when interpreting the results.
Overall, this study emphasizes the critical role that BMI plays in menstrual health, urging women to maintain a healthy weight for optimal reproductive function and a more regular menstrual cycle.
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