Relationships Between Estradiol Levels and Vaginal Lactobacillus Abundance Across the Menstrual Cycles in Women

Microbiota of the urogenital tract is crucial for women's reproductive and sexual health and is largely dominated by Lactobacillus strains. A reduction of these bacteria within the vaginal microbial community has been linked to gynecological disorders and infections. It is assumed that the urogenital microbiota composition is influenced by estrogens; however, in most studies estrogen levels were not reliably measured. This study aimed to investigate the relationship between salivary estradiol levels and the abundance of Lactobacillus species in the vaginal microbiota of healthy, reproductive-age women across three menstrual cycles.

The participants were 39 women between 20 and 35 years with regular menstrual cycles. Over three cycles, participants collected daily saliva samples for measuring estradiol levels and vaginal swabs at several time points. Three model approaches were tested: immediate, delayed, and cumulative effect models. Additional day-specific analyses and adjustments for body fat or BMI were conducted.

No statistically significant associations were found between estradiol concentrations and Lactobacillus abundance in any of the models.

This study did not support a link between salivary estradiol levels and Lactobacillus abundance in healthy, reproductive-aged women with regular menstrual cycles. These findings do not imply that estradiol is not an important factor regulating the vaginal Lactobacillus, but suggest that variation in estradiol levels does not correlate with the abundance of these bacteria. Studies that measure estrogen levels in multiple samples rather than assume their concentrations based on menopausal status or cycle phase are necessary to better understand the hormonal regulation of vaginal microbiota.