Vaginal microbiome composition by menopausal status and reported vitamin D supplement use in Thai women: an exploratory cross-sectional 16S rRNA study

The vaginal microbiome of reproductive-age women is typically dominated by Lactobacillus , which helps protect against urogenital infection. This dominance often declines in postmenopausal women as estrogen falls, yet data from Southeast Asian populations are scarce. Whether reported vitamin D supplement use, which is common among postmenopausal women for bone health, relates to vaginal microbiome composition is also unknown. We therefore conducted an exploratory, cross-sectional study of vaginal microbiome composition in relation to menopausal status and reported vitamin D supplement use in Thai women. In this cross-sectional, observational study, vaginal swabs from premenopausal women (Group A, n = 25), postmenopausal women reporting no supplement use (Group B, n = 21), and postmenopausal women reporting vitamin D supplement use (Group D, n = 17) were analyzed by V3–V4 16S rRNA gene sequencing. Supplement use was self-reported and not randomized. Taxonomic composition, alpha diversity (Shannon and Dominance indices), beta diversity (Bray–Curtis and Aitchison PCoA with PERMANOVA and PERMDISP), and differential taxonomic abundance were compared among groups. Mean Lactobacillus relative abundance was 49.1% in Group A, 39.5% in Group B, and 1.6% in Group D. The low- Lactobacillus profile of Group D was accompanied by a higher relative abundance of Gardnerella (26.7%, versus 18.7% in Group A and 2.8% in Group B) and a greater proportion of other genera. Among Lactobacillus species, L. iners was the most frequently classified. Group D showed significantly higher Shannon diversity and lower Dominance than Group A. Overall community composition differed significantly among groups (PERMANOVA, p = 0.001), with Group D differing significantly from both other groups, whereas Groups A and B did not differ significantly. Differential-abundance analyses identified candidate differences in Lactobacillus -associated and anaerobic taxa, but these findings were not robust after sensitivity analysis and age adjustment. To our knowledge, this exploratory, cross-sectional study provides the first characterization of the vaginal microbiome in Thai women in relation to menopausal status and reported vitamin D supplement use. Lactobacillus relative abundance was lowest, and Gardnerella and other anaerobes were more abundant, in the older postmenopausal group reporting vitamin D supplement use. Because the groups differed in age and supplement use was self-selected, these findings represent associations rather than effects. They provide a foundation for longitudinal studies integrating microbiome, hormonal, clinical, and metabolic measures to clarify the determinants of postmenopausal vaginal microbiome composition.