The Microbiome and Menstrual Health

Introduction

The human microbiome — the trillions of bacteria, fungi, and viruses that inhabit our bodies — is increasingly recognised as a major regulator of hormonal health, metabolism, and immune function. Emerging research is revealing fascinating connections between the microbiome (both gut and vaginal) and menstrual health, including how bacteria influence estrogen metabolism, inflammation, and PCOS pathophysiology.

The Vaginal Microbiome and Menstrual Cycle

The healthy vaginal microbiome is dominated by Lactobacillus species — particularly L. crispatus, L. iners, L. jensenii, and L. gasseri. These bacteria produce lactic acid, maintaining a low vaginal pH (3.8–4.5) that protects against infection by pathogens including STIs, BV (bacterial vaginosis

A 2022 study (sequencing vaginal microbiomes throughout the menstrual cycle) found that:

• During menstruation
• vaginal bacterial diversity increases as Lactobacillus dominance temporarily decreases — blood raises the pH and provides nutrients to non-Lactobacillus bacteria.
• In the follicular phase
• rising estrogen promotes glycogen deposition in vaginal epithelial cells
• feeding Lactobacillus and re-establishing their dominance.
• In the luteal phase
• Lactobacillus dominance is typically maintained by progesterone\'s supportive effects on the vaginal epithelium.
• Women with low Lactobacillus at baseline are more susceptible to recurrent BV
• particularly around menstruation.

Practical implications: Avoiding menstrual products that disrupt vaginal flora (harsh soaps, perfumed products) helps maintain protective Lactobacillus populations during the vulnerable menstrual phase.

The Estrobolome: Gut Bacteria and Estrogen Metabolism

The "estrobolome" refers to the collection of gut microbiome genes capable of metabolising estrogens. This is a critical concept for hormonal health. Here\'s how it works:

This estrobolome mechanism means that gut microbiome composition directly influences circulating estrogen levels — potentially affecting PMS severity, endometriosis progression, and even breast cancer risk. Diets rich in fibre support a diverse, healthy estrobolome that maintains appropriate estrogen cycling.

Gut Microbiome and PCOS

PCOS is associated with significant gut dysbiosis — altered microbiome composition compared to healthy controls. Consistent findings across multiple studies include:

These changes are associated with:

• Systemic low-grade inflammation (from circulating lipopolysaccharide stimulating immune cells)
• Worsened insulin resistance (inflammation impairs insulin signalling)
• Altered bile acid metabolism (bile acids are important insulin sensitivity regulators)
• Dysregulated androgen metabolism

A 2025 review in the Journal of Translational Medicine highlighted gut-derived LPS as a significant contributor to the metabolic inflammation driving insulin resistance and androgen excess in PCOS.

Modulating the Microbiome for Hormonal Health

Diet

The strongest tool for microbiome modification is diet. A high-fibre, plant-diverse diet (Mediterranean pattern) consistently improves microbiome diversity, reduces Gram-negative pathogen overgrowth, and supports the estrobolome. Fermented foods (yoghurt, kefir, kimchi, kombucha) provide live bacteria that can transiently colonise the gut and modulate immune responses.

Probiotics

Several RCTs have examined probiotics in PCOS. A 2020 meta-analysis (8 RCTs) found that probiotic supplementation significantly reduced BMI, fasting insulin, testosterone, and inflammatory markers in PCOS women. Specific strains (Lactobacillus acidophilus, L. rhamnosus, Bifidobacterium lactis) have the most evidence. However, study quality varies and larger trials are needed before firm recommendations can be made.

Fecal Microbiota Transplant (FMT)

Still experimental for PCOS, but early animal and pilot human studies suggest FMT from lean, healthy donors can improve insulin sensitivity and reduce androgen levels. Larger, well-controlled clinical trials are underway.

References: Thackray VG, Biol Reprod 2019 (PCOS microbiome); Baker JM et al. — Estrobolome, J Steroids Biochem Mol Biol 2017; Liang Y et al. — Probiotics in PCOS meta-analysis, Clin Nutr 2020.