Cancer Treatment and Research Communications · Oct 19, 2026

The association of the gut microbiome with intermediate markers of breast cancer risk-mammographic density and blood steroid hormones-in the multiethnic cohort study

By Maskarinec G, Laxamana MA, Lim U et al.

INTRODUCTION: The gut microbiota may influence breast cancer risk through different biologic pathways. This analysis examined the association of the gut microbial composition with percent mammographic density (PMD), a strong predictor of breast cancer risk. METHODS: Postmenopausal women in the Adiposity Phenotype Study, a cross-sectional subset of the Multiethnic Cohort, from five racial/ethnic groups, provided a stool sample. Extracted bacterial DNA was sequenced for the V1-V3 region of the 16S rRNA gene. Bioinformatic analysis determined alpha-diversity and the abundance of 10 phyla and 152 genera. PMD was assessed in routine mammograms taken before stool collection with a quantitative computer-assisted method. Blood levels of estrogen metabolites were assessed using standard assays. General linear models regressed PMD on gut microbial diversity, phyla, and genera while adjusting for relevant covariates. RESULTS: Of 649 women (69.1 ± 2.7 years), 311 had low (<15%) and 338 had high (>=15%) PMD. Whereas alpha diversity or phyla were not associated with PMD, 10 genera were nominally related to PMD (p < 0.05) but not after adjusting for multiple comparisons: 7 positively (Bifidobacterium, Blautia, Dialister, Eubacterium_fissicatena_group, Lachnospira, Lachnospiraceae_UCG-008, Lachnospiraceae_uncultured) and 3 inversely (Alistipes, Senegalimassilia, Eubacterium_oxidoreducens_group), potentially implicating estrogen metabolism and short-chain fatty acid production. All blood steroid metabolites were significantly related to PMD, but only SHBG was associated with 2 genera after Bonferroni adjustment. CONCLUSIONS: The nominal associations detected in this analysis suggest that certain gut microbiota may be associated with PMD, a marker of risk but not a measure of breast cancer incidence, due to their ability to affect circulating hormones or SCFA production.

doi.org/10.1016/j.ctarc.2026.101497 Open access PubMed 42914583 Biomarkers