Background
Breast cancer incidence and mortality have increased globally over recent decades. Estrogen plays a pivotal role in breast cancer development, and growing evidence suggests that gut microbiota, particularly Lactobacillus species, may influence estrogen metabolism through modulation of enterohepatic circulation. Understanding this interaction may provide insights into disease mechanisms and potential biomarkers. However, the relationship between circulating estrogen levels, gut microbiota, and breast cancer remains unclear and insufficiently characterized.

Methods
A case–control cross-sectional study was conducted on 180 women, including 90 patients with non-metastatic breast cancer and 90 healthy controls. Clinical and anthropometric data were collected using standardized procedures. Serum estrogen levels were measured using enzyme-linked immunosorbent assay (ELISA), and stool samples were analyzed for Lactobacillus isolation and identification using conventional microbiological methods. Statistical analyses evaluated differences in estrogen levels between groups and their association with Lactobacillus species.

Results
Serum estrogen levels were significantly lower in breast cancer patients compared to healthy controls. Receiver operating characteristic (ROC) analysis demonstrated high discriminatory performance of estrogen levels in distinguishing patients from controls. However, no significant association was identified between specific Lactobacillus species and circulating estrogen levels.

Conclusion
Circulating estrogen levels are altered in non-metastatic breast cancer and may serve as a complementary biomarker for disease characterization. The lack of association with individual Lactobacillus species underscores the complexity of the gut microbiota–estrogen–breast cancer axis, warranting further mechanistic studies and larger, well-designed investigations.