Gut microbiota and breast cancer: systematic review and meta-analysis.

Background: The gastrointestinal microbiota can modulate systemic estrogens, potentially influencing estrogen-induced breast neoplasia development. This study aimed to assess alterations in the gut microbiota in breast cancer patients.

Methods: A search strategy was developed using the terms: "Microbiota," "Gastrointestinal Microbiome," "Breast Cancer," and synonyms. Ten observational studies were included.

Results: The total sample was 1730 women (929 cases and 801 controls). The meta-analysis of alpha diversity, assessed by the Shannon index, displayed that in the breast cancer group, the diversity of the gut microbiota was reduced compared to controls, with a standardized mean difference (SMD) of - 0.34 (95% CI - 0.59, - 0.10, I² = 68%, p = 0.007). Regarding the premenopausal population, there was a significant reduction in the breast cancer group (SMD - 0.67, 95% CI - 1.06, - 0.28, I² = 77%, p = 0.0009). In women with a body mass index (BMI) between overweight or obesity, no statistically significant difference was observed (SMD - 0.20; 95% CI - 0.51, 0.11; I² 52%, p = 0.20). However, in women with a BMI greater than or equal to 18.5 and less than 25.0, there was lower diversity in women with breast cancer compared to controls (SMD - 0.49, 95% CI - 0.94, - 0.04; I² 78%, p = 0.03).

Conclusions: The study found a significant difference in gut microbiota diversity between women with breast cancer and controls, supporting the growing evidence that the gut microbiota may play a role in mammary carcinogenesis.