Firmicutes in gut microbiota: hormonal influence on breast cancer in obese individuals

Breast cancer (BC) is among the leading causes of death in women. The risk factors of BC are various, among which obesity has been proved to be closely related to the incidence and progression of BC, especially in postmenopausal women. Obesity leads to elevated estrogen levels, increased inflammatory factors, and insulin resistance, which are considered to increase the risk of BC. Microbial dysbiosis has emerged as an important element in the development and progression of various cancers, including BC. Meanwhile, Firmicutes, a phylum enriched in the gut microbiota, can affect the metabolism of estrogen and progesterone, which explains part of its role in promoting the development of BC. This review highlights the “obesity–microbiota–hormone–breast cancer” axis, focusing on Firmicutes influence on BC in hormone levels. We also explore potential methods to prevent obesity-related BC and improve patient survival outcomes by regulating gut microbiota. To identify relevant studies, we conducted a comprehensive literature search in PubMed, Web of Science, and Scopus up to April 2025 using a combination of MeSH terms and free-text keywords related to obesity, microbiota, hormones, and BC.