Oral Aromatase Inhibitors Modify the Gut Microbiome Effecting Estrogen Bioavailability

Inclusion Criteria

• Pathologic diagnosis in-situ or invasive breast cancer for which initiating aromatase inhibitor (AI) therapy is recommended or initiating AI for risk reduction
• Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, 2, 3
• Age >= 18 years
• HER2+ breast cancer is allowed if patients are on a monoclonal antibody therapy for which a change is not anticipated while on study (12 weeks) and they have been on the monoclonal antibody therapy for at least 4 weeks
• Ability to read, understand and evaluate study materials and willingness to sign a written informed consent document

Exclusion Criteria

• Have been on antibiotics within 4 weeks of enrollment
• Administered chemotherapy < 4 weeks prior to enrollment
• Hormone replacement (defined as only estrogen and/or progesterone replacement) or anti-estrogen therapy within 4 weeks of starting AI
• Use of antibody drug conjugate (HER2+ breast cancer)
• Male breast cancer
• Any serious and/or unstable pre-existing medical, psychiatric, or other existing condition that would prevent compliance with the trial or consent process