The goal of this pilot observational study is to learn about the feasibility of collecting patient-reported data and stool and blood samples from patients age 65 and older treated with aromatase inhibitor therapy for breast cancer. Participants will be treated with standard of care aromatase inhibitor therapy and will undergo serial phlebotomy, complete patient-reported questionnaires, and submit serial stool samples.

The main exploratory translational questions it aims to obtain preliminary data for are:

• What are the effects of aromatase inhibitor therapy on biomarkers of aging?
• What are the effects of aromatase inhibitor therapy on the microbiome? These data will be used for the development of future definitive studies.

Inclusion Criteria

• Female gender.
• Cohort 1: Age 65 or greater. Cohort 2: Age younger than 65 years
• Cohort 1 Participants (starting AI therapy): Histologically proven stage 0-III carcinoma of the breast or at high risk for development of breast cancer who are planning to undergo treatment with neoadjuvant or adjuvant aromatase inhibitor therapy (anastrozole, letrozole, or exemestane).
• Cohort 1 Controls (no endocrine therapy): Histologically proven stage 0-III carcinoma of the breast or high risk for development of breast cancer and who are not planning to undergo treatment with any endocrine therapy (AI or tamoxifen).
• Participants Cohort 2 (starting AI therapy): Histologically proven stage 0-III carcinoma of the breast or at high risk for development of breast cancer who are planning to undergo treatment with neoadjuvant or adjuvant aromatase inhibitor therapy (anastrozole, letrozole, or exemestane).
• Concurrent treatment with GnRHa therapy, radiation therapy, CDK4/6 inhibitor, bisphosphonate, PARP inhibitor, and/or anti-HER2 therapy is permitted. Prior tamoxifen is permitted.
• The patient is aware of the nature of her diagnosis, understands the study requirements, risks, and discomforts, and is able and willing to sign an informed consent form.
• Able to read and understand English.

Exclusion Criteria

• Distant metastatic disease
• Prior aromatase inhibitor therapy except in the context of fertility treatment.
• Use of exogenous estrogen supplementation other than vaginal estrogen preparations.

Patients with breast cancer or who are at high risk for breast cancer who are starting treatment with aromatase inhibitor therapy as standard of care for treating breast cancer or for chemoprevention will be eligible. In addition, women age 65 and older with breast cancer or at high risk for developing breast cancer who are not going to receive treatment with endocrine therapy are eligible for the control cohort.