Research Question: Do Aromatase Inhibitors Decrease Intestinal Calcium Absorption?

Study Design: Postmenopausal women with early stage breast cancer initiating aromatase inhibitor adjuvant therapy will participate in this two-month study. The primary study outcome is the change in intestinal calcium absorption following such therapy. Secondary outcomes are the changes in bone resorption markers and musculoskeletal signs and symptoms after initiation of therapy.

We will interview women and review their medical records to determine eligibility. Eligible subjects will undergo two calcium absorption studies. The first study will determine their baseline calcium absorption, and the second study will occur after taking an aromatase inhibitor daily for at least 6 weeks. Women will present to the research unit in the early morning and receive an oral and intravenous stable calcium tracer with breakfast. Over the next 24 hours, we will collect all urine for measurement of its calcium content. During each inpatient stay, we will assess musculoskeletal symptoms by questionnaire and joint examination. Each woman will complete a four-day diet diary twice during the study.

Inclusion Criteria:

• Women at least 5 years past menopause, defined as date of last menses or bilateral oophorectomy
• Newly diagnosed breast cancer and starting an AI as initial adjuvant therapy

Exclusion Criteria

• Recent, current or planned chemotherapy for breast cancer, as this may have independent effects on intestinal health, calcium homeostasis and bone turnover
• Allergy or intolerance to orange juice, as one isotope is given with orange juice
• Current use of over-the-counter or prescription antacids, as they may influence Ca-Ab
• Intestinal conditions associated with malabsorption or low gastric acid levels including Crohn's disease, ulcerative colitis, pernicious anemia, bacterial overgrowth, celiac sprue, chronic diarrhea or use of antibiotics within the past month
• Known Stage 4 or 5 Chronic Kidney Disease, defined as an estimated GFR <30 cc/minute
• Use of medications known to interfere with calcium or vitamin D metabolism, including oral steroids or anticonvulsants
• Significant lymphedema precluding adequate intravenous access