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