This research will examine the effectiveness of calcitriol in treating bone loss in women who are about to begin treatment for breast cancer. Twenty-five (25) subjects are expected to take part in this study. The investigators don't know if bone loss in breast cancer survivors should be treated differently than bone loss in other women.

The calcitriol intervention is aimed at reducing fracture risk by maintaining proper bone density. Calcitriol is efficacious in maintaining proper bone health and muscle mass among the general population, but little research has been done on breast cancer patients. In addition, calcitriol may be effective in reducing tumor proliferation and angiogenesis, while increasing tumor apoptosis. Each of those factors could have beneficial effects on breast cancer outcomes.

Inclusion Criteria:

• Must be female.
• Must have pathologically confirmed incident, primary invasive breast cancer.
• Must be awaiting surgical resection.
• Women of child-bearing potential (i.e. women who are pre-menopausal or not surgically sterile) must use acceptable contraceptive methods (abstinence, intrauterine device (IUD), or double barrier device) and must have a negative serum or urine pregnancy test within 1 week prior to beginning treatment on this trial. Contraceptive use needs to be continued at least 1 month after the trial has ended.
• Must provide informed consent.
• Must be willing to discontinue use of calcium and/or vitamin D supplements other than multivitamin supplementation.
• Participants must have an ionized serum calcium level within normal limits (1.19-1.29mmol/L) and a total corrected serum calcium of < 10.2mg/dl.

Exclusion Criteria:

• Subjects with life-threatening conditions that would preclude them from breast cancer treatment including: chronic cardiac failure, which is unstable despite medication use; uncontrolled hypertension; uncontrolled diabetes mellitus; or unstable coronary artery disease.
• Patients with severe metabolic disorders, which includes phenylketonuria (PKU), homocystinuria, and Fabry's disease, that would preclude them from taking calcitriol.
• Patients with a previous history of any other cancer except non-melanomous skin cancer within the past 5 years.
• Patients with impaired renal function (CRCL < 60 mL/min) or who had kidney stones (calcium salt) within the past 5 years.
• Patients with hypercalcemia (corrected serum CA > 10.2 mg/dl) or a history of hypercalcemia or vitamin D toxicity.
• Patients currently taking calcium supplements or aluminum-based antacids must immediately discontinue their use if they are to enroll in the study.
• Patients currently taking vitamin D supplements must immediately discontinue their use if they are to enroll in the study.
• Patients with a known sensitivity to calcitriol.
• Women who are pregnant or lactating.
• Women on antiresorptive drugs (e.g. bisphosphonates) within the past year.
• Women currently using oral contraception.
• Women with malabsorptive syndromes (i.e. cystic fibrosis, chronic pancreatitis) or taking medications that decrease the absorption of fat soluble vitamins (i.e. Orlistat, Questran).
• Participants assigned to calcitriol who are routinely taking a multivitamin supplement may continue the supplement as long as the amount of vitamin D in the supplement is not in excess of the RDA (recommended daily allowance) of 400 IU or 10 μg. If they are not taking a multivitamin supplement, they will be asked to not start supplementation while on study.

12 weeks