Inclusion Criteria:

- Must be female.

- 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.

- 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.

- Must have a functional capacity rating of ≤ 2 on the Eastern Cooperative Oncology
Group (ECOG) performance status when assessed at baseline.

- Must have the approval of their treating physician (or physician's nurse practitioner
or physician's assistant) to participate in sub-maximal physiological fitness testing
and a low to moderate home-based walking and progressive resistance exercise program
and to receive the 12-week supplementation of calcitriol 45 μg. Participants
assigned to either of the calcitriol treatment arms will be instructed to stop taking
calcium and/or vitamin D supplements.

- Must be less than five years from the diagnosis of breast cancer and have received
chemotherapy, radiation therapy, and/or hormonal therapy. Chemotherapy and radiation
therapy, if received, must have been completed prior to study enrollment. Hormonal
therapy may be ongoing.

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 who had a myocardial infarction within the past year.

- Patients with severe metabolic disorders, which includes phenylketonuria (PKU),
homocystinuria, and Fabry's disease, that would preclude them from taking calcitriol.

- 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 be
willing to 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.

- Previously verified diagnosed of osteoporosis.

- Women on antiresorptive drugs (e.g. bisphosphonates) within the past year.

- Patients not capable of participating in an exercise intervention due to severe knee
arthrosis or ligament/cartilage injuries of the lower extremities.

- 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.