Calcitriol, Physical Activity, and Bone Health in Cancer Survivors

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.