Inclusion Criteria:

- Must be female and have a primary diagnosis of Stage I, II, or III hormone-receptor
positive breast cancer.

- Women must be postmenopausal at time of enrollment.

- 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 < 10.6 mg/dl.

- Must have a functional capacity rating of ≤ 2 on the Eastern Cooperative Oncology
Group (ECOG) performance status when assessed at baseline.132

- 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 24-week supplementation of vitamin D.

- Must be less than five years from the diagnosis of breast cancer and must be within 6
months of starting treatment with aromatase inhibitors (AI) in accordance with ASCO
guidelines.

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 ≥ 10.6 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 vitamin D.

- Patients who are severely vitamin D deficient (<10 ng/ml).

- Previously verified diagnosed of osteoporosis or severe osteopenia (any t-score ≤
-2.0).

- 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 with malabsorptive syndromes (i.e. cystic fibrosis, chronic pancreatitis) or
taking medications that decrease the absorption of fat soluble vitamins (i.e.
Orlistat, Questran).