Inclusion Criteria  Subject must meet all of the following applicable inclusion criteria to participate in this study:  Written informed consent and HIPAA authorization for release of personal health information. NOTE: HIPAA authorization may be included in the informed consent or obtained separately. Age; 45 years at the time of consent Female Histological or cytological confirmation of breast cancer clinical or pathologic stages 0-III  Patient has been recommended to initiate systemic therapy for breast cancer. It is preferable for patient to enroll prior to systemic therapy initiation. However, enrollment will be allowed if systemic therapy has been initiated within 4 weeks prior to enrollment (randomization).  Note: Patients who undergo only surgery and/or radiotherapy alone would not qualify for the study.  Systemic therapy for breast cancer is planned As determined by the enrolling physician, ability of the subject to understand and comply with study procedures for the entire length of the study Previous vitamin D supplementation allowed, as long as patient is agreeable to stop previous dosing at the time of trial enrollment, to comply with trial procedures including a baseline 25(OH)D level, and is otherwise determined to be appropriate for enrollment  Exclusion Criteria  Subjects meeting any of the criteria below may not participate in the study:  Uncontrolled intercurrent illness/medical condition or psychiatric illness/social situations that would limit compliance with study requirements as determined by the Investigator Patients will undergo breast surgery and/or radiotherapy alone without planned neoadjuvant and/or adjuvant anti-cancer drug therapy. Baseline 25(OH)D level <10ng/mL or >80ng/mL Baseline serum corrected calcium level of >10.3mg/dL Bone mineral density less than the expected range for age on baseline DEXA scan (defined as Z-score </= -2.0) Breast cancer with distant metastasis History of previous breast cancer Postmenopausal, as confirmed by the lack of menses >/=12 months and/or ovarian function laboratories (estradiol, FSH) consistent with menopause (if any of two values outside of menopausal range and subject had menses within 12 months, subject would be considered perimenopausal or premenopausal and therefore eligible for enrollment) Pregnancy or lactation History of bone disease, including Paget's bone disease or osteomalacia Concurrent rheumatoid or other inflammatory arthritis Concurrent or prior treatment with bisphosphonates Use of oral corticosteroids within the last 30 days prior to randomization Concurrent treatment for thyroid deficiency BMI <18.5 Currently receiving treatment for tuberculosis, or planning to receive treatment for tuberculosis during breast cancer treatments History of another primary cancer that required systemic treatment within the last 5 years Any of the following kidney diseases at the time of randomization: active chronic kidney disease >/= stage 3, history of kidney stones, sarcoidosis Gastrointestinal disease that would limit the absorption of pill therapy (i.e. celiac disease, gastric bypass surgery)