The purpose of this study is to test the efficacy of a high-dose vitamin D supplementation regimen in reducing androgen deprivation therapy (ADT)-related side effects in older prostate cancer patients on ADT. The proposed study is a randomized, double-blind, 2-arm, controlled clinical trial that will accrue 76 prostate cancer patients without severe bone loss, aged 60 and older, beginning ADT, and scheduled to receive at least 6 months more of ADT. Participants will be randomized to: 1) weekly high-dose vitamin D3 (50,000 IU) or 2) vitamin D placebo only for a period of 24 weeks. Both groups will also receive a daily multivitamin and calcium supplement.

The investigators hypothesize high-dose vitamin D supplementation will reduce androgen deprivation therapy (ADT)-related side effects including ADT-induced bone loss, decreased muscle mass, falls, reduced muscle strength, and diminished physical performance in older prostate cancer patients. The vitamin D supplementation is aimed at reducing fracture risk by maintaining proper bone density, thereby preventing osteoporotic/osteopenic conditions and increasing muscle mass. Both vitamin D and exercise are efficacious in maintaining proper bone health and muscle mass among the general population, but little research has been done on prostate cancer patients and survivors. Vitamin D could reduce fracture risk among prostate cancer survivors.

Inclusion Criteria:

• Have a confirmed diagnosis of stage I-IIIA prostate cancer
• Within 6 months of starting ADT with an additional 6 more months planned.
• Participants must have sub-optimal vitamin D levels of <32 ng/ml.
• Participants must agree not to take calcium and/or vitamin D supplements for the duration of the intervention other than those provided.
• Participants must have an ionized serum calcium level within normal limits (1.19-1.29mmol/L) and a total corrected serum calcium of ≤10.5 mg/dl.
• No contraindications for fitness testing and no physical limitations (e.g. cardiorespiratory, orthopedic, nervous system) as assessed by their physician.
• Able to read English (since the assessment materials are in printed format).
• Able to swallow medication and provide written informed consent.
• 60 years of age or older.

Exclusion Criteria:

• Previously verified diagnosis of osteoporosis (any t-score ≤ -2.5).
• Patients on antiresorptive drugs (i.e. bisphosphonates) within the past year.
• Patients with hypercalcemia (corrected serum Ca > 10.5 mg/dl) or a history of hypercalcemia or vitamin D toxicity/sensitivity.
• Patients with impaired renal function (CrCl < 60 mL/min) or who had kidney stones (calcium salt) within the past 5 years.
• Myocardial infarction within the past year.