The long-term goal of our research program is to conduct a large phase III RCT to determine which dose of vitamin D3 supplementation optimally improves acute fracture healing outcomes in healthy adult patients (18-50 years). The current proposed phase II exploratory trial will perform important preliminary work to test the central hypothesis that vitamin D3 dose and timing of administration is critical for improving fracture healing at 3 months. This trial will also inform the feasibility of the large phase III RCT.