This study is to assess whether a single oral loading dose of 100,000 IU vitamin D3 prior to allogeneic hematopoietic stem cell transplant (aHSCT) can achieve optimal 25 hydroxy-vitamin D3(25-OH-D3) levels (>75nmol/L) at one month post aHSCT, and maintain adequate levels for at least three months, compared to our standard practice of 2000 IU vitamin D3 daily. 25-OH-D3 levels will be measured prior to vitamin D3 supplementation, and 30 as well as 100 days post vitamin D supplementation. We hypothesize that the intervention will prevent vitamin D insufficiency or deficiency in aHSCT recipients, therefore benefit aHSCT-associated morbidity, including osteoporosis.

This is a randomized non-blinded single centre prospective clinical trial designed to validate whether a single oral loading dose of 100,000 IU vitamin D3 prior to allogeneic hematopoietic stem cell transplant (aHSCT) can achieve optimal 25 hydroxy-vitamin D3(25-OH-D3) levels (>75nmol/L) at one month post aHSCT, and maintain adequate levels for at least three months, compared to our standard practice of 2000 IU vitamin D3 daily.

84 patients will be enrolled. Following completion of all baseline data collection, participants will be randomized 1:1 to the intervention group or the control group on the admission day for aHSCT

Patients in the intervention group will receive a single oral loading dose of 100,000 IU vitamin D3, with subsequent vitamin D3 2000 IU daily. Patients assigned to the control group will receive our current standard of care, 2000 IU vitamin D3 daily.

Inclusion Criteria:

1. Patients undergoing allogeneic stem cell transplantation at the Leukemia/BMT Program of British Columbia.
2. Age greater than or equal to 18 years.
3. Able to provide written informed consent.

Exclusion Criteria:

1. Hypercalcemia, hypervitaminosis D, or allergic/ sensitive to vitamin D.
2. Not meeting eligibility criteria to proceed with allogeneic stem cell transplantation as per the Leukemia/BMT Program of BC.