The investigators hypothesize that supplementation with vitamins A and D will reduce the incidence of acute gastrointestinal graft versus host disease (GI GVHD) compared with supplementation with vitamin D alone.

The investigators' preliminary data suggest that low levels of vitamin A directly impact risk of mucosal barrier injury laboratory-confirmed bloodstream infection (MBI-LCBI) and they believe supplemental vitamin A at the time of hematopoietic stem cell transplantation (HSCT) can reduce the risk of MBI-LCBI and gastrointestinal graft versus host disease (GI GVHD). In addition, the investigators' preliminary data suggest that a significant number of patients requiring HSCT have vitamin D deficiency even prior to transplantation, and that persistent and newly developed deficiency post-HSCT resulted in worse outcomes.

This study is a comparison of vitamin D supplementation comparing a single large dose of vitamin D "stoss therapy" with a placebo in the standard care arm with supplementation with single large doses of both vitamins D and A in the experimental arm. Participants will be randomly assigned to either the standard care arm or the experimental arm.

Inclusion Criteria:

• Scheduled for allogeneic stem cell transplant
• Vitamin D level < 50 ng/ml
• Vitamin A level < 75th centile for age
• Able to tolerate enteral vitamin dose administration

Exclusion Criteria:

• History of pathologic fractures
• Known history of nephrocalcinosis or nephrolithiasis
• Current granulomatous disease
• ALT > 10X ULN for age prior to administration of vitamin A
• Ongoing raised intracranial pressure
• Pregnancy