Vitamin D and Hemopoietic Stem Cell Transplantation: Clinical Guidance for GVHD Management and Post-Transplant Outcomes.

Vitamin D is a pleiotropic secosteroid with endocrine and intracrine actions that influence key phases of allogeneic hematopoietic stem cell transplantation. Epithelial barriers, antigen-presenting cells and effector lymphocytes express the vitamin D receptor and enzymes required for local activation, allowing circulating 25-hydroxyvitamin D to be converted into its active form and modulate immune interactions. During the peri-transplant period, sunlight deprivation, reduced intake, mucosal injury, cholestasis and corticosteroid exposure markedly reduce vitamin D levels at a time when antigen presentation and immune reconstitution occur. This review integrates mechanistic immunology with clinical observations and interventional data to outline strategies that prevent severe deficiency. It summarizes epidemiology before and after transplantation, associations with acute and chronic graft-versus-host disease, relapse, engraftment, infections, bone health and survival, and evaluates dosing approaches including pre-conditioning loading and reassessment at day thirty with escalation if needed. Absorption-savvy formulations such as oral thin-film and intramuscular cholecalciferol are considered when gastrointestinal function is compromised. Given the high prevalence of deficiency, biological plausibility, safety and low cost, a structured approach that includes screening, repletion and monitoring to achieve concentrations of at least thirty nanograms per milliliter by day thirty represents a pragmatic and low-risk component of supportive care pending definitive evidence.