Background: Pediatric patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) face several risk factors influencing transplantation success, including nutritional status as measured by body mass index (BMI). Methods: This study analyzed BMI data collected from patients transplanted between 2003 and 2023, and aimed to evaluate whether deviations from normal BMI are associated with poorer clinical outcomes. BMI levels assessed before and after first-line treatment and pre-transplantation were analyzed retrospectively to determine a correlation with survival and post-transplant complications. Results: Underweight patients had significantly lower 12- and 36-month overall survival rates compared to normal-weight and overweight patients (p = 1.22 × 10−8 and p = 8.88 × 10−8, respectively). Event-free survival was also lower for underweight patients at all time points. A higher pre-transplant BMI increases the risk of acute graft-versus-host disease (GVHD, p = 0.00068). Otherwise, pre-transplant BMI was not significantly correlated with early TRCs and cGVHD. As secondary objectives, this study identified differences in BMI across primary disease groups, with solid tumor patients having the highest BMI and myelodysplastic syndrome patients having the lowest. BMI cut-offs were identified to predict or protect against serious outcomes, including delayed engraftment, TRCs, and acute and chronic GVHD. Conclusions: This study highlights the importance of nutritional assessment and management in pediatric patients undergoing allo-HSCT to optimize post-transplant outcomes, as deviations from a normal BMI can significantly impact post-transplant health. These findings underscore the importance of integrating BMI assessment throughout the entire pre-HSCT therapeutic course to identify patients at higher risk for complications and to define more effective nutritional management strategies.
