Nel Dąbrowska-Leonik, Renata Grzywa-Czuba, Monika Wasiak, Mikołaj Wasil, Weronika Zaczek, Maria Żebrowska, Małgorzata Pac
Background: Vitamin D plays an important immunomodulatory role, and vitamin D deficiency has been associated with an increased risk of infections and autoimmune disorders. Although patients with some primary immunodeficiency disorders (PID) exhibit higher rates of vitamin deficiency, there are currently no evidence-based recommendations regarding whether children with PIDs require higher vitamin D supplementation than healthy peers. This study aimed to evaluate the nutritional status of vitamin D and vitamin D supplementation in children with PIDs. Methods: The study included 132 children diagnosed with PIDs according to the 2024 update of the phenotypic classification by the International Union of Immunological Societies (IUIS). The following variables were evaluated: body mass index (BMI), 25-hydroxyvitamin D (25(OH)D), calcium, phosphorus, immunoglobulins G, A and M, and number of neutrophils and lymphocytes in the blood. Results: Vitamin D deficiency occurred in 13% of all tested children, and in 22 and 33% of children with predominantly antibody deficiencies and complement deficiencies, respectively, with the median vitamin D supplementation of 1000 IU. The median serum 25(OH)D concentration was 30.75 ng/mL (32.7 ng/mL in girls and 30 ng/mL in boys). The multiple linear regression model showed that the concentration of serum phosphorus level (b = 0.312; p = 0.0497) and immunoglobulin G level (b = 0.433; p = 0.0009) had the greatest influence on the concentration of 25(OH)D. No statistically significant differences in 25(OH)D concentrations were observed among the different IUIS diagnostic groups or between children with normal and low IgG levels. Conclusions: In this retrospective cross-sectional study, children reportedly receiving median 1000 IU supplementation had median 25(OH)D of 30.75 ng/mL with 13% deficiency prevalence; however, causal attribution requires prospective evaluation.