Nonlinear and Sex-Specific Associations of Vitamin D Metabolites with Inflammatory Blood Markers in 125,537 Adults.

BACKGROUND: Vitamin D is increasingly recognized as a key immunomodulatory nutrient, influencing innate and adaptive immune responses. While 25-hydroxyvitamin D [25(OH)D] is widely used to assess vitamin D status, the active form, 1,25-dihydroxyvitamin D [1,25(OH)2D], may exert distinct effects on immune function. This study investigates the concentration-dependent and sex-specific relationships of both vitamin D metabolites with systemic inflammatory markers in a large clinical cohort. OBJECTIVES: To characterize the associations of 25(OH)D and 1,25(OH)2D with total white blood cell (WBC) count and leukocyte subtypes, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. METHODS: We conducted a retrospective cross-sectional analysis of 125,537 adults (37.9% male; age 18-89 years) from routine laboratory diagnostics collected between 2014 and 2020 in Germany. Serum 25(OH)D and 1,25(OH)2D were measured using standardized chemiluminescent immunoassays. Inflammatory markers were assessed via automated hematology. Multivariable-adjusted linear and non-linear regression models were used to assess associations, adjusting for age, sex, and season. RESULTS: Neutrophils and Monocytes: Displayed U-shaped associations with both 25(OH)D and 1,25(OH)2D. Neutrophil counts were lowest at 25(OH)D levels of ~40-60 nmol/L and increased significantly at both lower and higher extremes (p < 0.001). Lymphocytes: Inverse relationship with 25(OH)D (p < 0.001), and an inverse U-shaped relationship with 1,25(OH)2D, peaking at ~90 pmol/L, with counts decreasing at both lower and higher levels (p < 0.001). Sex-specific analysis revealed that the relationship between 1,25(OH)2D and lymphocyte count remained independent only in men, Eosinophils and Basophils: Demonstrated consistently negative correlations with both forms of vitamin D across all concentration ranges (p < 0.001). CONCLUSIONS: Our findings reveal distinct, concentration-dependent associations between vitamin D metabolites and leukocyte profiles, with evidence for nonlinear and sex-specific immunological effects. Both low and high levels of 25(OH)D and 1,25(OH)2D were linked to increased neutrophil and monocyte counts, suggesting that vitamin D excess, like deficiency, may be linked to low-grade inflammation. These data are hypothesis-generating and suggest that personalized monitoring of vitamin D status may be relevant for future research on immune health, particularly in populations at risk for inflammatory or metabolic disease, but they do not provide a basis for clinical decision-making.