Serum 25(OH)D concentrations are differentially associated with innate and adaptive immune cell markers, with threshold effects near 40 ng/mL: A cross-sectional study.

The nonlinear relationship and threshold effects of vitamin D on immune function in healthy adults remain unclear. This cross-sectional study of 316 healthy adults (January 2022 to June 2023) investigated associations between serum 25-hydroxyvitamin D (25(OH)D) concentrations and peripheral blood immune parameters (CD3+ T lymphocytes, CD4+ T cells, CD19+ B cells, and natural killer [NK] cells) quantified by flow cytometry. Restricted cubic spline and piecewise regression models evaluated nonlinear relationships and inflection points. Compared to the low 25(OH)D group (<28.45 ng/mL), the high group (>28.45 ng/mL) had higher NK cell counts (306.00 vs 272.50, P < .01) and percentages (15.52% vs 14.21%, P < .01), but lower CD3+ T cell percentage (69.99% vs 73.20%, P < .01). Multivariate regression confirmed that 25(OH)D was negatively associated with CD3+ T cell percentage (β = -0.16, P < .001), and positively with NK cell count (β = 3.62, P = .002) and NK cell percentage (β = 0.16, P < .001). Restricted cubic spline analyses revealed significant threshold effects, with inflection points at 40.94 ng/mL for CD3+ T cell percentage (P = .003), 37.86 ng/mL for NK cell count (P = .005), and 40.94 ng/mL for NK cell percentage (P = .022). Subgroup analyses showed stronger correlations in males and individuals 40 to 59 years of age (P < .05). Our findings suggest that vitamin D may exert threshold-dependent dual immunomodulatory effects-enhancing innate immunity (NK cells) while suppressing adaptive immune activation (CD3+ T cells). Higher 25(OH)D concentrations (around 40 ng/mL) were associated with distinct immune profiles in this healthy cohort. These associations may provide preliminary insights for future studies, although causal relationships cannot be inferred from this cross-sectional design.