The U-shaped correlation between serum VD concentration and systemic immune-inflammatory index: results from the NHANES database prospective cohort study.

Background: Vitamin D (VD) plays a critical role in regulating systemic inflammation, but its correlation with the systemic immune-inflammatory index (SII) remains unclear. This study aimed to explore the relationship between serum VD concentration, dietary VD intake, and SII using data from the National Health and Nutrition Examination Survey (NHANES).

Methods: Data from NHANES 2007-2018 and NHANES 2007-2020 were analyzed for serum VD levels and dietary VD intake, respectively. Restricted cubic splines (RCS) and logistic regression were used to assess associations between VD and SII. Mediation analysis was conducted to evaluate the role of SII in VD-related disease outcomes and mortality.

Results: Serum VD concentration exhibited a U-shaped correlation with SII (P-overall = 0.005; P-non-linear = 0.002). Severe VD deficiency significantly elevated SII levels compared to insufficiency or sufficiency groups. No association was observed between dietary VD intake and SII. Mediation analysis revealed that SII mediated the effects of VD on all-cause and cardiovascular disease-related mortality, but not on cancer, hypertension, or diabetes development.

Conclusion: A U-shaped relationship exists between serum VD and SII, with VD supplementation potentially reducing systemic inflammation and improving cardiovascular outcomes. Future studies should explore VD's role in systemic inflammation and its clinical implications.