Abstract / Summary
Abstract Cholecalciferol (Vitamin D₃) and α-tocopherol (Vitamin E) are lipophilic micronutrients that co-regulate key physiological processes in childhood, including immune function, antioxidant defense, and skeletal development. However, their interdependent dose-response relationship remains incompletely understood. This study aimed to characterize the relationship between serum concentrations of 25(OH)D₃ and α-tocopherol, including its shape and inflection point, in a community-based sample of children. This cross-sectional study enrolled 4,752 asymptomatic children during routine well-child visits in Ningbo, China, from 2018 to 2021. Serum 25-hydroxyVitamin D₃ [25(OH)D₃] and α-tocopherol concentrations were quantified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Multivariable linear regression, penalised spline models, and two-segment piecewise regression models were employed to identify potential threshold effects, with adjustments for sex, age, and season. Among 4,752 children, 25(OH)D₃ and α-tocopherol levels varied by status: Mean serum 25(OH)D₃ was lowest in the α-tocopherol-deficient group (23.9 ± 8.4 ng/mL) and highest in the α-tocopherol-excess group (31.4 ± 10.3 ng/mL); mean serum α-tocopherol was lowest in the severe 25(OH)D₃ deficiency group (4.2 ± 1.6 µg/mL) and increased with improved 25(OH)D₃ status. 25(OH)D₃ exhibited a positive, non-linear relation with α-tocopherol ( p < 0.001). Below the threshold of 29.5 ng/mL, each 1 ng/mL increment in serum 25(OH)D₃ was associated with a 0.044 µg/mL (95% CI 0.031–0.058, p < 0.001) increase in α-tocopherol; above this breakpoint the slope flattened to 0.003 µg/mL (95% CI − 0.008–0.014, p = 0.539). The likelihood-ratio test confirmed a significant slope change ( p < 0.001). The threshold was consistent across sex and season, with the steepest response in preschoolers (3–6 years). A 25(OH)D₃ concentration of ~ 30 ng/mL represents a metabolic saturation point beyond which additional 25(OH)D₃ no longer enhances α-tocopherol status. This threshold provides an evidence-based target for dual-micronutrient optimisation strategies in pediatric populations.