Two-phase linear relationship and threshold effects between Vitamin E and Vitamin A levels in children aged 0-14 years: a cross-sectional study

Background Vitamins E and A are essential fat-soluble micronutrients critical for immune regulation, bone metabolism, and cellular homeostasis. Current evidence highlights significant gaps in understanding their interdependent relationships and the modulatory effects of age, environmental exposures, and physiological status on their systemic bioavailability. Objective To investigate the relationship between Vitamin E and Vitamin A levels in children aged 0–14 years and identify any critical thresholds. Methods This cross-sectional study was conducted from 2018 to 2021 in Ningbo, Zhejiang, China. It involved children aged 0–14 years who attended community health service centers for routine health check-ups. To evaluate the relationship between serum Vitamin E and Vitamin A levels, we performed a multivariate linear regression analysis. Furthermore, a smooth curve fitting approach was employed to investigate the dose-response relationship between Vitamin E and Vitamin A. Results The study included a total of 4,752 participants. Significant variations in baseline Vitamin E and Vitamin A levels were observed across different deficiency categories. The mean Vitamin E level was lowest in the deficiency group (2.4 ± 0.5 µg/mL) and highest in the excess group (7.0 ± 1.6 µg/mL). Similarly, the mean Vitamin A level was lowest in the deficiency group (236.9 ± 60.4 ng/mL) and highest in the excess group (292.4 ± 72.3 ng/mL). A two-phase linear relationship was identified, with a significant threshold effect at a Vitamin E level of 3.579 µg/mL. Below this threshold, the association between Vitamin E and Vitamin A was robust ( β  = 35.829, 95% CI: 30.217, 41.441), while above the threshold, the association weakened significantly ( β  = 9.828, 95% CI: 8.250, 11.406). The likelihood ratio test confirmed the significance of this threshold effect ( p  < 0.001). Conclusions The study identified a significant threshold at 3.579 µg/mL for Vitamin E, beyond which the association with Vitamin A levels stabilizes. This threshold highlights the importance of maintaining optimal Vitamin E levels to support Vitamin A status, particularly in high-risk groups such as older children and those with limited access to Vitamin E-rich foods. Future longitudinal studies are needed to further validate these findings and explore their implications for public health interventions.