# Two-phase linear relationship of Vitamin D and Vitamin A among children aged 0–14 years: a cross-sectional study

## TL;DR

This study finds a two-phase relationship between Vitamin D and Vitamin A levels in children, with a key threshold at 28.289 ng/mL of Vitamin D.

## Contribution

The study identifies a specific threshold effect in the relationship between Vitamin D and Vitamin A levels in children.

## Key findings

- A significant threshold effect was found at 28.289 ng/mL Vitamin D, below which Vitamin D strongly correlates with Vitamin A.
- Above the threshold, the association between Vitamin D and Vitamin A becomes significantly weaker.
- Baseline vitamin levels varied significantly across deficiency categories.

## Abstract

Vitamin D and Vitamin A are crucial for children’s immune function, bone health, and cellular growth, but their interrelationship and the impact of various factors remain poorly understood.

To explore the relationship between Vitamin D and Vitamin A levels in children and identify any critical thresholds.

A cross-sectional study was conducted from 2018 to 2021 in Ningbo, Zhejiang, China. Participants were children aged 0 to 14 years attending community health service centers for routine health check-ups. Multivariate linear regression analysis was conducted to ascertain the association between serum Vitamin D and Vitamin A levels. A smooth curve fitting approach was employed to analyze the dose–response relationship between Vitamin D and Vitamin A.

A total of 4,752 participants were included. The study revealed significant variations in baseline Vitamin D and Vitamin A levels across different deficiency categories. Mean Vitamin D levels were lowest in the severe deficiency group (4.9 ng/mL) and highest in the normal group (34.8 ± 9.8 ng/mL), while mean Vitamin A levels were lowest in the deficiency group (226.6 ± 72.2 ng/mL) and highest in the normal group (263.3 ± 74.0 ng/mL). The study revealed a two-phase linear relationship with a significant threshold effect at a Vitamin D level of 28.289 ng/mL. Below this threshold, the association between Vitamin D and Vitamin A was strong (β = 2.935, 95% CI: 2.173, 3.696), while above the threshold, the association was significantly weaker (β = 0.737, 95% CI: 0.413, 1.061). The likelihood ratio test confirmed the significance of this threshold effect (p < 0.001).

The study concludes that a significant threshold at 28.289 ng/mL Vitamin D marks a point beyond which the association with Vitamin A levels plateaus, highlighting the importance of this threshold for optimizing vitamin status in children.