[Relationship between ambient air pollution and vitamin D in Chinese adults]

Objective: To investigate the relationship between individual and combined exposure to ambient air pollutants and vitamin D, as well as the potential role of ultraviolet radiation (UVR) in mediating the relationships. Methods: This study included 6 967 participants from the China Kadoorie Biobank, whose baseline (2004-2008) blood 25-hydroxyvitamin D had been tested. The average monthly concentrations of pollutants (PM_2.5, PM₁₀, NO₂, O₃) and the UVR estimates in the month of blood sample collection were derived from the ChinaHighAirPollutants and ultraviolet datasets covering China, respectively. Linear models and multinomial logistic regression models were used to analyze the associations of each air pollutant with vitamin D concentrations and groups. Principal component analysis integrated with quantile-based g-computation was applied to evaluate the co-effects and relative contribution weights of air pollutants. Mediation analyses were performed to investigate the potential role of UVR. Results: PM_2.5, PM₁₀, NO_2, and O₃ were individually associated with vitamin D concentrations, as well as vitamin D groups: insufficiency (20-30 ng/ml) and deficiency (<20 ng/ml). For each 10 μg/m³ increase in monthly PM_2.5, PM₁₀, NO_2, and O₃, the percentage changes (95%CIs) in vitamin D concentrations were -1.71% (-2.16% - -1.26%), -1.30% (-1.60% - -1.00%), -3.77% (-4.60% - -2.93%), and 1.27% (0.91%-1.63%), respectively, with corresponding ORs (95%CIs) for vitamin D deficiency of 1.17 (1.06-1.29), 1.12 (1.05-1.19), 1.66 (1.38-2.00), and 0.81 (0.74-0.89). The mixture of the first principal component of PM_2.5 and PM₁₀ (PM), along with NO₂ and O₃, was negatively correlated with vitamin D. The percentage change (95%CI) in vitamin D concentrations for a one-quintile increase in the mixture was -2.20% (-3.56%- -0.82%), with NO₂ contributing the most (83%), followed by PM (17%). UVR-mediated association was 12.2% (95%CI：2.5%-23.0%) for PM_2.5 and 4.9% (95%CI：2.2%-8.0%) for PM₁₀ with vitamin D concentrations, respectively. Conclusion: Higher concentrations of particulate matter, NO₂, and a mixture of air pollutants were associated with lower vitamin D concentrations in Chinese adults, with reduced UVR acting as a partial mediator in the particulate matter-vitamin D relationship.