Association between serum 25-hydroxyvitamin D concentrations and respiratory infections in US preschool children: a population-based study.

Vitamin D deficiency has been implicated in pediatric respiratory health, yet evidence regarding its role in preschool children remains inconsistent. This study investigated the association between serum 25-hydroxyvitamin D [25(OH)D] concentrations and respiratory infections in a nationally representative cohort of US preschool children. Cross-sectional analysis included 3316 children aged 2-6 years from NHANES (2007-2016). Vitamin D status was categorized as deficiency (< 50 nmol/L), insufficiency (50-74.9 nmol/L), and sufficiency (≥ 75 nmol/L). Respiratory outcomes included caregiver-reported head/chest colds and severe infections (influenza/pneumonia/ear infections) within 30 days. Multivariable logistic regression adjusted for sociodemographic, lifestyle, dietary, and clinical confounders. Dose-response relationships and subgroup heterogeneity were rigorously assessed. Vitamin D deficiency prevalence was 8.1%, with 52% of the cohort exhibiting suboptimal status (< 75 nmol/L). Deficient children had 56% higher odds of respiratory infections versus sufficient counterparts (adjusted OR: 1.56; 95% CI: 1.17-2.09). Stronger associations emerged for head/chest colds (OR: 1.47; 95% CI: 1.09-1.97) than severe infections (OR: 1.24; 95% CI: 0.70-2.18). Subgroup analyses revealed elevated risks in children aged 4-6 years (OR: 1.93; 95% CI: 1.35-2.76), female children (OR: 1.81; 95% CI: 1.17-2.80), low-income households (OR: 1.59; 95% CI: 1.10-2.30), and underweight individuals (OR: 1.61; 95% CI: 1.13-2.31). Exposure to secondhand smoke further amplified deficiency-related infection risks (OR: 1.54; 95% CI: 1.12-2.10). Adequate vitamin D intake (≥ 6.7 mcg/day) attenuated deficiency-related risks (OR: 1.37; 95% CI: 0.68-2.76). Conclusions: Vitamin D deficiency is independently associated with increased respiratory infection risk in preschool children, particularly in vulnerable subgroups. These findings support targeted screening and supplementation strategies to mitigate early childhood morbidity.