Pandemic-related changes in seasonal vitamin D disparities among preschool children: a community-based cross-sectional study

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Background: Preschool children are particularly vulnerable to vitamin D (VD) insufficiency owing to rapid skeletal development and limited dietary sources. Although COVID-19 restrictions reduced outdoor activity and theoretically increased deficiency risk, emerging evidence suggests that behavioral adaptations-especially intensified supplementation-may have offset these effects. Yet serial cross-sectional data spanning the pre-, during-, and post-pandemic periods in this population remain scarce, and age-specific vulnerabilities and seasonal disparities are poorly characterized. This study evaluated temporal trends in serum 25-hydroxyvitamin D [25(OH)D] status across three pandemic phases in children under 6 years, examining age-stratified associations and seasonal variations. Methods: In a community-based cross-sectional study, serum 25(OH)D was quantified by liquid chromatography-mass spectrometry in 6,681 children (<6 years). Participants were stratified into Pre-COVID (2019), During-COVID (2020-2022), and Post-COVID (2022-2023) phases. Multivariable linear and logistic regression models, adjusted for sex, age, and season, assessed pandemic phase associations on continuous 25(OH)D and sufficiency rates (≥30 ng/mL). Subgroup and interaction analyses were conducted by age and season. Results: for interaction <0.001). Spring samples demonstrated the largest increase (25.5 → 38.3 ng/mL), with the previously pronounced seasonal trough markedly attenuated. Conclusion: Despite reduced outdoor activity, VD status improved significantly during and after the pandemic, potentially reflecting increased supplementation practices and altered health behaviors, though these factors were not directly measured. The greatest gains occurred in infants and during spring, suggesting that targeted supplementation may help counteract sunlight deprivation. These findings generate the hypothesis that public health strategies promoting age-specific VD supplementation could help sustain such improvements; however, prospective interventional studies are warranted to confirm these observations and establish causality.

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