Serum 25-hydroxyvitamin D status in Ningbo children: age and seasonal determinants from an eight-year retrospective cross-sectional analysis

Background Suboptimal serum 25-hydroxyvitamin D [25(OH)D] status remains prevalent among pediatric populations, yet serial data on age-specific patterns and seasonal evolution in young children are limited. Objective To characterize age-stratified serum 25(OH)D status, seasonal patterns, sex-specific trends, and temporal changes in serum 25-hydroxyvitamin D [25(OH)D] among children aged 0–10 years. Methods This repeated cross-sectional study included 6,816 children undergoing well-child visits in Ningbo, China (2018–2025); a different sample of children was assessed in each calendar year. Serum 25(OH)D was measured by LC-MS/MS and categorized as deficient (< 20 ng/mL), 20–30 ng/mL, or sufficient (> 30 ng/mL). Multivariable linear regression (for 25(OH)D concentrations) and multivariable logistic regression (for 25(OH)D deficiency, < 20 ng/mL), together with stratified analyses (by season, sex, age group, and pandemic period), were performed. Results Overall, 38.1% of children had suboptimal serum 25(OH)D (< 30 ng/mL), including 9.4% with 25(OH)D deficiency (< 20 ng/mL). Sufficiency (> 30 ng/mL) declined markedly with age: 72.0% (< 1 year), 50.4% (1–3 years), 36.8% (3–6 years), and 8.6% (6–10 years). Seasonally, sufficiency was highest in autumn (71.7%) and lowest in spring (55.2%), with 25(OH)D < 30 ng/mL peaking in spring (44.8%). No significant sex differences were observed. Mean serum 25(OH)D concentrations showed a gradual upward trend from 2018 to 2025 in all seasons—an observed trend in this serially sampled cross-sectional sample; however, older children remained predominantly below 30 ng/mL, and the age-related gap widened over time. Conclusions Despite temporal improvements, suboptimal serum 25(OH)D status (< 30 ng/mL) remains common, with a pronounced age-dependent decline and persistent spring vulnerability; because only 93 children were aged 6–10 years, conclusions for school-aged children remain provisional. Age-targeted prevention strategies extending beyond early childhood warrant evaluation in intervention studies. Clinical trial number Not applicable.