More than a third of children in the eastern Chinese city of Ningbo have suboptimal vitamin D status, and the problem worsens sharply as children get older, according to an eight-year analysis of 6,816 pediatric blood samples. The study, published as an open-access research paper in BMC Public Health, tracked serum 25-hydroxyvitamin D, the standard laboratory marker of vitamin D status, in children aged 0 to 10 years who attended routine well-child visits between 2018 and 2025. Serum 25-hydroxyvitamin D was measured by LC-MS/MS, and the researchers classified vitamin D status using widely applied thresholds: concentrations below 20 nanograms per milliliter were considered deficient, values between 20 and 30 nanograms per milliliter were suboptimal but not deficient, and levels above 30 nanograms per milliliter counted as sufficient.
Overall, 38.1 percent of the children fell below the 30 nanograms per milliliter sufficiency threshold, and 9.4 percent were frankly deficient. Because each calendar year contributed a different sample of children, the analysis was a repeated cross-sectional design rather than a follow-up of the same individuals over time. The most striking finding was the steep age gradient.
Sufficiency was recorded in 72.0 percent of infants under one year old, but the proportion fell to 50.4 percent among children aged one to three years, 36.8 percent at ages three to six, and just 8.6 percent among children aged six to ten. In other words, by the time children reached school age, fewer than one in ten met the sufficiency threshold in this sample. Season shaped vitamin D status as well.
Sufficiency peaked in autumn at 71.7 percent and dropped to its lowest point in spring at 55.2 percent, when 44.8 percent of children had concentrations below 30 nanograms per milliliter. No significant differences between boys and girls emerged in the data. To move beyond simple proportions, the team applied multivariable linear regression to vitamin D concentrations and multivariable logistic regression to the odds of deficiency, defined as levels below 20 nanograms per milliliter.
They also ran stratified analyses by season, sex, age group, and pandemic period, allowing them to test whether patterns held within subgroups rather than only across the whole cohort. One genuinely encouraging signal was temporal. Mean serum 25-hydroxyvitamin D concentrations showed a gradual upward trend from 2018 to 2025 across all seasons.
The authors are careful to frame this as an observed trend in a serially sampled cross-sectional sample, not proof of improvement in individual children, since each year’s measurements came from a different group of youngsters. Still, the direction of the population-level shift was consistent. That improvement, however, was not shared equally across ages.
Older children remained predominantly below the 30 nanograms per milliliter threshold throughout the study period, and the gap between the youngest and oldest age groups actually widened over time. The study’s authors caution that the oldest age group rests on a small foundation: only 93 of the 6,816 children were aged six to ten years. Conclusions about school-aged children therefore remain provisional.
The cross-sectional design also means the analysis can describe associations with age and season but cannot establish why individual children’s levels change as they grow. The authors conclude that despite temporal improvements, suboptimal vitamin D status remains common in this pediatric population, with a pronounced age-dependent decline and a persistent springtime vulnerability.
They argue that prevention strategies targeting older children, extending beyond the infancy and early-childhood window where supplementation is routine, warrant evaluation in formal intervention studies. Until such trials are done, the Ningbo data stand as a detailed portrait of how age and season carve vitamin D status across a decade of childhood.
Subject of Research: Age and seasonal patterns of serum 25-hydroxyvitamin D in Chinese children aged 0 to 10 years
Article Title: Serum 25-hydroxyvitamin D status in Ningbo children: age and seasonal determinants from an eight-year retrospective cross-sectional analysis
Article References: Yi, J., Wang, Q., Bi, Y., & Liu, W. (2026). Serum 25-hydroxyvitamin D status in Ningbo children: age and seasonal determinants from an eight-year retrospective cross-sectional analysis. BMC Public Health. https://doi.org/10.1186/s12889-026-30115-3
Image Credits: AI Generated
DOI: 10.1186/s12889-026-30115-3
Keywords: vitamin D, 25-hydroxyvitamin D, children, Ningbo, seasonality, cross-sectional study, LC-MS/MS, pediatric health, BMC Public Health, deficiency, age gradient, public health
News Source: Ophelia Keating. (October 28, 2026). Vitamin D Sufficiency Plunges With Age in Chinese Children, Eight-Year Study Finds. Scienmag.
