Effect of Vitamin D supplementation and outdoor activity on serum 25-Hydroxyvitamin D among children in North China: an observational study

Background Living at high latitudes is one of the risk factors for vitamin D deficiency in children. However, evidence on vitamin D improvement for this pediatric population to date is limited. This study aims at evaluating the effect of different supplementation methods and outdoor activity time on the vitamin D status of children in North China. Methods A total of 55,925 children aged 1 month to 18 years old were recruited from pediatric outpatient departments from July 2016 to June 2017. Data on demographics, anthropometric measurements, vitamin D supplementation, and outdoor time were recorded. The serum levels of 25-hydroxycholecalciferol (25(OH)D) were determined by high performance liquid chromatography tandem–mass spectrometry. Logistic regression analysis was performed to assess the association of vitamin D supplementation or outdoor time with blood vitamin D status, adjusted for age, gender, BMI for age, and seasons. Results The overall rate of hypovitaminosis D was 65.60%. Of the children’s outdoor time, 35.63%, 31.95%, and 32.42% were below 30 min/d, 30–60 min/d and over 60 min/d, respectively. Furthermore, the proportion of iatrogenic supplementation, voluntary supplementation and no vitamin D supplementation among the children was 16.48%, 32.87%, and 50.65%, respectively. After adjusted for confounding factors, vitamin D supplementation was associated with a lower risk of hypovitaminosis D, with OR (95% CI) of 0.191 (0.180, 0.202) in children with iatrogenic supplementation and 0.423 (0.404, 0.443) in those with voluntary supplementation, compared with children without vitamin D supplementation. In addition, longer outdoor time was associated with a lower risk of hypovitaminosis D [0.479 (0.456, 0.504) for 60 min/d, 0.737 (0.701, 0.776) for 30–60 min/d], independent of vitamin D supplementation. Conclusions High prevalence of vitamin D deficiency was found in children living at high latitudes. Vitamin D supplementation and outdoor time are all negatively associated with children’s vitamin D deficiency. Routine 25(OH)D testing combined with vitamin D supplementation might be an effective approach to prevent hypovitaminosis D among children living at high latitudes.