25-Hydroxyvitamin D reference percentiles and the role of their determinants among European children and adolescents.

Background/objectives To provide age- and sex-specific percentile curves of serum 25-hydroxyvitamin D (25(OH)D) by determinants from 3-<15 year-old children, and to analyse how modifiable determinants influence 25(OH)D. Subjects/methods Serum 25(OH)D values from 2,171 children (N=3606 measurements) of the IDEFICS/I.Family cohort were used to estimate percentile curves using the generalized additive model for location, scale and shape. The association of 25(OH)D with time spent outdoors was investigated considering sex, age, country, parental education, BMI z-score, UV index (UVI), and dietary vitamin D in regressions models. Results The age- and sex-specific 5th and 95th percentiles of 25(OH)D ranged from 16.50 to 73.25 and 20.75 to 79.25 nmol/l in girls and boys, respectively. 63% had deficient (<50 nmol/l), 33% insufficient (50-<75 nmol/l) and 3% sufficient (>=75 nmol/l) levels. 25(OH)D increased with increasing UVI, time spent outdoors, and vitamin D intake and slightly decreased with increasing BMI z-score and age. The odds ratio (OR) for a non-deficient 25(OH)D status (reference category: deficient status) by one additional hour spent outdoors was 1.17, 95% CI [1.09-1.26], i.e. children who spent one more hour per day outdoors than other children had a 17% higher chance of a non-deficient than a deficient status. Conclusion UVI, outdoor time, and dietary vitamin D are important determinants of 25(OH)D. Since a majority of children suffer from deficient 25(OH)D their time spent outdoor should be extended. Vitamin D food fortification or supplementation should be considered. Serum 25(OH)D should be observed in pediatric routine to guide targeted supplementation in vitamin D deficient children.