Vitamin D status and its determinants during autumn in children at northern latitudes: a cross-sectional analysis from the optimal well-being, development and health for Danish children through a healthy New Nordic Diet (OPUS) School Meal Study.

Sufficient summer/autumn vitamin D status appears important to mitigate winter nadirs at northern latitudes. We conducted a cross-sectional study to evaluate autumn vitamin D status and its determinants in 782 Danish 8-11-year-old children (55 degrees N) using baseline data from the Optimal well-being, development and health for Danish children through a healthy New Nordic Diet (OPUS) School Meal Study, a large randomised controlled trial. Blood samples and demographic and behavioural data, including 7-d dietary recordings, objectively measured physical activity, and time spent outdoors during school hours, were collected during September-November. Mean serum 25-hydroxyvitamin D (25(OH)D) was 60.8 (sd 18.7) nmol/l. Serum 25(OH)D levels 50 nmol/l were found in 28.4 % of the children and 2.4 % had concentrations <25 nmol/l. Upon multivariate adjustment, increasing age (per year) ( -2.9; 95 % CI -5.1, -0.7 nmol/l), female sex ( -3.3; 95 % CI -5.9, -0.7 nmol/l), sampling in October ( -5.2; 95 % CI -10.1, -0.4 nmol/l) and November ( -13.3; 95 % CI -17.7, -9.1), and non-white ethnicity ( -5.7; 95 % CI -11.1, -0.3 nmol/l) were negatively associated with 25(OH)D (all P<0.05). Likewise, immigrant/descendant background was negatively associated with 25(OH)D, particularly in females ( -16.3; 95 % CI -21.9, -10.7) (P<0.001) (P-interaction=0.003). Moderate-to-vigorous physical activity (MVPA) (min/d) ( 0.06; 95 % CI 0.01, 0.12), outdoor walking during school hours (min/week) ( 0.4; 95 % CI 0.1, 0.6) and intake of vitamin D-containing supplements 3 d/week ( 8.7; 95 % CI 6.4, 11.0) were positively associated with 25(OH)D (all P<0.05). The high proportion of children with vitamin D status below the recommended sufficiency level of 50 nmol/l raises concern as levels expectedly drop further during winter months. Frequent intake of vitamin D supplements was strongly associated with status. MVPA and outdoor activity during school hours should be investigated further in interventions to improve autumn vitamin D status in children at northern latitudes.