Does inflammation explain the association between vitamin D and depression? Results of a cross-sectional study in children and adolescents.

Vitamin D has been associated with depression, potentially via anti-inflammatory mechanisms, yet data are scarce, particularly in adolescence. We investigated (1) whether lower vitamin D status is associated with greater depression severity and (2) whether this association is statistically moderated by inflammation in patients of a child and adolescent psychiatry department. At admission, fasting morning venous blood was drawn. Serum vitamin D (25-hydroxy-cholecalciferol (25(OH)D)) and C-reactive protein (CRP) were analysed in all participants (n 465 (64·7 %♀; 11·3-18·9 years)). In a subsample (n 177), we additionally measured tumor necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and interleukin (IL)-1β, IL-6, IL-8 and IL-10. Depression severity was assessed by the Beck Depression Inventory II (BDI-II) (n 450), the Diagnostic System for Mental Disorders in Childhood and Adolescence via self-assessment (DISYPS Self) (n 441) and parent-assessment (DISYPS Proxy) (n 422). Overall, 43·2 % (n 201) were at risk for vitamin D deficiency (< 30 nmol/l), and 73·5-83·2 % - depending on assessment tool - showed at least mild depression. Linear regression revealed an inverse association between 25(OH)D and BDI-II in both crude and CRP-adjusted full-sample models. Logistic regressions showed a robust inverse association between 25(OH)D and DISYPS Proxy, but not for DISYPS Self. Although 25(OH)D was inversely correlated with some pro-inflammatory markers, neither their inclusion in regression models nor formal mediation analyses supported inflammation as a mediator of the vitamin D-depression association. Overall, our results suggest that vitamin D relates modestly to both depression and inflammation in adolescence. However, based on the measured parameters, we cannot confirm that anti-inflammatory effects are the link between vitamin D and depression.