CXCL8 and obesity due to melanocortin 4 receptor deficiency: Thus, this study aims to answer the following research questions in a large cohort of European children and adolescents: (i) Are there longitudinal associations of serum 25(OH)D concentrations with single inflammatory markers (adiponectin, leptin, ghrelin, CRP, IL-1Ra, IL-6, IL-8, IL-15, IP-10, TNF-α and a derived inflammation score) adjusting for weight status and other relevant covariables?; (ii) Do the associations differ between children with normal weight and children with overweight/obesity?