“…These encompassed a broad spectrum, including not only sociodemographic factors, lifestyle factors, body mass index (BMI), biomarkers, diseases and health-related factors, but also vitamin D-specific considerations such as the geographic latitude of the assessment center and seasonality (i.e., the calendar month of attending assessment centers and blood draw) [ 26 ]. Adherence to a healthy lifestyle and the season of assessment are also relevant for LBP and are crucial aspects to adjust for [ 15 , 27 , 28 , 29 ]. The covariates included in the final full models were selected from these determinants of vitamin D deficiency or vitamin D supplement use, along with two additional factors specific to LBP: a history of other musculoskeletal diseases before baseline enrollment (ICD-10: M00-M53, M55-M99) and injuries to the abdomen, lower back, lumbar spine, and pelvis prior to baseline (ICD-10: S30-S39) from primary care data [ 15 ].…”