Third, due to the register-based nature of the study, we did not have complete information on all protective or risk factors for IBD, including lifestyle factors (e.g., smoking, body mass index, and physical activity) [5,46], medical conditions (e.g., antibiotic exposure and vitamin D deficiency) [5,32], and genetic factors (e.g., ethnicity [1,47] and risk loci (e.g., NOD2 in European populations) [48,49]), leaving residual confounding as a concern. Here, NOD2 is linked to vitamin D deficiency.