BackgroundThe association between schizophrenia and decreased vitamin D levels is well documented. Low maternal and postnatal vitamin D levels suggest a possible etiological mechanism. Alternatively, vitamin D deficiency in patients with schizophrenia is presumably (also) the result of disease-related factors or demographic risk factors such as urbanicity.MethodsIn a study population of 347 patients with psychotic disorder and 282 controls, group differences in vitamin D concentration were examined. Within the patient group, associations between vitamin D, symptom levels and clinical variables were analyzed. Group × urbanicity interactions in the model of vitamin D concentration were examined. Both current urbanicity and urbanicity at birth were assessed.ResultsVitamin D concentrations were significantly lower in patients (B = −8.05; 95% confidence interval (CI) −13.68 to −2.42; p = 0.005). In patients, higher vitamin D concentration was associated with lower positive (B = −0.02; 95% CI −0.04 to 0.00; p = 0.049) and negative symptom levels (B = −0.03; 95% CI −0.05 to −0.01; p = 0.008). Group differences were moderated by urbanicity at birth (χ2 = 6.76 and p = 0.001), but not by current urbanicity (χ2 = 1.50 and p = 0.224). Urbanicity at birth was negatively associated with vitamin D concentration in patients (B = −5.11; 95% CI −9.41 to −0.81; p = 0.020), but not in controls (B = 0.72; 95% CI −4.02 to 5.46; p = 0.765).ConclusionsLower vitamin D levels in patients with psychotic disorder may in part reflect the effect of psychosis risk mediated by early environmental adversity. The data also suggest that lower vitamin D and psychopathology may be related through direct or indirect mechanisms.