Risk of colorectal cancer by socioeconomic position and history of mental illness – a national nested case-control study

Abstract Aim Whether socioeconomic factors are associated with colorectal cancer (CRC) risk remains unknown, with inconsistent results from different countries. The hypothesis was that both socioeconomic factors and the presence of psychiatric comorbidities are risk factors for CRC. Method CRCBaSe, a registry linkage project, was used for a nested case‒control study with 78,043 CRC patients and 431,105 controls matched by age, sex, and county of residence. Socioeconomic factors (income, education, civil status, and birth country) and history of mental illness were used to obtain incidence rate ratios (hazard ratios). Results Patients in the middle-income quartile had an increased rate of CRC compared with those in the lowest-income quartile (HR 1.05, 95%CI 1.03–1.08 for Q2 vs. Q1, and HR 1.04 95%CI 1.021.07 for Q3 vs. Q1), whereas there was no association with high income or educational level. Compared with those born in Sweden, those born in other Nordic countries had an increased rate of CRC (HR 1.06 95%CI 1.02–1.10), and those born outside of the EU had a decreased rate (HR 0.80 95%CI 0.77–0.83). A history of mental illness was associated with the rate of CRC even after adjusting for socioeconomic factors: mild mental illness, HR 0.81 (95%CI 0.78–0.83); and severe mental illness, HR 1.85 (95%CI 1.74–1.96). Conclusion These findings confirm that the effect of socioeconomic position on CRC risk is heterogeneous and small in Sweden and that severe mental illness is a separate risk factor. Further investigations into the mechanisms that drive the increased risk of CRC in patients with mental illness are warranted.