Inflammatory bowel diseases in Faroese-born Danish residents and their offspring: further evidence of the dominant role of environmental factors in IBD development.

BACKGROUND: The incidence of inflammatory bowel disease (IBD) is record high in the Faroe Islands, and many Faroese emigrate to Denmark, where the IBD incidence is considerably lower.

AIM: To study the IBD incidence in first-, second- and third-generation immigrants from the Faroe Islands to Denmark to assess the extent to which the immigrants adopt the lower IBD incidence of their new home country.

METHODS: Data on Faroese-born Danish residents and their children were retrieved from the Danish Central Population Register for 1980-2014. Incident IBD cases were identified from the Danish National Patient Register. Standardised Incidence Ratios (SIRs) were used to compare the IBD risk in immigrants with that of Danes. 95% confidence intervals (CI) were calculated using the square-root transform.

RESULTS: First-generation Faroese immigrants had a higher IBD incidence than Danes, SIR 1.25 (95% CI, 0.97-1.59) for men and 1.28 (95% CI, 1.05-1.53) for women. This excess risk derived from ulcerative colitis (UC), SIR 1.44 (95% CI, 1.10-1.87) for men and 1.36 (95% CI, 1.09-1.68) for women. No excess risk was found for Crohn's disease (CD). The UC risk was nearly doubled during the immigrants' first 10 years in Denmark; SIR 2.13 (95% CI, 1.52-2.92) for men and 1.63 (95% CI, 1.19-2.18) for women.

CONCLUSIONS: Although some impact of genetic dilution cannot be excluded, our findings indicate importance of gene-environment interplay in UC, as the excess UC risk in Faroese immigrants to Denmark disappeared over time and over one generation in men and over two generations in women.