The trends and the risk of type 1 diabetes over the past 40 years: an analysis by birth cohorts and by parental migration background in Sweden

Objective To investigate the trends and the risk of developing type 1 diabetes in the offspring of Swedes and immigrants by specific parental migration background, age, sex and birth cohort. Design Registry-based cohort study. Setting Using Swedish nationwide data we analysed the risk of developing type 1 diabetes in 3457486 female and 3641304 male offspring between 0 and 30years of age, born to native Swedes or immigrants and born and living in Sweden between 1969 and 2009. We estimated incidence rate ratios (IRRs) with 95% CIs using Poisson regression models. We further calculated age-standardised rates (ASRs) of type 1 diabetes, using the world population as standard. Results We observed a trend of increasing ASRs among offspring below 15years of age born to native Swedes and a less evident increase among offspring of immigrants. We further observed a shift towards a younger age at diagnosis in younger birth cohorts in both groups of offspring.Compared with offspring of Swedes, children (0-14years) and young adults (15-30years) with one parent born abroad had an overall 30% and 15-20% lower IRR, respectively, after multivariable adjustment. The reduction in IRR was even greater among offspring of immigrants if both parents were born abroad. Analysis by specific parental region of birth revealed a 45-60% higher IRR among male and female offspring aged 0-30years of Eastern Africa. Conclusions Parental country of birth and early exposures to environmental factors play an important role in the aetiology of type 1 diabetes.