Incidence of Type 1 Diabetes in Sweden Among Individuals Aged 0–34 Years, 1983–2007: An analysis of time trends

OBJECTIVE: To clarify whether the increase in childhood type 1 diabetes is mirrored by a decrease in older age-groups, resulting in younger age at diagnosis. RESEARCH DESIGN AND METHODS: We used data from two prospective research registers, the Swedish Childhood Diabetes Register, which included case subjects aged 0–14.9 years at diagnosis, and the Diabetes in Sweden Study, which included case subjects aged 15–34.9 years at diagnosis, covering birth cohorts between 1948 and 2007. The total database included 20,249 individuals with diabetes diagnosed between 1983 and 2007. Incidence rates over time were analyzed using Poisson regression models. RESULTS: The overall yearly incidence rose to a peak of 42.3 per 100,000 person-years in male subjects aged 10–14 years and to a peak of 37.1 per 100,000 person-years in female subjects aged 5–9 years and decreased thereafter. There was a significant increase by calendar year in both sexes in the three age-groups <15 years; however, there were significant decreases in the older age-groups (25- to 29-years and 30- to 34-years age-groups). Poisson regression analyses showed that a cohort effect seemed to dominate over a time-period effect. CONCLUSIONS: Twenty-five years of prospective nationwide incidence registration demonstrates a clear shift to younger age at onset rather than a uniform increase in incidence rates across all age-groups. The dominance of cohort effects over period effects suggests that exposures affecting young children may be responsible for the increasing incidence in the younger age-groups.