Incidence of childhood type 1 diabetes mellitus in Yamanashi Prefecture, Japan, 1986‐2018

INTRODUCTION: Several studies have examined the incidence of childhood T1DM in Japan from the 1970s onwards, but none have been long‐term studies using registration data. We estimate the incidence of childhood type 1 diabetes mellitus (T1DM) from 1986 to 2018 in Yamanashi Prefecture, Japan. METHODS: We began a population‐based, long‐term study of childhood T1DM in 1986 involving every hospital paediatrics department in Yamanashi Prefecture. In the Prefecture, every child newly diagnosed with T1DM is referred to a hospital, and therefore, almost 100% of new patients aged <15 years are registered. We calculated the incidence of T1DM among children aged <15 years from 1986 to 2018. All cases met the Japan Diabetes Society diagnostic criteria and were tested for T1DM‐related autoantibodies whenever possible. RESULTS: Ninety‐nine patients (44 boys and 55 girls) were newly diagnosed with T1DM. The annual incidence among 5‐ to 9‐year‐olds increased by 5.35% over the study period (95% confidence interval 2.34%‐8.35%, p = .0005), and there was a trend towards increasing 3‐year incidence (15.52% increase, p = .0516). There were also trends towards increasing annual and 3‐year incidence among 0‐ to 14‐year‐olds. However, there were no changes over time in annual or 3‐year incidence in the 0–4 year or 10–14 year age groups. CONCLUSIONS: The incidence of T1DM in Yamanashi Prefecture increased among children aged 0‐14 years over the study period, with the most significant increase occurring among 5‐ to 9‐year‐olds. These data suggest that the number of children aged <15 years with T1DM is gradually increasing in one of the local prefectures in Japan, Yamanashi Prefecture and that the age of onset is decreasing.