The changing incidence of childhood-onset type 1 diabetes in Wales: Effect of gender and season at diagnosis and birth

Aims Determinants of the changing incidence of childhood-onset type 1 diabetes remain uncertain. We determined the recent time-trend of type 1 diabetes incidence in Wales and explored the role of vitamin D by evaluating the influence of season both at diagnosis and at birth. Methods Data from all Welsh paediatric units 1990–2019, and from primary care to determine ascertainment. Results Log-linear modelling indicated a non-linear secular trend in incidence with peak and subsequent decline. The peak occurred around June 2010: 31∙3 cases/year/100,000 children aged 12y but more in females aged 9–10 y. More were diagnosed in winter. Also, children born in winter had less risk of future diabetes. Conclusions The risk of developing type 1 diabetes before age 15y in Wales is no longer increasing. The data on season are consistent with a preventative role for vitamin D both during pregnancy and later childhood. Metereological Office data shows increasing hours of sunlight since 1980 likely to increase vitamin D levels with less diabetes. Additional dietary supplementation with vitamin D might further reduce the incidence of type 1 diabetes