The Incidence of Adult-Onset Type 1 Diabetes: A Systematic Review From 32 Countries and Regions

BACKGROUND: The epidemiology of adult-onset type 1 diabetes (T1D) incidence is not well-characterized due to the historic focus on T1D as a childhood-onset disease. PURPOSE: We assess the incidence of adult-onset (≥20 years) T1D, by country, from available data. DATA SOURCES: A systematic review of MEDLINE, Embase, and the gray literature, through 11 May 2021, was undertaken. STUDY SELECTION: We included all population-based studies reporting on adult-onset T1D incidence and published from 1990 onward in English. DATA EXTRACTION: With the search we identified 1,374 references of which 46 were included for data extraction. Estimates of annual T1D incidence were allocated into broad age categories (20-39, 40-59, ≥60, or ≥20 years) as appropriate. DATA SYNTHESIS: Overall, we observed the following patterns: 1) there is a paucity of data, particularly in low- and middle-income countries; 2) the incidence of adult-onset T1D is lowest in Asian and highest in Nordic countries; 3) adult-onset T1D is higher in men versus women; 4) it is unclear whether adult-onset T1D incidence declines with increasing age; and 5) it is unclear whether incidence of adult-onset T1D has changed over time. LIMITATIONS: Results are generalizable to high-income countries, and misclassification of diabetes type cannot be ruled out. CONCLUSIONS: From available data, this systematic review suggests that the incidence of T1D in adulthood is substantial and highlights the pressing need to better distinguish T1D from T2D in adults so that we may better assess and respond to the true burden of T1D in adults.