OBJECTIVE—Incidence of type 1 diabetes is considered to be low in adults, but no study has been performed in Mediterranean countries.

RESEARCH DESIGN AND METHODS—We extended the study base of the registry of the province of Turin, Italy, to subjects aged 30–49 years in the period 1999–2001 to estimate the incidences of type 1 and type 2 diabetes. Diagnosis of type 1 diabetes was based on permanent insulin treatment or a fasting C-peptide level ≤0.20 nmol/l or islet cell (ICA) or GAD (GADA) antibody positivities.

RESULTS—We identified 1,135 case subjects with high completeness of ascertainment (99%), giving an incidence rate of 58.0 per 100,000 person-years (95% CI 54.7–61.5). The incidence of type 1 diabetes was 7.3 per 100,000 person-years (6.2–8.6), comparable with the rates in subjects aged 0–14 and 15–29 years (10.3 [9.5–11.2] and 6.8 [6.3–7.4]). Male subjects had a higher risk than female subjects for both type 1 (rate ratio [RR] 1.70 [95% CI 1.21–2.38]) and type 2 (2.10 [1.84–2.40]) diabetes. ICA and/or GADA positivities were found in 16% of the cohort. In logistic regression, variables independently associated with autoimmune diabetes were age 30–39 years (odds ratio [OR] 2.39 [95% CI 1.40–4.07]), fasting C-peptide <0.60 nmol/l (3.09 [1.74–5.5]), and BMI <26 kg/m² (2.17 [1.22–3.85]).

CONCLUSIONS—Risk of type 1 diabetes between age 30 and 49 years is similar to that found in the same area between age 15 and 29 years. Further studies are required to allow geographical comparisons of risks of both childhood and adulthood autoimmune diabetes, the latter being probably higher than previously believed.