Type 1 diabetes in insulin-treated adult-onset diabetic subjects.

The frequency of autoimmune features was compared in adult-onset diabetic subjects either requiring insulin treatment within 12 months of diagnosis or progressing to insulin therapy after a latency of at least 5 years. Adult-onset insulin-treated diabetic subjects were sampled from the population-based Canterbury Diabetes Registry (n = 1580). There were 237 (15%) registrants who met the study criteria of age < 75 years at 1 January 1993, age at diagnosis of diabetes > or = 45 years and duration of diabetes between 5 and 15 years; 101 subjects commenced insulin 5-15 years after diagnosis (group 1) and 80 subjects commenced insulin within 1 year of diagnosis (group 2). C-peptide levels, islet cell antibodies (ICA) and antibodies against glutamic acid decarboxylase (anti-GAD) were determined in all individuals from group 1 (n = 27) and group 2 (n = 23) who agreed to be recruited to the study. The group 1 and group 2 samples did not differ significantly in their demographic characteristics, nor were they different from the two groups from which they were drawn (mean age, 64.2 years; age at diagnosis, 53.5 years; duration of diabetes, 10.7 years; body mass index, 28.6 kg/m2). Overall, 12 of the 50 (24%) study subjects tested positive for anti-GAD; 43% (10) of group 2 subjects were anti-GAD positive compared with only 7.4% (2) of group 1 subjects (P < 0.01). Postprandial C-peptide levels were significantly lower in group 2 subjects compared with group 1 subjects (627 vs 1124 pM, P < 0.05). All subjects were ICA negative. These observations suggest that autoimmune destruction of beta-cells explains early requirement for insulin in adult-onset diabetes.