Fall in C-Peptide During First 4 Years From Diagnosis of Type 1 Diabetes: Variable Relation to Age, HbA1c, and Insulin Dose

OBJECTIVE We aimed to describe the natural history of residual insulin secretion in Type 1 Diabetes TrialNet participants over 4 years from diagnosis and relate this to previously reported alternative clinical measures reflecting beta-cell secretory function. RESEARCH DESIGN AND METHODS Data from 407 subjects from 5 TrialNet intervention studies were analyzed. All subjects had baseline stimulated C-peptide values of >= 0.2 nmol/L from mixed-meal tolerance tests (MMTTs). During semiannual visits, C-peptide values from MMTTs, HbA(1c), and insulin doses were obtained. RESULTS The percentage of individuals with stimulated C-peptide of >= 0.2 nmol/L or detectable C-peptide of >= 0.017 nmol/L continued to diminish over 4 years; this was markedly influenced by age. At 4 years, only 5% maintained their baseline C-peptide secretion. The expected inverse relationships between C-peptide and HbA(1c) or insulin doses varied over time and with age. Combined clinical variables, such as insulin-dose adjusted HbA(1c) (IDAA1C) and the relationship of IDAA1C to C-peptide, also were influenced by age and time from diagnosis. Models using these clinical measures did not fully predict C-peptide responses. IDAA1C <= 9 under-estimated the number of individualswith stimulated C-peptide >= 0.2 nmol/L, especially in children. CONCLUSIONS Current trials of disease-modifying therapy for type 1 diabetes should continue to use C-peptide as a primary end point of beta-cell secretory function. Longer duration of follow-up is likely to provide stronger evidence of the effect of disease-modifying therapy on preservation of beta-cell function.