OBJECTIVE This study investigated the effect of parenteral insulin therapy on endogenous insulin secretion in the Diabetes Prevention Trial of Type 1 Diabetes (DPT-1). RESEARCH DESIGN AND METHODS

In the parenteral insulin arm of DPT-1, subjects without diabetes at high risk of future type 1 diabetes randomized to active treatment received a yearly 4-day intravenous insulin infusion (IV-I) and daily subcutaneous insulin (SC-I). To ex-amine the effects of these insulin therapies on endogenous insulin secretion, C-peptide and glucose concentrations were compared during oral glucose toler-ance tests (OGTTs) performed on and off IV-I and SC-I. Forty-six paired OGTTs were performed in 30 subjects from DPT-1 to determine the effect of IV-I. Twenty paired OGTTs were performed in 15 subjects from DPT-1 to determine the effect of SC-I. RESULTS IV-I suppressed fasting and OGTT-stimulated C-peptide (62 % and 40%, respec-tively), and it significantly lowered fasting glucose (67.4 6 4.5 mg/dL during IV-I vs. 90.9 6 1.8 mg/dL off insulin; P < 0.05). By contrast, post-OGTT glucose levels were significantly higher during IV-I: Glucose during IV-I versus off insulin at 120 minutes was 203.9 6 15.1 vs. 151.8 6 10.2 mg/dL, respectively (P < 0.05). O