The Effect of DPT-1 Intravenous Insulin Infusion and Daily Subcutaneous Insulin on Endogenous Insulin Secretion and Postprandial Glucose Tolerance
OBJECTIVE
To investigate the effect of parenteral insulin therapy on endogenous insulin secretion in the Diabetes Prevention Trial-Type 1 (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 examine the effects of these insulin therapies on endogenous insulin secretion, C-peptide and glucose levels were compared during oral glucose tolerance 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%, respectively), and it significantly lowered fasting glucose (67.4 ± 4.5 mg/dL during IV-I vs. 90.9 ± 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 min was 203.9 ± 15.1 vs. 151.6 ± 10.2 mg/dL, respectively (P < 0.05); 49% of OGTTs became transiently diabetic (>200 mg/dL at 120 min) when receiving IV-I. Fasting glucose was significantly lower when receiving SC-I versus when off insulin (85 ± 3 vs. 94 ± 2 mg/dL, respectively; P < 0.05), but SC-I did not significantly alter fasting or OGTT-stimulated C-peptide compared with being off insulin.


CONCLUSIONS
These data demonstrate that the IV-I used in the DPT-1 markedly suppressed endogenous insulin secretion, which was frequently associated with postprandial glucose intolerance. SC-I, however, did not.