Insulin resistance & secretion in subjects with normal fasting plasma glucose.

Background & Objectives: Lowering the diagnostic threshold of normal fasting plasma glucose (FPG) from 6.1 to 5.56 mmol/l has been proposed by American Diabetes Association. As early detection of subjects with risk of diabetes is important, it is crucial to understand the relationship between different levels of FPG, insulin resistance (IR) and insulin secretion. We measured IR and insulin secretion in subjects with different levels of FPG to investigate the relationship between FPG and IR and insulin secretion, and to see whether lowering the cut-off improves the sensitivity of impaired fasting glucose to predict diabetes.

Methods: Apparently healthy subjects (294) were enrolled and divided into 3 groups according to their FPG levels; group 1 (<5.56 mmol/l, n=129), group 2 (5.56-6.09 mmol/l, n=99) and group 3 (6.1-6.9 mmol/l, n=66). Steady state plasma glucose (SSPG) derived from insulin suppression test was used to quantify insulin resistance and 30 min insulinogenic index during an oral glucose tolerance test to measure acute insulin secretion.

Results: The blood pressure was higher in group 3 than group 2 and fasting plasma insulin and triglyceride were higher in group 3 than that in group 1. The insulinogenic index was higher in the group 1 than group 3 and the SSPG was significantly higher in group 3 compared to group 1.

Interpretation & Conclusion: Our study showed a trend of progressive deterioration of both insulin action and secretion with increasing FPG level. These are supportive evidences for lowering the FPG to 5.56 mmol/l as suggested by the ADA.