Added this information: The Inter99 Study was a five year study. 3,145 subjects who started out with normal glucose tolerance but developed some form of abnormal blood sugar who were given glucose tolerance tests and Insulin sensitivity index (ISI), homeostasis model assessment of insulin sensitivity (HOMA-IS), early-phase insulin release (EPIR), and insulin secretion relative to insulin action (disposition index) were estimated. The researchers conclude,
A stationary reduced insulin secretion followed by a decline in primarily hepatic insulin sensitivity characterizes the transition from N[ormal] G[lucose] T[olerance] to i-I[impaired]F[asting]G[lucose]. In contrast, low whole-body insulin sensitivity with a secondary lack of ß-cell compensation is associated with the development of i-I[mpaired]G[lucose]T[olerance]. Thereby, i-IFG and i-IGT appear to result from different underlying mechanisms, which may have implications for the prevention and treatment of the diabetes that succeeds them.
Natural History of Insulin Sensitivity and Insulin Secretion in the Progression From Normal Glucose Tolerance to Impaired Fasting Glycemia and Impaired Glucose Tolerance: The Inter99 Study.
Kristine Færch, et al. Diabetes Care, 32:439-444, 2009.