Aims/hypothesis: Proinsulin concentrations are increased relative to insulin concentrations in subjects with Type II (non-insulin-dependent) diabetes mellitus. This could be secondary to hyperglycaemia or insulin resistance or due to a defect in insulin secretion.
Methods: We investigated the association between fasting insulin, intact proinsulin and the intact proinsulin: insulin ratio with insulin sensitivity, estimated by a frequently sampled intravenous glucose tolerance test and the minimal model and with acute insulin response (AIR) in 182 newly diagnosed Type II diabetic subjects aged 40 to 69 years. None of the subjects was receiving hypoglycaemic medication.
Results: Insulin sensitivity correlated inversely with fasting insulin (r(s) = -0.42) and intact proinsulin (r(s) = -0.32) (p < 0.001). The intact proinsulin:insulin ratio was not correlated with insulin sensitivity. AIR correlated positively with intact proinsulin (r(s) = 0.23) and inversely with the intact proinsulin:insulin ratio (r(s) = -0.29, p < 0.001). Fasting glucose correlated positively with intact proinsulin (r(s) = 0.34) and the intact proinsulin:insulin ratio (r(s) = 0.24, p < 0. 001). The intact proinsulin:insulin ratio increased by decreasing AIR (quartiles of AIR from high to low: 7.8, 8.2, 9.7 and 12.1 %, p < 0.001). This association was independent of age, sex, ethnicity, body mass index, fasting glucose, and insulin sensitivity.
Conclusion/interpretation: Insulin resistance (low insulin sensitivity) was not related to the intact proinsulin:insulin ratio in subjects with Type II diabetes. In contrast, both low AIR and high fasting glucose concentrations were associated with a disproportionate increase in proinsulin concentration. These results suggest that increased intact proinsulin:insulin ratio is a marker of a defect in insulin secretion in Type II diabetic subjects.