Serum Insulin, Proinsulin and Proinsulin/Insulin Ratio in Type 2 Diabetic Patients: As an Index of β-Cell Function or Insulin Resistance

BACKGROUND: Although insulin resistance and decreased insulin secretion are characteristics of established type 2 DM, which of these metabolic abnormalities is the primary determinant of type 2 DM is controversial. It is also not well known how insulin resistance and beta cell dysfunction influence serum insulin, proinsulin, proinsulin/insulin ratio in type 2 DM. METHODS: We compared serum insulin, proinsulin and proinsulin/insulin ratio in type 2 diabetic patients and control subjects. We also investigated the relationship between serum insulin, proinsulin and proinsulin/insulin ratio and several biochemical markers which represent insulin resistance or beta cell function. RESULTS: Insulin, proinsulin and proinsulin/insulin ratio were significantly higher in type 2 DM than control (p < 0.001). In diabetic patients, total insulin level was correlated with urinary albumin excretion rates (r = 0.224, p = 0.025) and body mass index (r = 0.269, p = 0.014). Proinsulin level was correlated with fasting C-peptide (r = 0.43, p = 0.002), postprandial 2 hour blood glucose (r = 0.213, p = 0.05) and triglyceride (r = 0.28, p = 0.022). Proinsulin/insulin ratio was positively correlated with fasting C-peptide (r = 0.236, p = 0.031), fasting blood glucose (r = 0.264, p = 0.015), postprandial 2 hour blood glucose (r = 0.277, p = 0.001) and triglyceride(r = 0.428, p < 0.001). In control subjects, insulin level was correlated with triglyceride (r = 0.366, p = 0.002). Proinsulin/insulin ratio was correlated with age (r = 0.241, p = 0.044). CONCLUSION: The serum levels of insulin and proinsulin seem to be associated with several markers of insulin resistance, Whereas proinsulin/insulin ratio might represent beta cell function rather than insulin resistance. But more studies are needed to clarify the mechanisms of elevated proinsulin/insulin ratio in type 2 DM.