Background: Type 2 diabetes is a common multigenic disorder which caused by impaired β-cell function and insulin sensitivity. However, which of the two is the primary defect is much debated. In addition, the rapid increase in diabetes in Asia in a short period may be due to the increase in obesity. Aim: We evaluated the clinical characteristic of insulin resistance and the insulin secretion capacity in newly diagnosed, drug-naive Korean persons with type 2 diabetes by analyzing a nationwide cross-sectional primary care clinic-based study. Method: We conducted a study of 912 drug-naive patients (512 men and 400 women) with newly diagnosed diabetes from primary care clinics nationwide in 2015-2016. All participants underwent a 75g OGTT after an overnight fast of 10–12h. We assessed insulin resistance using the homeostasis model assessment of insulin resistance (HOMA-IR) and insulin secretion based on fasting serum C-peptide level. The presence of insulin resistance was defined as a HOMA-IR ≥2.5 and insulin secretory dysfunction was classified based on fasting plasma C-peptide levels: severe (<1.10 ng/ml), moderate (1.10–1.69 ng/ml), and mild to non-secretory defect (≥1.70 ng/ml). Results: A total of 75.1 and 22.6% of participants had insulin resistance and β-cell dysfunction, respectively. The proportion of participants with insulin resistance but no β-cell dysfunction increased and the proportion of participants with β-cell dysfunction but no insulin resistance decreased as body mass index (BMI) increased. People diagnosed with diabetes before 40 years of age had significantly higher HOMA-IR and BMI than those diagnosed over 65 years of age (HOMA-IR, 5.0 vs. 3.0; BMI, 28.7 vs. 25.1 kg/m2.). Discussion: We observed that the main pathogenic mechanism of type 2 diabetes is insulin resistance in participants with newly diagnosed type 2 diabetes. In addition, young adults with diabetes are more likely to have higher insulin resistance with obesity and have higher insulin secretory defect with severe hyperglycemia in the early period of diabetes than older populations.
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