Improved β-cell function rather than increased

β-cell dysfunction and decreased insulin sensitivity are believed to be two chief mechanisms that participate in deterioration of glycemic control in Type 2 diabetes. Meformin is widely accepted as the first-line oral agent in the treatment of Type 2 diabetes. However, the relative contributions of improved β-cell function and increased insulin sensitivity to reduction in hemoglobin A1c (HbA1c) are unclear in newly diagnosed Type 2 diabetic patients treated with metformin. We investigated β-cell function and insulin sensitivity in relation to reduction in HbA1c in 20 newly diagnosed Type 2 diabetic patients (17 men and 3 women, mean age 49.1 ± 10.1 years, mean body mass index 26.4 ± 5.2 kg/m) treated with metformin for 16 weeks. We used homeostasis model assessment (HOMA) 2%B and HOMA2%S as estimates of β-cell function and insulin sensitivity, respectively. Median HOMA2%B and HOMA2%S significantly increased from 38.8 to 68.8 (p < 0.001) and from 50.2 to 67.1 (p = 0.004), respectively. In univariate regression analysis, reduction in HbA1c was highly correlated with change in HOMA2%B (r = −0.866, p < 0.001), but not with that in HOMA2%S (r = −0.264, p = 0.260). Furthermore, multivariate regression analysis with reduction in HbA1c as a dependent variable showed that increase in HOMA2%B but not that in HOMA2%S was a significant dependent variable (β = −0.847, p < 0.001). In conclusion, our findings demonstrate that improved β-cell function rather than increased insulin sensitivity is associated with reduction in HbA1c. These results suggest that metformin reduces HbA1c chiefly through improved β-cell function rather than increased insulin sensitivity in patients with newly diagnosed Type 2 diabetes.