Reduced glucose effectiveness as a feature of glucose intolerance: evidence in elderly type-2 diabetic subjects

One of the factors determining glucose tolerance is glucose disappearance independent from the dynamic insulin (glucose effectiveness); the debate on its role in the development of Type-2 diabetes is still open. The aim of the present study was to evaluate insulin delivery, insulin sensitivity (SI), and glucose effectiveness (SG) in a group of elderly Type-2 diabetic patients (D, 4/6 F/M, age 67 +/- 2 years, 64 +/- 2 kg, BMI 23.8 +/- 0.5 kg/m2), compared to young controls (C, 4/6 F/M, 25 +/- 2 years, 72 +/- 4 kg, 23.7 +/- 1.1 kg/m2) and elderly controls (E, 2/4 F/M, 73 +/- 3 years, 63 +/- 4 kg, 23.1 +/- 0.5 kg/m2). We performed oral (OGTT) and intravenous (FSIGT) glucose tolerance tests. The OGTT showed that C and E were normotolerant, while D had a markedly reduced glucose tolerance. This was also confirmed in the FSIGT where the glucose tolerance index (KG) was 0.6 +/- 0.1% min-1 in D vs 1.8 +/- 0.2 in C and 1.5 +/- 0.2 in E (p < 0.0002). Total insulin area of D and the overall insulin delivery were not different from those of the control groups. The early phase area was instead significantly reduced (0.19 +/- 0.02 mU min/mL vs 0.61 +/- 0.06 of C and 0.46 +/- 0.06 of E, p < 0.001) given the reduction in the dynamic first-phase insulin delivery (0.86 +/- 0.17 min(microU/mL)/(mg/dL) vs 3.95 +/- 0.61 in C (p < 0.005) and 2.61 +/- 0.66 (p < 0.001) in E). SI of D was 3.4 +/- 0.4 10(-4) min-1/(microU/mL), not different from that of C (4.7 +/- 0.6) and E (3.5 +/- 0.2). This study showed a marked difference between SG of D and that of both control groups [0.010 +/- 0.001 min-1 vs 0.026 +/- 0.004 (p < 0.001) of C and 0.020 +/- 0.003 (p < 0.002) of E], mostly due to the zero-insulin component GEZI which was 0.006 +/- 0.001 in D vs 0.021 +/- 0.004 in C and 0.016 +/- 0.003 in E (p < 0.003). In the elderly groups, when taken together, SG exhibited a positive correlation with the area under insulin concentration during the early phase and with KG (r = 0.69, p = 0.0032 and r = 0.90, p = 0.0001, respectively), demonstrating the importance of the first-phase insulin delivery in modulating glucose effectiveness and glucose tolerance.