Adaptation of insulin clearance to metabolic demand is a key determinant of glucose tolerance

With the development of insulin resistance (IR), there is a compensatory increase in the plasma insulin response to offset the defect in insulin action to maintain normal glucose tolerance. The insulin response is the result of two factors: insulin secretion and metabolic clearance rate of insulin (MCR_I). Subjects (104 with normal glucose tolerance [NGT], 57 with impaired glucose tolerance [IGT], and 207 with type 2 diabetes mellitus [T2DM]), divided in nonobese and obese groups, received a euglycemic insulin-clamp (40 mU/m² $ min) and an oral glucose tolerance test (OGTT) (75 g) on separate days. MCR_I was calculated during the insulin-clamp performed with [3-³H]glucose and the OGTT and related to IR: peripheral (glucose uptake during the insulin clamp), hepatic (basal endogenous glucose production 3 fasting plasma insulin [FPI]), and adipocyte (fasting free fatty acid 3 FPI). MCR_I during the insulin clamp was reduced in obese versus nonobese NGT (0.60 6 0.03 vs. 0.73 6 0.02 L/min $ m², P < 0.001), in nonobese IGT (0.62 6 0.02, P < 0.004), and in nonobese T2DM (0.68 6 0.02, P < 0.03). The MCR_I during the insulin clamp was strongly and inversely correlated with IR (r 5 20.52, P < 0.0001). During the OGTT, the MCR_I was suppressed within 15–30 min in NGT and IGT subjects and remained suppressed. In contrast, suppression was minimal in T2DM. In conclusion, the development of IR in obese subjects is associated with a decline in MCR_I that represents a compensatory response to maintain normal glucose tolerance but is impaired in individuals with T2DM.