Peripheral hyperinsulinemia of simple obesity: pancreatic hypersecretion or impaired insulin metabolism?

Insulin and C-peptide levels in peripheral blood in the fasting state and after an oral glucose load were measured in 65 nondiabetic, obese subjects and 65 age- and sex-matched nondiabetic normal weight subjects. Fasting insulin and C-peptide levels were significantly higher in obese than in nonobese subjects, whereas 1 and 2 h after the oral glucose load only insulin concentrations were significantly higher in the obese subjects. C-peptide to insulin molar ratios, as well as the relation between the incremental areas of the two peptides, were used as relative measures of hepatic insulin extraction. In the fasting state the ratios between C-peptide and insulin were similar in obese and nonobese subjects, whereas after glucose they were significantly lower in the obese individuals. Similarly, the relations between C-peptide and insulin incremental areas were significantly lower in obese than in nonobese subjects. The comparison of the corresponding plasma levels and areas of C-peptide and insulin after glucose showed that for the same C-peptide value, the insulin value was higher in the obese group. Last, in obese subjects the parameter used as an estimate of hepatic removal of insulin after oral glucose inversely correlated with the fasting insulin concentration and the insulin incremental area after glucose. These results suggest that in obesity peripheral hyperinsulinemia depends on pancreatic hypersecretion of insulin in the fasting state and impaired hepatic insulin metabolism after oral glucose loading.