C-Peptide and Insulin Secretion in Pima Indians and Caucasians: Constant Fractional Hepatic Extraction over a Wide Range of Insulin Concentrations and in Obesity*

Peripheral serum insulin and C-peptide concentrations during oral glucose tolerance tests were measured in 10 nondiabetic Pima Indians and 10 nondiabetic Caucasians with varying degrees of obesity. Although both insulin and C-peptide levels were elevated in the Indians compared to the Caucasians (p less than 0.05), hepatic insulin extraction, measured by comparing the C-peptide to insulin ratios, was similar over a wide range of insulin concentrations in both groups. The ratios of C-peptide to insulin were independent of the degree of obesity. These studies indicate that the peripheral hyperinsulinemia in Pima Indians and obese subjects is due in general to pancreatic hypersecretion rather than to diminished hepatic extraction of insulin.