Relation of free and specifically bound leptin to insulin secretion in patients with impaired glucose tolerance (IGT)

Impaired glucose tolerance (IGT) is frequently associated with an increased fat mass and an altered fat distribution. The adipocyte derived hormone, l eptin has been shown to interact with insulin at various levels and may be intimately involved in this process. However, only limited data concerning the interaction of insulin, glucose tolerance and leptin are available and no data exist on the potential influence of bound vs, free circulating lept in. We therefore studied free and bound leptin in 136 patients (77 males, 5 9 females) with IGT, in relation to plasma glucose, insulin, proinsulin and C-peptide levels as well as serum free and bound leptin concentrations dur ing an oral glucose tolerance test (oGTT). The expected positive relation o f free serum leptin levels with body mass index (BMI) was found. Free lepti n concentrations were higher in women than in men. Analysis in tertiles rev ealed a significant relation between free leptin (16-58, 60-160, and 169-93 2 pmo/l) and mean fasting insulin levels (65, 93, and 100 pmol/l). This rel ationship remained significant in a multiple regression analysis with BMI a nd gender as covariates. Similar independent relationships to leptin serum levels were observed for HbAlc and plasma C peptide levels and the proinsul in/insulin ratio bur not for plasma glucose and proinsulin levels. These da ta suggest a fine tuning of leptin by small changes in circulating insulin levels observed in impaired glucose tolerance.