Evaluation of insulin secretion through Intravenous Glucose Tolerance Test (IVGTT) in normoglycaemic lean individuals and in obese subjects with normoglycaemia or varying degrees of dysglycaemia

We investigated the insulin secretion and glucose dynamics using intravenous glucose tolerance test (IVGTT) in obese subjects with varying metabolic profiles. We enrolled 68 subjects divided into 5 groups of obese individuals: normal glucose tolerance (Ob-NGT, n = 29), impaired fasting glucose (Ob-IFG, n = 2), impaired glucose tolerance (Ob-IGT, n = 16), Ob-IGT/IFG (n = 6), Ob-T2D (n = 6), and one control group of lean subjects with NGT (Lean-NGT, n = 9). Through IVGTT, we evaluated the acute insulin response (AIR) and insulin sensitivity was assessed using the calculated sensitivity index (CSi). β-cell function was determined using the Disposition Index (DI). Ob-T2D subjects exhibited a marked reduction in the first-phase insulin secretion (FPIS) compared with both Lean-NGT and Ob-NGT. In contrast, Ob-IGT, Ob-IFG and Ob-IFG/IGT subjects retained FPIS but demonstrated a pronounced enhancement of the second phase insulin secretion relative to both Ob-T2D and Ob-NGT. The secretion curves for Ob-NGT closely mirrored those of Ob-IGT in terms of overall pattern. We observed a significant difference between obese groups and Lean-NGT group in CSi (P = 0.000). Similarly, DI was markedly reduced in obese subjects compared with Lean-NGT and showed a progressive decline with worsening glucose tolerance within the obese cohort (P = 0.000). These findings highlight several alterations in insulin sensitivity and secretion in obese individuals relative to Lean-NGT controls.