Differential mirroring between in vivo insulin secretion and ex vivo islet function in individuals with and without diabetes

Insulin secretion measured in vivo reflects the integrated output of pancreatic islets within a multi-organ regulatory network, whereas ex vivo stimulation of isolated human islets captures intrinsic, islet-autonomous secretory competence. Whether these two levels of β cell function mirror each other in humans remains unclear. We studied 63 individuals undergoing partial pancreatectomy who underwent preoperative OGTT with model-based assessment of β cell function and provided pancreatic tissue for ex vivo islet isolation and glucose stimulation. Ex vivo function was quantified as the stimulation index at 16.7 mmol/l glucose (SI16.7) and related to in vivo indices of insulin secretion. SI16.7 was reduced in islets from people with T2D but showed marked inter-individual variability with overlapping distributions between individuals with and without T2D. In non-diabetic individuals, no significant associations were detected between SI16.7 and OGTT-derived indices. In contrast, in people with T2D, SI16.7 was positively associated with multiple measures of glucose-driven insulin secretion and inversely with potentiation. In multivariable analysis, a composite index of intrinsic glucose-dependent β cell competence remained independently related to SI16.7. These findings indicate that a direct correspondence between systemic β cell function and intrinsic islet secretory capacity emerges only in T2D. This state-dependent alignment suggests that loss of network-mediated regulation may unmask islet-intrinsic defects, allowing ex vivo islet performance to better reflect in vivo insulin secretion and supporting integrated in vivo–ex vivo phenotyping as a tool for functional stratification of T2D.