Oral glucose tolerance test

All individuals underwent an extended 75-g frequently sampled OGTT according to the BIGTT protocol, where the estimated insulin sensitivity and insulin secretion capacity were validated against an intravenous glucose tolerance test in non-DM individuals (28). After a 12-h overnight fast, venous blood samples were drawn at −20, −10 and 0 min before the OGTT and at 10, 30, 50, 60, 90, 105, 120, 140, 180 and 240 min from the start of the glucose load and stored at −80°C until analysis of p-glucose, p-lactate, s-insulin levels and s-c-peptide. Plasma glucose and plasma lactate were determined on ABL800 FLEX^® (Radiometer Medical, Brønshøj, Denmark) and serum insulin, and c-peptide was measured on Cobas e411^® (Roche) by electro-chemiluminescence immunoassay (ECLIA). Samples from mutations carriers and the matched control subjects were analyzed in pairs.