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Improved glucose tolerance to sequential glucose loading (Staub-Traugott effect) is an important determinant of day-to-day glycemic exposure. Its mechanisms have not been clearly established. We recruited 17 healthy volunteers to receive two sequential oral glucose tolerance tests (OGT1's), at time 0 mm and 180 mm (Study 1). The protocol was repeated on a separate day (Study Ii) except that plasma glucose was clamped at 8.3 mmol/l between 60 and 180 mm. 3-Cell function was analyzed by mathematical modeling of C-peptide concentrations. In a subgroup, glucose kinetics were measured by a triple-tracer technique (infusion of [6,6-2H2]glucose and labeling of the 2 glucose loads with [l-2H]glucose and [U-13C]glucose). In both Studies I and II, the plasma glucose response to the second OGTT equaled 84 ± 2% (P = 0.003) of the response to the first OGTT. Absolute insulin secretion was lower (37.8 ± 4.3 vs. 42.8 ± 5.1 nmol/m2, P = 0.02), but glucose potentiation (i.e., higher secretion at the same glycemia) was stronger (1.08 ± 0.02- vs. 0.92 ± 0.02-fold, P = 0.006), the increment being higher in Study!! (+36 ± 5%) than Study! (+19 ± 6%, P < 0.05). In pooled data, a higher glucose area during the first OGTT was associated with a higher potentiation during the second OGTT (rho0.60, P = 0.002). Neither insulin clearance nor glucose clear- ance differed between loads, and appearance of glucose over 3 h totalled 60 ± 6 g for the first load and 52 ± 5 g for the second load (P not significant). Fasting endogenous glucose production [13.3 ± 0.6 μmol·min-1·kg fat-free mass (FFM)-1] averaged 6.0 ± 3.8 μmol·min-1kg FFM-1 between 0 and 180 mm and 1.7 ± 2.6 between 180 and 360 mm (P < 0.03). Glucose potentiation and stronger suppression of endogenous glucose release are the main mechanisms underlying the Staub-Traugott effect. [ABSTRACT FROM AUTHOR]