Tracer methodology has been applied extensively to the estimation of endogenous glucose production (R_a) during euglycemic glucose clamps. The accuracy of this approach has been questioned due to the observation of significantly negative estimates for R_a when insulin levels are high. We performed hyperinsulinemic (300 μU/ml)-euglycemic glucose clamps for 180 min in normal dogs and compared the standard approach, an unlabeled exogenous glucose infusate (cold GINF protocol, n = 12), to a new approach in which a tracer (D-[3-³H]glucose) was added to the exogenous glucose used for clamping (hot GINF protocol, n = 10). Plasma glucose, insulin and glucagon concentrations, and glucose infusion rates were similar for the two protocols. Plasma glucose specific activity was 20 ± 1% of basal (at 120–180 min) in the cold GINF studies, and 44 ± 3 to 187 ± 5% of basal in the hot GINF studies. With the one-compartment, fixed pool volume model of Steele, R_a, for the cold GINF studies was –2.4 ± 0.7 mg · min⁻¹ · kg⁻¹ at 25 min and remained significantly negative until 110 min (P < .05). For the hot GINF studies, R_a was never significantly less than zero (P > .05) and was greater than in the cold GINF studies at 20–90 min (P < .05). There was substantially less between-(78%) and within- (40%) experiment variation for the hot GINF studies compared with the cold GINF studies. An alternate approach (regression method) to the application of the one-compartment model, which allows for a variable and estimable effective distribution volume, yielded R_a estimates that were suppressed 60–100% from basal.

In conclusion, the one-compartment, fixed pool volume model of glucose kinetics is inadequate for the estimation of R_a during euglycemic glucose clamps. Two new strategies for estimating R_a from the one-compartment model, the hot GINF protocol and the regression method calculation, yielded more accurate and physiologically plausible estimates of R_a than currently used methodology.