Comparison of extracellular and net glucose oxidation measured isotopically and by indirect calorimetry during high and low glucose turnover

To determine the extent to which glucose oxidation measured by indirect calorimetry reflects glucose oxidation measured isotopically, subjects were studied during a 6-h hyperinsulinemic euglycemic clamp (1 mU · kg^-1 · min^-1) and during infusion of saline. [6^-14C]glucose was infused on both occasions. Breath was collected for determination of the specific activity of carbon dioxide, oxygen consumption, and carbon dioxide production. Glucose turnover during hyperinsulinemia was approximately eightfold higher than during saline infusion. During the final 1.5 h of the hyperinsulinemic glucose clamp, oxidation measured isotopically remained slightly but consistently lower (P < 0.05) than that measured by indirect calorimetry(13.8 ± 1.1 vs 16.5 ± 1.7 μmol · kg^-1 · min^-1, respectively). In contrast, during the saline infusion, glucose oxidation measured isotopically did not differ from that measured by indirect calorimetry (8.3 ± 0.6 vs 7.2 ± 2.8 μmol · kg^-1 · min^-1, respectively). We conclude that although net glucose oxidation measured isotopically was slightly lower than that measured by indirect calorimetry, both techniques provide similar estimates of glucose oxidation over a wide range of glucose disposal.