Comparison of intraduodenal and intravenous glucose metabolism under clamp conditions in humans.

To determine whether the uptake and metabolic partition of glucose are influenced by its delivery route, 12 normal volunteers underwent two 3-h euglycemic (approximately 93 mg/dl) hyperinsulinemic (approximately 43 mU/l) clamps at a 3- to 5-wk interval, one with intravenous (i.v.) and the other with intraduodenal (i.d.) glucose labeled with [3-3H]- and [U-14C]glucose. Systemic glucose was traced with [6,6-2H2]glucose in eight subjects. During the last hour of the clamps, the average glucose infusion rate (5.85 +/- 0.37 vs. 5.43 +/- 0.43 mg.kg(-1).min(-1); P = 0.02) and exogenous glucose uptake (5.66 +/- 0.37 vs. 5.26 +/- 0.41 mg.kg(-1).min(-1); P = 0.04) were borderline higher in the i.d. than in the i.v. studies. The increased uptake was entirely accounted for by increased glycolysis (3H2O production), which was attributed to the stimulation of gut metabolism by the absorptive process. No difference was observed in glucose storage whether it was calculated as glucose uptake minus glycolysis (i.d. vs. i.v.: 2.44 +/- 0.28 vs. 2.40 +/- 0.31 mg.kg(-1).min(-1)) or as glucose uptake minus net glucose oxidation (2.86 +/- 0.33 vs. 2.81 +/- 0.35 mg.kg(-1).min(-1)). Because peripheral tissues were exposed to identical glucose, insulin, and free fatty acid levels under the two experimental conditions, we assumed that their glucose uptake and storage were similar during the two tests. We therefore suggest that hepatic glycogen storage (estimated as whole body minus peripheral storage) was also unaffected by the route of glucose delivery. On the other hand, in the i.d. tests, the glucose splanchnic extraction ratio calculated by the dual-isotope technique averaged 4.9 +/- 2.3%, which is close to the figures published for i.v. glucose. Despite the limitations related to whole body measurements, these two sets of data do not support the idea that enteral glucose stimulates hepatic uptake more efficiently than i.v. glucose.