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 (~93 mg/dl) hyperinsulinemic (~43 mU/l) clamps at a 3- to 5-wk interval, one with intravenous (IV) and the other with intraduodenal (ID) 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 plus or minus 0.37 vs. 5.43 plus or minus 0.43 mg kg-1 min-1; P = 0.02) and exogenous glucose uptake (5.66 plus or minus 0.37 vs. 5.26 plus or minus 0.41 mg kg-1 min-1; P = 0.04) were borderline higher in the ID than in the IV 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 (ID vs. IV: 2.44 plus or minus 0.28 vs. 2.40 plus or minus 0.31 mg kg-1 min-1) or as glucose uptake minus net glucose oxidation (2.86 plus or minus 0.33 vs. 2.81 plus or minus 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 ID tests, the glucose splanchnic extraction ratio calculated by the dual-isotope technique averaged 4.9 plus or minus 2.3%, which is close to the figures published for IV 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 IV glucose. [PUBLICATION ABSTRACT]