Abstract
To determine if bypassing both intestinal absorption and hepatic glucose uptake by intravenous glucose infusion might increase the rate of muscle glucose oxidation above 1 g · min−1, ten endurance-trained subjects were studied during 125 min of cycling at 70% of peak oxygen uptake (VO2 peak). During exercise the subjects ingested either a 15 g · 100 ml−1 U-14C labelled glucose solution or H2O labelled with a U-14C glucose tracer for the determination of the rates of plasma glucose oxidation (Rox) and exogenous carbohydrate (CHO) oxidation from plasma14C glucose and14CO2 specific activities, and respiratory gas exchange. Simultaneously, 2-3H glucose was infused at a constant rate to measure glucose turnover, while unlabelled glucose (25% dextrose) was infused into those subjects not ingesting glucose to maintain plasma glucose concentration at 5 mmol · l−1. Despite similar plasma glucose concentrations [ingestion 5.3 (SEM 0.13) mmol · l−1; infusion 5.0 (0.09) mmol · l−1], compared to glucose infusion, CHO ingestion significantly increased plasma insulin concentrations [12.9 (1.0) vs 4.8 (0.5) mU · l−1;P<0.05], raised total Rox values [9.5 (1.2) vs 6.2 (0.7) mmol · 125 min−1 kg fat free mass−1 (FFM);P<0.05] and rates of CHO oxidation [37.2 (2.8)vs 24.1 (3.9) mmol · 125 min−1 kg FFM−1;P<0.05]. An increased reliance on CHO metabolism with CHO ingestion was associated with a decrease in fat oxidation. Whereas the contribution from fat oxidation to energy production increased to 51 (10)% with glucose infusion, it only reached 18 (4)% with glucose ingestion (P<0.05). Despite these differences in plasma insulin concentration and rates of fat oxidation, the rates of glucose oxidation by muscle were similar after 125 min of exercise for both trials [ingestion 93 (8); infusion 85 (5) μmol · min−1 kg FFM−1], suggesting that peak rates of muscle glucose oxidation were primarily dependent on blood glucose concentration which, in turn, regulated the hepatic appearance of ingested CHO.
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Hawley, J.A., Bosch, A.N., Weltan, S.M. et al. Effects of glucose ingestion or glucose infusion on fuel substrate kinetics during prolonged exercise. Europ. J. Appl. Physiol. 68, 381–389 (1994). https://doi.org/10.1007/BF00843733
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DOI: https://doi.org/10.1007/BF00843733