Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise

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Abstract
This study examined effects of ingesting a 10% carbohydrate (CHO) drink (CI) or placebo (PI) at 500 ml/h on total (splanchnic) glucose appearance (endogenous + exogenous; Ra), blood glucose oxidation, and muscle glycogen utilization in 14 male endurance-trained cyclists who rode for 180 min at 70% of maximal O-2, uptake after CHO loading [starting muscle glycogen 203 +/- 7 (SE) mmol/kg wet wt]. Total CHO oxidation was similar in CI and PI, but Ra increased significantly during the trial in both groups, with CI reaching a plateau after 75 min. Pa was significantly greater in CI than in PI at the end of exercise. Blood glucose oxidation also increased significantly during the trial to a plateau in CI and was significantly higher in CI than in PI at the end of exercise. However, mean endogenous Ra was significantly lower in CI than in PI throughout exercise, as was oxidation of endogenous blood glucose, which remained almost constant in CI and reached 43 +/- 8 and 73 +/- 13 mu mol.min(-1).kg fat-free mass(-1) in CI and PI, respectively, at the end of exercise. At 0.83 g/min of CHO ingestion, 0.77 +/- 0.03 g/min was oxidized. Muscle glycogen utilization was identical in both groups and was higher during the Ist h of exercise. We conclude that ingestion of a 10% CHO solution at 500 ml/h during prolonged exercise 1) had a marked liver glycogen-sparing effect and/or caused a reduction in gluconeogenesis that should ultimately extend exercise time to exhaustion by delaying onset of hypoglycemia, 2) increased blood glucose oxidation, 3) provided CHO at a rate closely matched by the rate of oxidation of ingested CHO, and 4) did not have a muscle glycogen-sparing effect.