Fuel substrate turnover and oxidation and glycogen sparing with carbohydrate ingestion in non-carbohydrate-loaded cyclists

Oct 1, 1996Pflugers Archiv : European journal of physiology

Fuel use, burning, and glycogen saving with carbohydrate intake in cyclists without extra carbs

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Abstract

Ingestion of 500 ml/h of a 10% carbohydrate drink during exercise resulted in a marked liver glycogen-sparing effect.

Splanchnic glucose appearance rate significantly increased during the trial in both carbohydrate and placebo groups.
Plasma glucose oxidation was significantly higher in the carbohydrate group compared to the placebo group at the end of exercise.
Mean endogenous glucose appearance rate was significantly lower in the carbohydrate group throughout the exercise.
No net muscle glycogen disappearance occurred during the final hour in the carbohydrate group, while it continued in the placebo group.
Approximately 50% of the ingested carbohydrate was oxidized during the trial.

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This study examined the effects of ingesting 500 ml/h of either a 10% carbohydrate (CHO) drink (CI) or placebo (PI) on splanchnic glucose appearance rate (endogenous + exogenous) (Ra), plasma glucose oxidation and muscle glycogen utilisation in 17, non-carbohydrate-loaded, male, endurance-trained cyclists who rode for 180 min at 70% of maximum oxygen uptake. Mean muscle glycogen content at the start of exercise was 130 +/- 6 mmol/kg ww; (mean +/- SEM). Total CHO oxidation was similar in CI and PI subjects and declined during the trial. Ra increased significantly during the trial (P < 0.05) in both groups. Plasma glucose oxidation also increased significantly during the trial, reaching a plateau in the PI subjects, but was significantly (P < 0.05) higher in CI than PI subjects at the end of exercise [(98 +/- 14 vs. 72 +/- 10 micromol/min/kg fat-free mass) (FFM) (1.34 +/- 0.19 vs. 0.93 +/- 0. 13 g/min)]. However, mean endogenous Ra was significantly (P < 0.05) lower in the CI than PI subjects throughout exercise (35 +/- 7 vs. 54 +/- 6 micromol/min/kg FFM), as was the oxidation of endogenous plasma glucose, which remained almost constant in CI subjects, and reached values at the end of exercise of 42 +/- 13 and 72 +/- 10 micromol/min/kg FFM in the CI and PI groups respectively. Of the 150 g CHO ingested during the trial, 50% was oxidised. Muscle glycogen disappearance was identical during the first 2 h of exercise in both groups and continued at the same rate in PI subjects, however no net muscle glycogen disappearance occurred during the final hour in CI subjects. We conclude that ingestion of 500 ml/h of a 10% CHO solution during prolonged exercise in non carbohydrate loaded subjects has a marked liver glycogen-sparing effect or causes a reduction in gluconeogenesis, or both, maintains plasma glucose concentration and has a muscle glycogen-sparing effect.

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Fuel substrate turnover and oxidation and glycogen sparing with carbohydrate ingestion in non-carbohydrate-loaded cyclists

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