Ingestion of glucose or sucrose prevents liver but not muscle glycogen depletion during prolonged endurance-type exercise in trained cyclists

The purpose of this study was to define the effect of glucose ingestion compared with sucrose ingestion on liver and muscle glycogen depletion during prolonged endurance-type exercise. Fourteen cyclists completed two 3-h bouts of cycling at 50% of peak power output while ingesting either glucose or sucrose at a rate of 1.7 g/min (102 g/h). Four cyclists performed an additional third test for reference in which only water was consumed. We employed C-13 magnetic resonance spectroscopy to determine liver and muscle glycogen concentrations before and after exercise. Expired breath was sampled during exercise to estimate whole body substrate use. After glucose and sucrose ingestion, liver glycogen levels did not show a significant decline after exercise (from 325 +/- 168 to 345 +/- 205 and 321 +/- 177 to 348 +/- 170 mmol/l, respectively; P > 0.05), with no differences between treatments. Muscle glycogen concentrations declined (from 101 +/- 49 to 60 +/- 34 and 114 +/- 48 to 67 +/- 34 mmol/l, respectively; P <0.05), with no differences between treatments. Whole body carbohydrate utilization was greater with sucrose (2.03 +/- 0.43 g/min) vs. glucose (1.66 +/- 0.36 g/min; P <0.05) ingestion. Both liver (from 454 +/- 33 to 283 +/- 82 mmol/l; P <0.05) and muscle (from 111 +/- 46 to 67 +/- 31 mmol/l; P <0.01) glycogen concentrations declined during exercise when only water was ingested. Both glucose and sucrose ingestion prevent liver glycogen depletion during prolonged endurance-type exercise. Sucrose ingestion does not preserve liver glycogen concentrations more than glucose ingestion. However, sucrose ingestion does increase whole body carbohydrate utilization compared with glucose ingestion.