Liver and muscle glycogen oxidation and performance with dose variation of glucose-fructose ingestion during prolonged (3 h) exercise

Purpose This study investigated the effect of small manipulations in carbohydrate (CHO) dose on exogenous and endogenous (liver and muscle) fuel selection during exercise. Method Eleven trained males cycled in a double-blind randomised order on 4 occasions at 60% (V)over dot O-2max for 3 h, followed by a 30-min time-trial whilst ingesting either 80 g h(-1) or 90 g h(-1) or 100 g h(-1 13)C-glucose-C-13-fructose [2:1] or placebo. CHO doses met, were marginally lower, or above previously reported intestinal saturation for glucose-fructose (90 g h(-1)). Indirect calorimetry and stable mass isotope [C-13] techniques were utilised to determine fuel use. Result Time-trial performance was 86.5 to 93%, 'likely, probable' improved with 90 g h(-1) compared 80 and 100 g h(-1). Exogenous CHO oxidation in the final hour was 9.8-10.0% higher with 100 g h(-1) compared with 80 and 90 g h(-1) (ES = 0.64-0.70, 95% CI 9.6, 1.4 to 17.7 and 8.2, 2.1 to 18.6). However, increasing CHO dose (100 g h(-1)) increased muscle glycogen use (101.6 +/- 16.6 g, ES = 0.60, 16.1, 0.9 to 31.4) and its relative contribution to energy expenditure (5.6 +/- 8.4%, ES = 0.72, 5.6, 1.5 to 9.8 g) compared with 90 g h(-1). Absolute and relative muscle glycogen oxidation between 80 and 90 g h(-1) were similar (ES = 0.23 and 0.38) though a small absolute (85.4 +/- 29.3 g, 6.2, -23.5 to 11.1) and relative (34.9 +/- 9.1 g, -3.5, -9.6 to 2.6) reduction was seen in 90 g h(-1) compared with 100 g h(-1). Liver glycogen oxidation was not significantly different between conditions (ES < 0.42). Total fat oxidation during the 3-h ride was similar in CHO conditions (ES < 0.28) but suppressed compared with placebo (ES = 1.05-1.51). Conclusion 'Overdosing' intestinal transport for glucose-fructose appears to increase muscle glycogen reliance and negatively impact subsequent TT performance.