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

This study compared liver glucose turnover, blood glucose oxidation, and muscle glycogen utilization in 15 male endurance-trained cyclists who rode for 180 min at 70% of maximal O2 consumption in either a carbohydrate-(CHO) loaded (CL) or a non-CHO-loaded (NL) state. Total CHO oxidation during exercise was similar in the CL and NL subjects (492 +/- 77 vs. 448 +/- 43 g, respectively), as were blood glucose oxidation (103 +/- 19 vs. 99 +/- 7 g, respectively) and liver glucose appearance (110 +/- 15 vs. 127 +/- 16 g, respectively). However, total muscle glycogen utilization was greater in CL than NL subjects (134 +/- 11 vs. 95 +/- 12 mmol/kg wet wt; P < 0.05), the former of which had higher muscle glycogen content at the start (194 +/- 4 vs. 124 +/- 7 mmol/kg wet wt; P < 0.05) and throughout the trial. Whereas high rates of muscle glycogen breakdown were maintained throughout the trial in CL subjects, rates of muscle glycogenolysis in NL subjects decreased to 26 mmol.kg wet wt-1.h-1 after 60 min of exercise (P < 0.05) when their muscle glycogen content had declined to 70 mmol/kg wet wt. Comparable rates of blood glucose and overall CHO oxidation in CL and NL subjects, despite a slowing of muscle glycogenolysis in the NL group, could be explained by an accelerated breakdown of glycogen in the nonworking muscles to redistribute CHO (lactate) to the working muscles for oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)