Reduced oxidation rates of ingested glucose during prolonged exercise with low endogenous CHO availability.

This study investigated the effect of endogenous carbohydrate (CHO) availability on oxidation rates of ingested glucose during moderate-intensity exercise. Seven well-trained cyclists performed two trials of 120 min of cycling exercise in random order at 57% maximal O2 consumption. Preexercise glycogen concentrations were manipulated by glycogen-lowering exercise in combination with CHO restriction [low-glycogen (LG) trial] or CHO loading [moderate-to-high-glycogen (HG) trial]. In the LG and HG trials, subjects ingested 4 ml/kg body wt of an 8% corn-derived glucose solution of high natural 13C abundance at the start, followed by boluses of 2 ml/kg every 15 min. The third trial, in which potato-derived glucose was ingested, served as a control test for background correction. Exogenous glucose oxidation rates were calculated from the 13C enrichment of the ingested glucose and of the breath CO2. Total CHO oxidation was lower in the LG trial than in the HG trial during 60-120 min of exercise [84 +/- 7 (SE) vs. 116 +/- 8 g; P < 0.05]. Exogenous CHO oxidation in this period was 28% lower in the LG trial compared with the HG trial. Maximal exogenous oxidation rates were also lower (P < 0.05) in the LG trial (0.64 +/- 0.05 g/min) than in the HG trial (0.88 +/- 0.04 g/min). This decreased utilization of exogenous glucose was accompanied by increased plasma free fatty acid levels (2-3 times higher) and lower insulin concentrations. It is concluded that glycogen-lowering exercise, performed the evening before an exercise bout, in combination with CHO restriction leads to a reduction of the oxidation rate of ingested glucose during moderate-intensity exercise.