Purpose: This study determined the effect of ingesting galactose and glucose 30 min before exercise on exogenous and endogenous fuel use during exercise. Methods: Nine trained male cyclists completed three bouts of cycling at 60% Wmax for 120 min after an overnight fast. Thirty minutes before exercise, the cyclists ingested a fluid formulation containing placebo, 75 g of galactose (Gal), or 75 g of glucose (Glu) to which 13C tracers had been added, in a double-blind randomized manner. Indirect calorimetry and isotope ratio mass spectrometry were used to calculate fat oxidation, total carbohydrate (CHO) oxidation, exogenous CHO oxidation, plasma glucose oxidation, and endogenous liver and muscle CHO oxidation rates. Results: Peak exogenous CHO oxidation was significantly higher after Glu (0.68 ± 0.08 g/min, P < 0.05) compared with Gal (0.44 ± 0.02 g/min); however, mean rates were not significantly different (0.40 ± 0.03 vs. 0.36 ± 0.02 g/min, respectively). Glu produced significantly higher exogenous CHO oxidation rates during the initial hour of exercise (P < 0.01), whereas glucose rates derived from Gal were significantly higher during the last hour (P < 0.01). Plasma glucose and liver glucose oxidation at 60 min of exercise were significantly higher for Glu (1.07 ± 0.1 g·min-1, P < 0.05, and 0.57 ± 0.08 g·min-1, P < 0.01) compared with Gal (0.64 ± 0.05 and 0.29 ± 0.03 g/min, respectively). There were no significant differences in total CHO, whole body endogenous CHO, muscle glycogen, or fat oxidation between conditions. Conclusion: The preexercise consumption of Glu provides a higher exogenous source of CHO during the initial stages of exercise, but Gal provides the predominant exogenous source of fuel during the latter stages of exercise and reduces the reliance on liver glucose.
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