PURPOSE: To investigate the effect of three CHO doses on fuel selection during exercise, in particular exogenous and endogenous (liver and muscle) CHO oxidation.
METHODS: Eleven trained male cyclists (V[Combining Dot Above]O2max: 60.0 ± 4.3 ml/kg/min) cycled on 4 occasions at 60% V[Combining Dot Above]O2max for 3 hours after an overnight fast. From 15 min into exercise and every 15 min thereafter, either 80 g.h-1 (LOW), 90 g.h-1 (MED), 100 g.h-1 (HIGH) of GF (all 2:1 ratio) or a placebo (PLA) was ingested in a double-blind randomised order. The formulations contained 13C isotope tracers and were designed to span the reported saturation levels for SGLT1 and GLUT5. Total, exogenous, endogenous (muscle and liver) CHO oxidation, and total fat oxidation were computed using indirect calorimetry and isotope ratio mass spectrometry.
RESULTS: Total CHO oxidation was elevated, and total fat oxidation suppressed in all CHO conditions relative to PLA (CHO range 94.0-125.0 g higher, Fat 29.7-34.3 g lower: both ES > 1.05). Exogenous oxidation in the final hour was greatest in HIGH (91.1 g.h-1), a moderate effect to LOW (81.6 g.h-1, ES = 0.64, P = 0.10) and MED (82.9 g.h-1, ES = 0.70, P = 0.39) a moderate increase from the second hour in all conditions (ES = 1.38-2.00, P < 0.014). However, increasing GF dose beyond intestinal saturation increased muscle glycogen utilisation in the final hour (101.6 ± 16.6 g.h-1 in HIGH; 6.2, -23.5 to 11.1 g.h-1 higher [95% CI] vs. LOW, ES = 0.47, P = 0.61 & 16.1, 0.9 to 31.4 g.h-1 [95% CI]) higher vs. MED, ES = 0.68, P = 0.16), and second hour (ES = 0.51 & 0.48, P > 0.05). A small, non-significant reduction was seen in liver glycogen oxidation with HIGH in the last hour compared with LOW (-2.6, -5.6 to 0.4 g.h-1, ES = 0.40) and MED (-2.6, -6.8 to 1.6 g.h-1, ES = 0.42).
CONCLUSIONS: Increasing CHO ingestion beyond previously reported saturation rates produces higher exogenous oxidation, but results in an increased reliance on muscle glycogen. Ingestion of 90 g.h-1 GF can attenuate the rate of muscle glycogen oxidation by the end of 3 hours prolonged exercise, but recommendations should remain in the region of 80 to 90 g.h-1.