Overnight fasting compromises exercise intensity and volume during sprint interval training but improves high-intensity aerobic endurance

BACKGROUND: The combined effects of sprint interval training (SIT) and exercising in the fasted state are unknown. We compared the effects of SIT with exogenous carbohydrate supplementation (SITCHO) and SIT following overnight fast (SITFast) on aerobic capacity (peak oxygen consumption: (V) over dotO(2peak)) and high-intensity aerobic endurance (time-to-exhaustion at 85% (V) over dotO(2peak) [T85%]). METHODS: Twenty male cyclists were randomized to SITCHO and SITFast. Both groups performed 30-second all-out cycling followed by 4-minute active recovery 3 times per week for 4 weeks, with the number of sprint bouts progressing from 4 to 7. Peak power output (PPO) and total mechanical work were measured for each sprint interval bout. The SITCHO group performed exercise sessions following breakfast and consumed carbohydrate drink during exercise, whereas the SITFast group performed exercise sessions following overnight fast and consumed water during exercise. Before and after training, (V) over dotO(2peak) and T-85% were assessed. Blood glucose, non-esterified fatty acids, insulin and glucagon concentrations were measured during T85%. RESULTS: Overall PPO and mechanical work were lower in SITFast than SITCHO (3664.9 vs. 3871.7 J/kg; P= 0.021 and 10.6 vs. 9.9 W/kg; P= 0.010, respectively). Post-training (V) over dotO(2peak) did not differ between groups. Baseline-adjusted post-training T-85% was longer in SITFast compared to SITCHO (19.7 +/- 3.0 vs. 16.6 +/- 3.0 minutes, ANCOVA P= 0.038) despite no changes in circulating energy substrates or hormones. CONCLUSIONS: Our results suggest that SITFast compromises exercise intensity and volume but still can have a greater impact on the ability to sustain high-intensity aerobic endurance exercise compared to SITCHO.