Author(s): MAO, W., Institution: LOUGHBOROUGH UNIVERSITY, Country: UNITED KINGDOM, Abstract-ID: 1594

INTRODUCTION:
Morning high-intensity interval training (HIIT) sessions are often performed after an overnight fast, yet there is conflicting evidence on whether pre-exercise carbohydrate feeding improves HIIT performance, substrate utilisation and perceptions of effort. Existing studies are limited by open-label designs, narrow exercise tasks and inadequate expectancy control. This study examined whether a carbohydrate breakfast alters HIIT performance, acute physiological responses, and perceived exertion using an ecologically valid, expectancy-controlled protocol.
METHODS:
Twenty-seven trained male participants (age: 23 ± 3 years; VO2peak: 54.2 ± 5.5 mL/kg/min) with endurance or team-sport backgrounds completed two main trials in a randomised, double-blind, crossover design. After an overnight fast, participants consumed an individualised semi-solid breakfast 90 min pre-exercise in each main trial: carbohydrate (maltodextrin 1.5 g/kg; 600 mL) or a visually identical non-caloric placebo. The main trial comprised six 4-min treadmill intervals separated by 2 min of low-intensity jogging. Intervals 1-3 were performed at 85% vVO2peak. Intervals 4-6 began at 85% vVO2peak and then became self-paced, with participants encouraged to maximise distance covered. Total distance across intervals 4-6 was the primary performance outcome. Gas exchange, rating of perceived exertion (RPE), blood lactate and blood glucose, and post-session perceptions were assessed. Outcomes were analysed using paired t-tests and 2-way repeated-measures ANOVA.
RESULTS:
Total distance covered in intervals 4-6 was numerically greater after the carbohydrate breakfast than after placebo (2658 ± 324 vs 2622 ± 311 m), but this difference was not statistically significant (p = 0.115, Cohen's d = 0.31). During intervals 1-3, carbohydrate increased the respiratory exchange ratio (RER) and carbohydrate oxidation (CHOox) and reduced fat oxidation (Fatox) versus placebo (condition main effects, all p < 0.001). Resting blood lactate and blood glucose did not differ by condition (p = 0.801 and p = 0.848). Across intervals, blood lactate and blood glucose increased (both p < 0.001) without significant condition effects (p = 0.341 and p = 0.070), and RPE increased across intervals (p < 0.001) with no condition effect (p = 0.236). Sessional RPE, enjoyment, and likelihood to repeat did not differ between conditions.
CONCLUSION:
A carbohydrate-rich breakfast consumed 90 minutes before morning HIIT significantly altered substrate utilisation, shifting metabolism towards greater CHOox and reduced Fatox, but did not produce statistically significant improvements in HIIT performance or perceived exertion compared with an energy-free placebo. These findings suggest that, during an approximately 30-min morning HIIT session after an overnight fast, pre-exercise carbohydrate intake altered substrate utilisation without a clear improvement in brief HIIT performance.