The effect of a breakfast's glycaemic index and type of hydration on metabolism and cycling performance: a crossover, randomized, controlled clinical trial

The purpose of this study was to compare the effect of the glycaemic index of breakfast on metabolic parameters and performance of cyclists with different types of hydration (water or isotonic beverage). Twelve male recreational cyclists participated in four experimental trials where they consumed either a high glycaemic index (HGI) or low glycaemic index (LGI) meal, 30 min before exercise on a cycle ergometer. Exercise was performed at 70% maximal oxygen uptake for 90 min followed by a 6 km performance. During each trial, 3 mL.kg(-1) body mass of either water or isotonic beverage was provided. The postprandial glycaemic response and areas under the blood glucose curve 30 min after ingestion were higher after the consumption of the HGI meals than that after the consumption of the LGI meals. The glycaemic response and carbohydrate oxidation during the trials with isotonic beverage consumption were higher than that in trials with water consumption during exercise (p<0.05). There was no significant difference on exercise performance among all trials (p=0.409). This study demonstrated that, despite significant metabolic changes, neither LGI nor HGI meals consumed for breakfast, 30 min before exercise on a cycle ergometer, affect subsequent cycling performance.