The effect of a high carbohydrate diet on running performance during a 30-km treadmill time trial

Summary The purpose of the present study was to examine the influence of a high carbohydrate diet on running performances during a 30-km treadmill time trial. Eighteen runners (12 men and 6 women) took part in this study and completed a 30-km time trial on a level treadmill without modifying their food intake (trial 1). The runners were then randomly assigned to a control or a carbohydrate (CHO) group. The CHO group supplemented their normal diets with additional carbohydrate in the form of confectionery products during the 7 days before trial 2; the control group matched the increased energy intake of the CHO group by consuming additional fat and protein. The mean (SEM) carbohydrate intake of both groups was 334 (22) g before trial 1, after which the CHO group consumed 566 (29) g · day^–1 for the first 3 days and 452 (26) g · day^–1 for the remaining 4 days of recovery. Although there was no overall difference between the performance times for the two groups during trial 2, the CHO group ran faster during the last 5 km of trial 2 than during trial^–1 [3.64 (0.24) m · s^–1 vs 3.44 (0.26) m · s^–1P < 0.05] . Furthermore, the 6 men in the CHO group ran the 30 km faster after carbohydrate loading [131.0 (5.4) min vs 127.4 (4.9) min;P < 0.05], whereas there was no such improvement in times of the men in the control group. Blood glucose concentrations of both groups decreased below pre-exercise values during trial^–1 (P < 0.001), but only the control group had a decrease in blood glucose concentrations during trial 2 (P < 0.001). There were no differences between the concentrations of plasma catecholamines of the control group during the two trials. However, the adrenaline concentrations of the CHO group were lower (P < 0.05) during trial 2 than during trial 1, even though they ran faster during trial 2. These results confirm that dietary carbohydrate loading improves endurance performance during prolonged running and that confectionery can be used as an effective means of supplementing the normal carbohydrate intake in preparation for endurance races.