Effect of carbohydrate ingestion on exercise endurance and metabolism after a 1-day fast.

Fasting before an exercise event has been demonstrated to decrease endurance. The purpose of this study was to investigate whether this decrement in performance after fasting could be reversed by ingestion of a carbohydrate solution before and during exercise. Nine fit male subjects ran to exhaustion at approximately 70% VO2max in two counterbalanced trials. The subjects were fasted for 21 h before both trials, and the trials were arranged so that the subjects ingested either a carbohydrate (CHO) or placebo (PL) solution. Although ratings of perceived exertion were significantly lower in the CHO trial, there were no differences in endurance time to exhaustion in the two trials (102 +/- 8 min in the PL trial and 106 +/- 8 min in the CHO trial). There were no differences between trials for the VO2, heart rate, and blood lactate concentrations. As expected, the blood glucose and insulin concentrations were higher in the CHO trial. The respiratory exchange ratio was significantly higher in the CHO trial at 40 min of exercise and tended to be higher at all other times, suggesting a greater reliance on carbohydrate and less on fat as an energy source. This seemed to be confirmed by the significantly lower plasma glycerol concentration, which suggested less fat mobilization in the CHO trial. Ingestion of a glucose polymer solution increased carbohydrate utilization in fasted subjects, but exercise performance was not improved.