The effect of pre-exercise glucose ingestion on performance during prolonged swimming

The purpose of this study was to determine if pre-exercise glucose ingestion would improve distance swimming performance. Additionally, pre-exercise glucose was provided at 2 different feeding intervals to investigate the affects of the timing of administration. Ten male triathletes (mean +/- SD: age, 29.5 +/- 5.0 years; VO2peak, 48.8 +/- 3.2 ml.kg-1.min-1) swam 4000 m on 3 occasions following the consumption of either a 10% glucose solution 5 min prior to exercise (G5), a 10% glucose solution 35 min prior to exercise (G35), or a similar volume of placebo (PL). Despite a significant difference (p < .01) in blood glucose concentration prior to exercise (mean +/- SD in mmol.L-1: G35 8.4 +/- 1.1 vs. G5 5.2 +/- 0.5 or PL 5.3 +/- 0.4), no significant differences were observed in total time (mean +/- SD in minutes: G35 70.7 +/- 7.6, G5 70.1 +/- 7.6, PL 71.9 +/- 8.4), post-exercise blood glucose (mean +/- SD in mmol.L-1: G35 5.1 +/- 1.1, G5 5.1 +/- 0.9, PL 5.3 +/- 0.4), and average heart rate (mean +/- SD in bpm: G35 155.8 +/- 10.8, G5 153.6 +/- 12.6, PL 152.0 +/- 12.5; p > .05). While not reaching statistical significance, glucose feedings did result in improved individual performance times, ranging from 24 s to 5 min in 8 of the 10 subjects compared to the placebo. These results were found despite significant differences in blood glucose between trials immediately prior to exercise.