Failure of fluid ingestion to improve self-paced exercise performance in moderate-to-warm humid environments

It has been previously proposed that fluid ingestion might enhance performance and thermoregulation through the heat storage capacity of the ingested fluid. While accurate under certain conditions, in some situations this cannot account for differences in thermoregulatory and performance responses. To test this hypothesis seven subjects performed a 60 min self-paced cycling time trial on four occasions, differentiated by ambient temperature (moderate 19.8±0.6°C, warm 33.2±0.2°C; 63.3±0.6% relative humidity) and fluid ingestion regime (no fluid, NF; or sufficient fluid, F, to prevent any change in body mass). No differences were observed for total distance cycled or final core temperature during exercise where for the moderate-NF, moderate-F, warm-NF and warm-F conditions were 32.6±6.4, 30.8±5.7, 30.5±4.8, 30.1±5.0 km and 38.9±0.3°C, 38.6±0.4°C, 38.9±0.5°C, 38.7±0.4°C, respectively. Furthermore, pacing strategy, as indicated by distance covered during maximal sprint and submaximal sections of the trial were similar among conditions. Although this result is not dissimilar to previous findings, the data show that complete fluid replacement during exercise of 1 h does not provide the proposed heat sink sufficient to attenuate thermoregulatory strain and improve performance over no fluid replacement. The findings indicate that the ingestion of fluids replacing 100% of sweat losses has no effect on 1 h of self-paced cycling performance or thermoregulation in moderate and warm conditions.