Dugas, J. P., Oosthuizen, U., Tucker, R., & Noakes, T. (2006). Drinking “ad libitum” optimizes performance and physiological function during 80 km indoor cycling trials in hot and humid conditions with appropriate convective cooling. Medicine and Science in Sports and Exercise, 38(5), Supplement abstract 1340.

The effects of graded levels of fluid replacement on an 80-km cycling time-trial performance in hot, humid conditions with appropriate facing wind speeds were evaluated. Highly-trained, non-acclimatized male cyclists (N = 6) performed six self-paced 80-km individual time-trials on an air-braked cycle ergometer. Ingested fluid volume was manipulated to produce six conditions in which subjects replaced 0%; 33%; 66%; or 100% of the weight lost during the familiarization trial (FT), in which subjects drank “ad libitum”. In the sixth condition (WET), subjects rinsed their mouths with similar volume of water that was ingested (66%) at 10 km intervals. Energy intake remained constant at 100 g CHO per trial.

Percent dehydration was significantly different between trials. There was no trial effect on time-trial performance time; peak-Tre, heat storage, or sweat rate. However, when trials with lower rates of fluid ingestion were compared to those with higher rates, time-trial performance was significantly faster in high compared to low replacement.

Implication. Replacing 33% or less of the weight lost during an 80-km cycling time trial in hot, humid environmental conditions was associated with an impaired performance compared to higher rates of fluid ingestion. However, this effect was not dose-dependant. Rates of fluid ingestion greater than ad libitum were not associated with further improvements in time-trial performance. Therefore, ad libitum fluid ingestion appears to be the best practice for competitive cyclists in the heat.

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