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Drinking “ad libitum” Optimises Performance and Physiological Function During 80 km Indoor Cycling Trials in Hot and Humid Conditions with Appropriate Convective Cooling: 1340Board #4

Dugas, Jonathan P.; Oosthuizen, Ursula; Tucker, Ross; Noakes, Timothy FACSM

Medicine & Science in Sports & Exercise: May 2006 - Volume 38 - Issue 5 - p S176
Thematic Poster Session Format: First 30 minutes of session – View posters: Remaining 90 minutes of session – Chair leads discussion: D-58 Thematic Poster – Hypohydration and Performance: THURSDAY, JUNE 1, 2006 3:15 PM – 5:15 PM ROOM: 101

University of Cape Town, Cape Town, South Africa.

Email: jdugas@sports.uct.ac.za

PURPOSE: To examine the effects of graded levels of fluid replacement on 80 km cycling time trial performance in hot, humid conditions with appropriate facing wind speeds (33 °C; 50% relative humidity; 38.5 ± 1.0 km/h wind speed

METHODS: Six highly-trained, non-acclimatized male cyclists (PPO = 426 ± 39 W) 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 familiarisation 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 in 66 at 10 km intervals. Energy intake remained constant at 100 g CHO per trial.

RESULTS: Percent dehydration was significantly different between trials (FT, 2.1 ±0.6%; 0,4.3 ±0.4%; 33, 2.9 ±0.4%; 66, 1.9 ±0.3%; 100, 0.5 ± 0.3%; WET, 3.9 ± 0.2%; p <0.05, trial effect). There was no trial effect on time-trial performance time (FT, 124.2±5.8;0, 128.3 ± 6.3; 33, 129.9 ± 6.1; 66, 126.1 ±4.8; 100, 125.4±5.8; WET, 129.4 ±8.1 min); peak-Tre, heat storage (Qs), or sweat rate. However, when trials with lower rates of fluid ingestion (WET, 0, 33: LO) were compared to those with higher rates (FT; 66; 100: HI), time-trial performance was significantly faster in HI compared to LO (125.23 ±5.23 vs. 129.16 ±6.51 min, p <0.05).

CONCLUSIONS: 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.

© 2006 American College of Sports Medicine