Drinking ad libitum during exercise often leads to dehydration ranging from -1% to -3% of body weight.
Purpose: This article aimed to study the effect of a prescribed hydration protocol matching fluid losses on a simulated 30-km criterium-like cycling performance in the heat (31.6°C ± 0.5°C).
Methods: Ten elite heat-acclimatized male endurance cyclists (30 ± 5 yr, 76.5 ± 7.2 kg, 1.81 ± 0.07 m, V˙O2peak = 61.3 ± 5.2 mL·min·kg, body fat = 10.5% ± 3.3%, Powermax = 392 ± 33 W) performed three sets of criterium-like cycling, which consisted of a 5-km cycling at 50% power max followed by a 5-km cycling all out at 3% grade (total 30 km). Participants rode the course on two separate occasions and in a counterbalanced order, during either ad libitum drinking (AD; drink water as much as they wished) or prescribed drinking (PD; drink water every 1 km to much fluid losses). To design the fluid intake during PD, participants performed a familiarization trial to calculate fluid losses.
Results: After the exercise protocol, the cyclist dehydrated by -0.5% ± 0.3% and -1.8% ± 0.7% of their body weight for the PD and AD trial, respectively. The mean cycling speed for the third bout of the 5-km hill cycling was greater in the PD trial (30.2 ± 2.4 km·h) compared with the AD trial (28.8 ± 2.6 km·h) by 5.1% ± 4.8% (P < 0.05). Gastrointestinal, mean skin, and mean body temperatures immediately after the last hill climbing were greater in the AD compared with the PD trial (P < 0.05). Overall, sweat sensitivity during the three climbing bouts was lower in the AD (15.6 ± 5.7 g·W·m) compared with the PD trial (22.8 ± 3.4 g·W·m, P < 0.05).
Conclusion: The data suggested that PD to match fluid losses during exercise in the heat provided a performance advantage because of lower thermoregulatory strain and greater sweating responses.