In the first trial, they drank as much as they wanted; in the other five, they were assigned varying levels of hydration ranging from nothing to enough to fully replace all their sweat losses. Sure enough, being hydrated improved performance: in the three trials where the cyclists were forced to drink less than they’d chosen to in the first trial, they were slower than the three higher-hydration trials. But there was no further improvement when they drank more than they had chosen to in the first trial. Avoiding thirst, rather than avoiding dehydration, seems to be the most important key to
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