But Joseph Costello, an exercise physiologist at the University of Portsmouth in England who has long studied the effects of heat and cold on athletes, was skeptical. He and his colleagues suspected that this so-called “permissive dehydration” would be stressful for athletes’ bodies, but not advantageous.

So for the new study, which was published recently in Cytokine, he and his colleagues had a group of eight, fit young men complete two different courses of heat acclimation.

Both heat acclimations took place inside, in a physiology lab with the thermometer cranked up unpleasantly high. There, the men rode stationary bicycles at increasingly higher intensities for 11 consecutive days.

On most of these days, they rode for 90 minutes, but on several days, the exercise was briefer and the lab’s temperature higher to test how well they were acclimating.

The scientists drew blood and weighed riders at the start of the study and before and after each ride.

The only difference between the two 11-day acclimation sessions (which took place three months apart) was that, during one, the men drank plenty of fluids before and during each ride, downing 1.75 liters (almost 60 ounces) in every workout.

In the other 11-day session, the cyclists permissively dehydrated, drinking no fluids before or during their exercise. They did drink afterward, swallowing the same 1.75 liters as in the other acclimation program and more if they still felt thirsty.