Some athletes have begun to eschew fluids during hot weather workouts, in hopes that the privation might somehow make them stronger. A new study suggests it doesn’t.

Don’t skip drinking during exercise in hot weather, a new study reminds us.

This advice might seem obvious. But apparently some athletes, especially in team sports, have begun to eschew fluids during hot weather workouts, in hopes that the privation might somehow make them stronger.

But the new study finds that it is likely only to make them more physically stressed. And very, very thirsty.

Working out in the heat is inherently difficult, as any of us who exercise outside in summer knows. When ambient temperatures are high, we generate internal heat more quickly than if the weather is cool.

To remove this heat and maintain a safe body temperature, our hearts pump warm blood toward the skin’s surface, where heat can dissipate, and we sweat copiously, providing evaporative heat loss.

These reactions become more pronounced and effective with practice, a process known as heat acclimation (also referred to as acclimatization). During heat acclimation, which can require several weeks of sultry exercise, we begin to sweat earlier and in greater volume.

This and other changes help our hearts to labor less, so that, in general, the effort of being physically active in high temperatures starts to feel less wearing. A run on a sizzling summer day in August should feel easier than a similar run on an equally hot evening in June, if we have been running outside in the meantime, because our bodies will have acclimated to the heat.

But athletes being athletes, some of them and their coaches began to wonder in recent years whether, if heat acclimation taxes the body and makes it stronger, would exacerbating the physical difficulties of acclimation lead to greater adaptations, in approved Machiavellian style?

They settled on not drinking fluids during exercise in the heat as the way to increase efforts, since mild dehydration is known to be hard on the body.

They had some evidence to back up that plan. At least one study, from 2014 had suggested that athletes adjusted more quickly to the heat if they avoided fluids during the first days of acclimation.

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.

The scientists then checked a variety of blood markers of stress, inflammation and heat acclimation.

They found that markers of inflammation mounted in the men after rides in each session, suggesting that the process of heat acclimation strains the body, whether people hydrate while exercising or not.

But, more interesting, they also found that levels of cortisol, a hormone that indicates physiological stress, rose only when the men acclimated without also hydrating.

The riders also reported feeling quite thirsty.

There were no signification differences in how quickly or fully the men’s bodies acclimated to exercising in the heat, whether they drank often while riding or drank nothing.

So, in effect, not drinking during hot workouts had caused “more physical stress but no particular physical benefit,” such as faster acclimation, Dr. Costello says.

The extra stress might also contribute to heightened risks for infections like colds or general fatigue, he says.

Of course, this study was small and involved only men, all of whom were young and in shape.

“Women and older people thermoregulate, or control their body temperature, slightly differently than young men,” Dr. Costello says.

But there is no reason to think that anyone is likely to benefit from deliberately desiccating themselves during workouts, he says.

“Have a drink before you exercise in the heat,” he says, and carry and consume fluids throughout the activity.

“I would suggest that everyone try to stay hydrated when performing in the heat,” he says.

By Gretchen Reynolds

Aug. 8, 2018

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