• It’s well-known that dehydration makes holding a given pace more difficult.
  • But does dehydration make running a given pace feel harder?
  • Not necessarily, according to a new review of research on the topic.

Summer means hot, sweaty runs that feel tougher than the same pace in moderate weather. Heat is the clear culprit, but dehydration is also often blamed. Yet the effect of dehydration on how our runs feel hasn’t been systematically explored. . . until now.

So, does dehydration contribute to that lead-legged feeling during midsummer runs? According to a recent study in Journal of Exercise Science & Fitness, it’s unlikely.

Dehydration 101

Physiologically, dehydration all comes down to blood.

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During exercise, your finite blood supply needs to deliver nutrients and oxygen to two places: your heart and your working muscles. When you get hot—due to the heat generated by your muscles, your environment, or both—two things happen. First, rather than being split just two ways, your blood supply is split a third way: It’s diverted to your skin to cool you off. Second, the way you cool off is by sweating, and your sweat comes from the liquid element of your blood (plasma). Therefore, as you sweat, your blood supply shrinks.

Now you have a decreasing amount of blood trying to service your heart, muscles, and skin. This creates stress. The question of whether this stress becomes perceptible enough to impact running performance is what Éric Goulet, Ph.D., the first author of the new study and director of the Performance, Hydration and Thermoregulation Laboratory at the University of Sherbrooke, and colleagues set out to investigate.

“Researchers believe that there is a strong relationship between perceived exertion and endurance performance,” Goulet said. “A lot of factors can influence perceived exertion; scientists believe that dehydration can be one of them. So we wanted to test that.”

Dehydration and perceived exertion

For their study, Goulet and his team analyzed 16 exercise science studies that included two conditions: one where participants were permitted to drink during exercise, and another where they weren’t. The researchers looked at how dehydrated participants became in either condition. When they were permitted to rehydrate, the participants lost, on average, 0.5 percent of their body mass; when they weren’t, they lost 3 percent. The scientists then examined whether higher levels of dehydration correlated with higher levels of perceived exertion—that is, when the athletes were more dehydrated, did they report that exercise felt harder?

The answer is yes, but barely. Goulet and his colleagues found that when the athletes were most dehydrated and rated their exertion on the 6–20-point Borg Scale (6 being little or no effort and 20 being the hardest possible effort), dehydration impacted their rating by one point—a statistically significant finding, except for the fact that the scale has a one-point margin of error. (Think about trying to rate your last run on the 6–20 scale. Does a rating of 16 or 17 seem all that different?)

“Our conclusion was that if dehydration plays a role in increasing perceived exertion, then the effect is probably really small,” Goulet said. “We concluded that, in the end, the impact of dehydration on perceived exertion should not be strong enough to have an influence on performance.”

Body temperature matters

Now, this doesn’t mean you should promptly ditch your water bottle, especially if you’re going to be running in the heat for a long time. That’s because Goulet’s study specifically measured the impact of dehydration alone on perceived exertion.

“Hydration really becomes vital during intense exercise in the heat,” said Douglas Casa, Ph.D., professor of kinesiology and CEO of the Korey Stringer Institute at the University of Connecticut. “Take any one of them—intensity, exercise, or heat—out of the mix, and hydration is not as important. But it’s very important when all three of those are combined.”

The reason has to do with core body temperature. On average, our bodies like to maintain a core temperature of 98.6 degrees Fahrenheit. Running raises that temperature, and running in the heat does so even more.

“We call it exercise-induced hyperthermia,” Casa said. “At the end of a hot-weather run, everyone is living in the 103 range. That’s normal.”

According to Casa, your core body temperature rises by half a degree Fahrenheit for every 1 percent body mass you lose to sweat. So, if you were to go on a 90-minute summer run and stayed well hydrated, you might finish with a core temperature around 102 degrees Fahrenheit. But if you finished having sweated out 4 percent of your body mass, you could easily be above 104 degrees Fahrenheit—the danger zone for heat stroke.

How to hydrate

While hydrating might not make your runs feel much easier, you can help keep your core temperature down during intense workouts and summer long runs with these practices.

Pay attention to thirst. Contrary to the common maxim that “once you’re thirsty, it’s too late,” Goulet said that you can reliably use thirst to guide your hydration.

“Thirst is a powerful mechanism to maintain body water,” he said. “If you rely only on that, it should be enough to perform optimally and to protect your health.”

He cautions, however, that you must have access to water for this to work. Otherwise, planning when and how much you’ll drink may be a better strategy.

Don’t forget electrolytes. Water may quench your thirst, but it won’t rehydrate you as well as a beverage that includes electrolytes. The reason is that when you sweat, you’re essentially losing the liquid part of your blood, or blood plasma.

“The main electrolyte in blood plasma is sodium, which is why we lose so much sodium with sweat,” said Nina Sieh, an endurance coach and Ph.D. student studying environmental ergonomics. “So that’s why we need to hydrate with electrolytes.”

Typical sport drinks such as Gatorade and Powerade are good options if you want carbohydrates along with your water and electrolytes, but in hot conditions where you’re drinking a lot of fluid, a less carb-concentrated option like Scratch or Nuun may sit better in your stomach.

Consider your purpose. When advising on hydration, Sieh tells her athletes to be mindful of hot conditions, but to also factor in the purpose of the run.

“If it’s a recovery run and their recovery strategy is pretty good—they’ve had good meals, good hydration throughout the day—and the run is, like, 60 minutes in the heat, they can probably handle that, no problem,” she said. It might not feel like no problem, but that’s part of training.

“A lot of endurance stuff is about that conversation you have with yourself in your head,” she said. “You want to get mentally prepared to tolerate the heat.”

At the end of the day, dehydration alone probably isn’t making your runs feel harder. Even if it is, that extra little bit of perceived exertion is probably okay on an easy run. For hotter, harder, longer workouts, drink when you’re thirsty, drink enough, and include electrolytes.

Then, wring out your shirt and keep on running.

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