Do we need to drink as much during exercise as we have been told?
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A strenuous workout should be accompanied by frequent water breaks, right? Not so fast, says Brock University physiologist Stephen Cheung.

While that certainly is the received wisdom, Cheung points out that top-performing athletes almost always speed past water stops in an effort to shave seconds off their time.

“Elite marathon runners barely touch water, even on a hot day,” he says. “And they lose more body weight through sweat (than others) and have higher core temperatures at the end of a race.”

In the mid-1990s, the American College of Sports Medicine recommended at least the full replacement of sweat loss during exercise. In 2007, it revised its guidelines to 400 to 800 millilitres of water per hour, and recommended dehydration be kept to less than 2 per cent of body weight for health and performance.

Yet when Cheung looked more closely at the studies backing these guidelines, he found a crucial flaw: they didn’t distinguish between being dehydrated and being thirsty. Instead, studies denied participants water while exercising in the heat, making them hyper-aware of their lack of hydration.

“Doing that changes not only your physical hydration status, but your psychology,” says Cheung, a competitive cyclist. “You are setting up a mental template for how hard you’re going to cycle or run.”

So Cheung set up his own study, putting 11 competitive cyclists and triathletes through a 90-minute bike ride at half their aerobic capacity, followed by a 20 kilometre time trial. All participants had an IV in their arms, but only some received saline to replace lost sweat. The saline was warmed to body temperature so they couldn’t tell. All participants also sweated out at least 2 per cent of their body weight before the time trial.

Crucially, neither the participants nor the researchers knew if they were being hydrated. A paramedic tasked with monitoring their vital signs was the only one who knew, and he remained carefully hidden behind a curtain so as not to influence the outcome.

The results? There was no difference in performance between those receiving saline rehydration and those receiving nothing.

And there was no difference when Cheung further divided the group into those who were hydrated and not thirsty, hydrated but thirsty, dehydrated and thirsty and dehydrated but not thirsty.

The takeaway message, says Cheung, is that losing 2 to 3 per cent body weight due to dehydration after two hours of exercise does not impair performance.

“So drink according to your thirst, but don’t obsess about drinking or worry that you can’t perform at your best without fluids,” he says.

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