Mild Dehydration Won’t Slow You Down

Trust your sense of thirst during a run.

Alex Hutchinson |

You’ve heard the warning: By the time you notice you’re thirsty, you’re already (cue ominous music) dehydrated – and that will slow you down. So it’s worth considering the counterintuitive results of a 2009 experiment. French researchers weighed 643 runners before and after a marathon to estimate how much fluid they’d lost.

The fastest runners, it turned out, were the most dehydrated: Sub-3:00 finishers lost an average of 3.1 percent of their starting weight, those between 3:00 and 4:00 lost 2.5 percent, and those slower than 4:00 lost just 1.8 percent. This effect is even more pronounced at the elite level: When Haile Gebrselassie became the first sub-2:04 marathoner in 2008, he lost 10 percent of his starting weight – far more than the 2 percent loss that the American College of Sports Medicine says “degrades aerobic exercise.” So what explains this apparent contradiction?

The key is to understand the difference between dehydration, the physiological state of having lost fluid, and thirst, the desire to drink. For decades, researchers lumped the two together, conducting studies in which volunteers were denied water for hours before undergoing exercise tests. Those subjects were both thirsty and dehydrated – and their endurance suffered even with 2 percent dehydration.

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But as the familiar warning implies, it’s also possible to be dehydrated, at least temporarily, without feeling thirsty. Does this matter? In a 2016 study, athletes completed a 20K trail run while either drinking an amount chosen to replace their expected sweat losses or simply drinking when they felt like it – a plan that, in the latter case, left them dehydrated by 2.6 percent. The finishing times in the two conditions were essentially identical.

One theory is that the disconnect between dehydration and thirst isn’t an evolutionary bug – it’s a feature. As you sweat out water, you also sweat out electrolytes like sodium, which keeps your blood concentration relatively constant. That disconnect, the theory goes, allowed our ancestors to keep hunting without constantly needing to stop for water.

This rethink has two practical consequences: You can trust your sense of thirst during a run, but you have to repay that fluid debt after you finish – otherwise, the next day, you’ll have nothing left to borrow.

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