“If you wait until you’re thirsty to drink, it’s too late.”
That unambiguous adage has become standard advice for runners seeking to avoid dehydration. The thing is, it’s wrong.
A growing body of research supports the idea that you can avoid the performance-harming effects of dehydration simply by drinking when you’re thirsty, even on long runs in hot weather.
The latest pillar in the drink-to-thirst defence is a study, published in the European Journal of Applied Physiology, in which runners did a series of moderate-pace two-hour runs in different temperatures and with different hydration strategies. There were two key takeaways.
First, when 10 experienced runners (six men, four women) drank water whenever they wanted during three two-hour runs, they finished the runs in a similar state of adequate hydration, despite the runs occurring at different temperatures (71, 86, and 95 degrees Fahrenheit). Body weight loss ranged from 1.7 per cent to 1.9 per cent; a loss of more than 2 per cent because of dehydration is often cited as the threshold beyond which endurance performance starts to suffer. (More on that below.)
The runners’ internal measures of hydration status, such as plasma density, and their perceived exertion were similar among the three runs. In other words, drinking to thirst during each of these runs was an adequate hydration strategy, even in extreme conditions, when the runners had a much higher sweat rate.
A second takeaway was that following a set hydration plan during another series of two-hour runs didn’t improve markers such as heart rate and perceived effort, nor did doing so lead to the runners going farther in two hours in 90-degree heat. The set hydration plan of taking in a carbohydrate drink or whey protein drink every 20 minutes, plus as much water as desired, led to less dehydration at the end of these runs (body weight loss of .6 to .7 per cent). But that improved hydration status had no obvious benefit compared to when the runners drank to thirst in the first series of tests.
Of course, too much dehydration is bad because it can raise your core temperature to dangerous levels. Less extreme but still-significant dehydration will make it increasingly difficult to maintain a given pace—and could keep you from hitting that race goal.
And drinking to thirst doesn’t mean being lackadaisical. After all, it’s hard to drink to thirst if you don’t have access to drinks. Having a set hydration plan, such as drinking something at each aid station, can help in situations like a long race, when it can be difficult to drink as much as you want at a given time.
At the same time, lead study author Martin Hoffman, M.D., is among those experts who believe the oft-cited 2-per cent-dehydration threshold isn’t relevant in the real world. As Hoffman wrote in 2016, many lab studies finding performance decreases from greater than a 2 per cent loss have serious flaws. Those flaws include not allowing participants to drink before and during exercise and not providing a means of convective cooling (such as the air that moves over your skin when you run outside).
More nuanced studies have supported what has been observed anecdotally in individual runners: It’s possible to perform well at hydration losses of greater than 2 per cent. Haile Gebrselassie lost almost 10 per cent of his body weight while winning the 2009 Dubai Marathon. In one study of more than 600 runners at a 2009 marathon, the fastest lost the greatest amount of weight during the race; all of those who broke 3 hours lost more than 3 per cent.
One reason that it’s possible to perform at a high level despite seemingly inadequate fluid intake: When you run, your body releases water as carbohydrates and other energy sources are burned. (About three grams of water are stored with every gram of glycogen, the storage form of carbohydrate, a fact that helps explain pre-marathon carboloading bloating.) “This effect adds up to a situation where, under common exercise conditions, the water generated in this manner could replace around 10 per cent of the water lost in sweat,” Hoffman wrote in an email.
In Hoffman’s new study, there was no apparent benefit when the runners followed a set hydration plan and finished less dehydrated. But is there harm? Other than the rare but dangerous condition of hyponatremia, not really.
By Scott Douglas