In the past few years, there’s been growing evidence that the conventional wisdom concerning dehydration and endurance performance is wrong. A new research review adds to that evidence, and provides more support for drinking to thirst as an adequate way to maintain performance in the heat.
For a long time we were told that performance starts to suffer after losing as little as two per cent of bodyweight to dehydration. For a 70kg runner, a two per cent loss represents 1.4L. Some people can sweat that much in an hour under not terribly oppressive conditions; in weather like the Aussie summer, you could sweat that much on a slow 8K.
The thing is, the decline-after-two-per-cent-loss formula came from observations made in exercise labs. In these studies, subjects maintain a fixed intensity throughout the test. Writing in the British Journal of Sports Medicine, Eric Goulet of McGill University in Montreal points out that studies in more real-world settings, like time trials, don’t produce this finding. Under those conditions, he notes, performance doesn’t seem to suffer with dehydration-induced losses of up to four per cent of bodyweight.
As Goulet notes, there are numerous cases of elite runners producing amazing performances while what would seem to be dangerously dehydrated. Tim Noakes, author of Lore of Running, makes the same point in his new book Waterlogged. For example, Haile Gebrselassie is reported as having lost close to 10 per cent of his bodyweight during the 2008 Berlin Marathon, where he became the first to break 2:04 for the distance.
“Drink to thirst during endurance exercise and you’ll be fine” isn’t a bad summary of Noakes’ 448-page tome on the topic. What that means in practical terms will vary by runner and the day’s weather, and over time you’ll get a general idea of how much works best for you. But don’t feel compelled to stash a water bottle around every corner.
