Do You Need to Drink During a One-Hour Run?

New research revives debate on when dehydration begins to hurt performance.

I  just got back from a very sweaty run in hot and humid conditions, and downed several glasses of cold water before hopping in the shower. But, as is my habit, I didn’t drink anything during my run. Am I an idiot?

That question (whether you need to drink during short or medium-length runs, not whether I’m an idiot) has been debated for years now – and new research is still appearing. A couple of presentations on the topic at the American College of Sports Medicine annual meeting in 2016, from researchers at Elon University, North Carolina, USA, caught my attention.

Both presentations were based on the same study, which involved 23 female recreational runners, who ran two 15K time trials in moderate, humid conditions (20 degrees Celsius, 87 per cent humidity). In one of the trials, they drank 350ml of water during the run; in the other, they rinsed their mouths with water then spat it out every three kilometers.

One presentation summarised the physiological and perceptual differences between the two runs. In brief, there wasn’t much difference. The drinking group sweated more, by roughly the same amount as the water they drank. The mouth rinse group was thirstier after the run. But their run times were the same (79.8 versus 79.7 minutes), and so were their heart rates and sense of effort.

The second presentation looked at affective responses – things like pleasure, arousal, energy, calmness and so on – and found ‘similar affective responses’ during and after both runs, though the mouth rinse group reported more tension.

So what does this tell us? For these particular runners, under these particular conditions, it didn’t really seem to matter whether they drank or just wet their mouths during the run. If they didn’t drink during the run, they were more thirsty afterwards – like I was this morning – but it didn’t slow them or make them feel worse.

One conclusion is that pausing to wet your mouth, or maybe take a sip or two, when you pass a water fountain may be all you need on some runs. Or it may just be that as long as you make sure you don’t feel thirsty (whether by wetting your mouth, drinking a little, or doing nothing at all), you don’t have to worry about mild levels of dehydration.

Conversely, you could argue that the real problem in this study is that the subjects didn’t drink enough, because even when they drank water they had a net sweat loss of 1.2 litres from an average starting weight of 59kg, indicating roughly two per cent dehydration. But I’m not convinced.

A similar study published earlier in 2016 by researchers at the University of Connecticut in the US compared two other hydration protocols for runners doing a pair of 20K trail races in hot conditions. In one run, they drank ‘ad libitum’, meaning as much as they wanted; in the other run, they were assigned an individualised amount to drink in an attempt to replace as much of their sweat losses as possible.

As you’d expect, those who merely drank as much as they wanted lost more fluids: 2.6 per cent of their starting weight, compared to 1.3 per cent for the well-hydrated group. So what were the results of the race?

For context, I should mention that I appeared on a WNPR radio program with two of the co-authors of this study, Lawrence Armstrong and Rebecca Stearns. They were adamant that drinking when you’re thirsty is a recipe for disaster; that you have to drink early and drink often to avoid dehydration; and, perhaps most worryingly from a safety perspective, that your core temperature rises by about 0.1-0.2 degrees Celsius for every per cent of dehydration.

The results of their own study, on the other hand, found no differences in race performance: the hydrated runs averaged 1:44:39, while the ‘drink as much as you want’ runs averaged 1:44:09. Moreover, the subjects swallowed thermometer capsules to measure their core temperatures, and there was no difference at any point between the two groups, despite the difference in hydration status.

In the paper, the authors try hard to argue that the fully hydrated approach is still better than drinking when you feel like it. For example, they point out that at some of the intermediate points of the race, there was a non-significant trend for the hydrated group to be faster. Of course, basic logic tells us that if the two groups had the same overall time, and one group was faster at some points, then the other group must have been faster at other points.

The reason they’re so surprised not to see any difference is that their previous studies, like this Gatorade-funded one from 2010, suggested that dehydration really does slow you down during trail running. To understand why, take a look at this figure from the 2010 study:

Image courtesy of Journal of Athletic Training
Image courtesy of Journal of Athletic Training

The chart shows how thirsty the subjects were before, during and after a pair of 12K races, one of which they ran with no water and the other with two drinks of 400 ml. The dehydrated group (the squares and dashed line) had their fluid consumption restricted for 22 hours before the race (they were allowed 200 ml of water to help them swallow the thermometer pill with breakfast). As a result, they were more thirsty before the race even started than the other group ever got.

So yes, drinking aggressively during a run is important if you haven’t had anything to drink for the last 22 hours. No dispute there. But under the more realistic conditions of the 2016 study, drinking when and as much as you feel like during a 20K trail run in hot conditions seems to be fine – even in the hands of researchers who don’t believe it.

Moreover, all of these studies involved time trials or races. If you’re just out for a typical training run, it stands to reason that dehydration will be even less of an issue.

The overall conclusion here, I should emphasise, isn’t that hydration doesn’t matter. It’s important to health and to performance. When a runner like Haile Gebrselassie sets a marathon world record while losing 10 per cent of his body weight, it’s not because he’s not paying attention to hydration – it’s because that’s the best balance he’s able to strike between drinking and running comfortably.

The shift in thinking about hydration over the past few years has involved a greater recognition of perceptual variables – how you feel – in addition to physiological variables; a realisation that the body can adjust and cope with surprisingly high levels of temporary dehydration; and a move away from the idea that you should always aspire to replace every drop you sweat as soon as possible. Studies like the ones above suggest that this shift is a good one.


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