THANK YOU FOR VISITING SWEATSCIENCE.COM!
My new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Also check out my new book, THE EXPLORER'S GENE: Why We Seek Big Challenges, New Flavors, and the Blank Spots on the Map, published in March 2025.
- Alex Hutchinson (@sweatscience)
***
The “right” amount of hydration during exercise is a hot topic these days. For years, lab studies showed that if you forcibly dehydrate someone and then stick them on a treadmill, their performance suffers. But more recently, “real-world” studies (like the one I blogged about here) have shown that in many cases, the fastest finishers in races tend to be the most dehydrated. The difference, according to researchers like Tim Noakes, is that in the real world your brain is able to make pacing decisions that keep your body in a safe state (using the thirst mechanism); on a treadmill at a fixed pace in a lab, your brain is cut out of the loop.
The result is that there are two bodies of research — lab and field — that appear to be answering the same question (how much should we drink?) but that produce completely different answers. So it’s nice to see a study from the “lab” camp that tries to bridge this gap by doing some experiments on an outdoor trail run. Researchers from the University of Connecticut had 14 runners perform two 12K trails runs, one in a hydrated state and the other in a dehydrated state. Here’s what they found:
Pretty straightforward, right? The runners were slower when they were dehydrated. As expected. Case closed. And we should disregard those inconvenient studies that found that faster runners in real races are more dehydrated, according to the researchers:
Although some field studies have found runners to be extremely successful despite considerable body fluid losses, these runners were not compared with a control condition where these same runners remained more optimally hydrated. Therefore, one cannot conclude that performance in these elite runners may have been enhanced if they had maintained or at least attenuated some of their fluid losses while racing.
But hang on a sec. Does this new study really offer valid “field” conditions? Not quite. It may have been conducted outdoors, on nice trails in a local state park, but it nonetheless managed to reproduce all the usual problems of lab studies. First of all, the runners weren’t freely paced: they were instructed to run at a set heart rate, which imposes a rather arbitrary limitation. So they didn’t go slower because they were unable to keep the pace, but because they weren’t allowed to increase their heart rate. More importantly, the dehydrated runners weren’t allowed to drink or eat “high water content foods” for 22 hours before the test run! The result:
So what can we conclude from these results? If you subject volunteers to a punishing dehydration regimen before your experiment even starts, their performance will suffer. This is very important to bear in mind next time you’re stranded in the desert. But as for how much water you should drink during your next run, this study has basically nothing to say.
(I should point out that the researchers did measure a number of other physiological parameters in the study, like gastrointestinal temperature. This is useful data. They also argue that holding heart rate steady is useful because it’s “similar to how a cross country or track coach may advise their athletes to maintain a certain intensity level during a run.” Sure, I guess. But if that’s what they’re measuring, then why are they using the results to make claims about what happens in real-world marathons, where nobody starts after a full day of dehydration?)
If you are asked to run at a constant heart rate, you will have to slow your pace to do this. As you run, you experience “cardiac drift” which increases your heart rate due to dehydration, reduced blood volume, and fatigue. Your blood becomes thicker and harder to pump, increasing your heart rate. During long run or a marathon, I usually expect my heart rate to increase by about 1 BPM per mile, even if I maintain the same pace.
If you withhold fluids and are dehyrated to begin with, the amount of cardiac drift will be increased.
I do think it is possible for people to accidently become dehydrated before a marathon. If they are carbo-loading, sometimes they don’t drink enough water (2.7 grams of water for every gram of carbs) or are worried about the small amount of weight that they are gaining and stop drinking.
In the abstract they mention the purpose of the study was “…to determine the effects of dehydration at a controlled relative intensity on physiological responses and trail running speed.” And in the conclusion, they mention the results were “…clinically meaningful for athletes using HR as a gauge for exercise effort and performance.”
My question to you, Alex, is what is your take on using heart rate as a measure of intensity in a study like this? Anyone familiar with cardiac drift in a dehydrated state (as Mike mentions) could have easily predicted the results of this study. With heart rate being affected by factors such as heat, dehydration, caffeine, etc. do you feel it is a good measure of exercise intensity in a study where they are manipulating one of the variables known to affect heart rate?
@Mike(s): Yes, you could interpret the study as confirmation of the existence of cardiac drift, which is in part how they spin it in the discussion. This is, of course, a well-known phenomenon — but hey, maybe it will help get the message out to some people who are unaware of it.
The bigger issue to me, though, was the whole question of lab versus field, and whether most of the literature on hydration has any broad ecological validity. It looks to me like these researchers are trying to respond to those criticisms by taking their studies outside — but they’ve missed the whole point by reproducing exactly the same flaws that make so many lab studies irrelevant to the real world.
This result — that cardiac drift is a real phenomenon, and is exacerbated by a full day of pre-exercise dehydration — is true and internally valid, as far as I can tell. But who cares? The basic design of the study immediately restricts its relevance to a tiny subset of the population under very specific and unusual conditions. That doesn’t mean the study is “bad” or “wrong”; it’s just not as interesting or useful as it could have been.
Two other major differences between the study and dehydration during a race:
1. the physiological impact of spending 22 hours dehydrating may have major ramifications on physical ability that are unrelated to 2 hours of dehydrating during a race, including muscle-fiber-firing, potassium channel damage (and possible arrhythmias), ion imbalance, and millions more conditions I am not aware of. Short-term dehydration will not create the same solution throughout the body as will long term, and damage from it will definitely not be as extensive in less time.
2. Given the amount of time dehydrating, and perhaps even more importantly, the fact that the dehydration took place before the exercise, likely brain mechanisms kicked in to protect the body against further damage. I find it analogous to going into a race when you are already hurting and therefore mentally you just are not prepared, whereas if you go into a race peaked by a taper you can usually end up hurting yourself far more because you are mentally prepared for the battle without the pre-race exhaustion (physical and mental).
While on the hydration topic I’ll share that recently ran my first marathon and found the “drink to thirst advice” irrelevant (at least for this event at my ability). I needed to ingest several packs of gels to consume enough carbs to complete the race and found myself drinking pre- and post gel in order to dissolve the carbs, whether I was thirsty or not. Perhaps I was overly concerned with dissolving the carbo goo but I had to drink more than what my thirst suggested. (Gatorade does not provide enough carbs, especially when half of the few ounces they hand me falls down my shirt instead of my throat.)
I’ll also add how much I appreciate this blog. I’m a competitive recreational runner and very curious about exercise science. This blog hits the spot almost every day. Thanks.
Sub maximal Heart Rates aren’t fixed though – when dehydrated (ie you have lower blood volume) you would expect a higher Heart Rate for the same cardiac output. So all the study has shown is that if you reduce cardiac output your performance suffers. That’s completely obvious and well proven.
The question that needs answering is if you are suffering from the higher HR a dehydrated state forces you to produce for the same cardiac output.