John Davis

Written by John Davis


Dehydration and Performance: The Right Way To Rehydrate When Running

While it’s not quite feeling like spring yet in most of the country, one concern that will surely be on many runners’ minds once it does is hydration.

Over the winter, it’s easy to get lulled into drinking minimal amounts of water, and once the temperature heats up, your first few runs in the heat can entail a rough reminder that drinking enough water is important to your performance.

This week, we’ll take a look at what the scientific literature has to say about staying hydrated while running and the kind of performance deficits you can expect when your body has less-than-optimal water reserves.

Dehydration and running

It’s certainly clear that being dehydrated can hurt your ability to run.

This was proved rather bluntly in a 1985 study by Lawrence Armstrong, David Costill, and William Fink. Using diuretic drugs, powerful agents which artificially induce dehydration by increasing urine production, the researchers examined the effects of dehydration on the performance of eight male runners over 1500m, 5000m, and 10,000m. Not surprisingly, the performance of the runners degraded significantly.

The results indicate that runners slowed nearly 80 seconds during a 5k and lost 2 minutes and 40 seconds over a 10k due to dehydration.

For half marathoners and marathoners, this data also makes it clear that hydration becomes more important as the race distance increases.

While Armstrong, Costill, and Fink measured blood plasma volume and urine output, the best predictor of each individual subject’s performance drop was how much their body weight dropped. The average amount of weight lost due to diuretic-induced dehydration was around 2% of the subjects’ total body weight, or three pounds for your average 150 lb runner.

The truth behind the Math

But, while measuring body weight is an often-recommended strategy for calculating your own water losses during exercise, it can be misleading in the real world, as R. J. Maughan and S. M. Shirreffs point out in a 2010 paper.

As they describe, changes in body weight reflect not only water loss due to sweating and respiration, but also the loss of fuel—just like your car gets lighter as the gas tank empties, your body weight drops as you burn carbohydrates and fats while you are out running.

Additionally, some water is generated by burning calories during exercise when the oxygen in the air combines with the carbs or fats in your muscle cells. So not all the water and weight you lose comes from sweat!

While these fuel-related losses only represent about a pound or so of weight during a long run or workout, it can muck up body weight and water loss calculations, causing people to overestimate how much water they need to stay hydrated.

Further complicating the issue are the facts that 1) dehydration necessarily induces a loss of body weight, which can slightly speed performance and 2) runners instructed to drink water ad libitum—that is, as much as they want—don’t drink as much water as they are losing in sweat during exercise.

These issues are explored by Tim Noakes, a doctor and researcher in South Africa whose work has done much to upend the “drink to replace body weight losses” mantra. In Lore of Running, a 2001 book reviewing a huge number of scientific studies on athletic performance, he cites several papers which show no advantage in consuming large quantities of water during exercise when compared to drinking water as desired.

Drinking enough

While doctors, trainers, and sports performance specialists often encourage 25 ounces of water per hour or more (enough to replace sweat loss), most athletes only drink about 13.5 ounces per hour when instructed to drink “to thirst.”

In a 2007 paper on hydration in the marathon, Noakes makes the case that hydration during exercise is designed to maintain stable levels of blood plasma—not necessarily body weight—and that losses of a few percent in body weight are normal and tolerable.

This might be the key to why the diuretic study we examined saw such a big difference in performance: The diuretic drugs not only regulated urine output, but blood plasma concentration as well.

While ad libitum water consumption, sipping a few ounces of water every 10-15 minutes, might be ideal in a laboratory setting, many training sessions don’t take place in well-controlled environments. Often, it may only be practical to rehydrate once or twice during a long run or workout.

While the average runner’s ad libitum water consumption per hour (13.5 ounces) lines up conveniently with the 16-ounce size of most athletic water bottles, the amount of fluids you need will also depend on the temperature, your body size, and your individual fluid needs.

Final tips

Through experience, you can find the ideal rehydrating strategy for your workouts, but one water bottle per hour is a good enough place to start once your workouts begin to stretch further than 60 minutes. Doing a loop course or dropping off a water bottle in a car before you run will allow you to strategically time your rehydration so you can get through your training feeling good.

The best takeaway from all of this research is that you don’t need to worry about calculating water losses or guzzling huge amounts of water per hour.

Just drink enough so that you aren’t thirsty during your run—for most people in moderately warm conditions, just under one 16 oz water bottle per hour—and worry about making up the lost fluid after your workout is over.

Noakes cites a study of army recruits training in the desert that showed that body weight is fully restored after athletically-induced dehydration once you’ve eaten some food and consumed fluids along with it.

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1. Armstrong, L. E.; Costill, D. L.; Fink, W. J., Influence of diuretic-induced dehydration on competitive running performance. Medicine & Science in Sports & Exercise 1985, 17 (4), 456-461.
2. Maughan, R. J.; Shirreffs, S. M., Dehydration and rehydration in competitive sport. Scandanavian Journal of Medicine & Science in Sports 2010, 20 (3), 40-47.
3. Noakes, T. D., Lore of Running. 4th ed.; Human Kinetics: Cape Town, 2001.
4. Noakes, T. D., Hydration in the marathon: using thirst to gauge safe fluid replacement. Sports Medicine 2007, 37 (4-5), 463-466.

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5 Responses on “Dehydration and Performance: The Right Way To Rehydrate When Running

  1. any recommendations for carrying water on those longer runs, say three hours or so? do you know if the hydration vests cause you to sweat more?

    • Hi Alex, you can try products such as handheld water bottles (Nathan and fuelbelt) a good brand) or hydration packs (camel backs are the most popular).

      I don’t think these would cause you to sweat more other than they are another layer of clothing/material that would prevent a breeze from reaching your skin.

  2. I have not read that first study, so may be incorrect. But if they dehydrated the athletes prior to the time trials, the decreased pace may have more to do with lack of sweating ability than actual dehydration. I believe I’ve also read of studies that found the most dehydrated athletes at a race are the ones that finish at the front of the race! They are not uselessly losing that weight, but it’s being used to cool their body. You could even argue that the ones that are able to cool their body the best are the ones that sweat the most, and thus become the most “dehydrated”.

    • Great points, Kyle. I didn’t think about the first fact that per-deyhdration would reduce the ability to sweat due to lack of water to actually sweat. I don’t think that has ever been approached in a study.

      As to your second point, definitely true. I believe Geb was once measured as losing about 10% of his body weight at the end of a race. Definitely goes to show those 2% warnings are baseless.

      Thanks for commenting!

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