The Basics: Running, sweating, and drinking in warm weather

Posted by Dave on July 19, 2011 | 17 Comments

Few topics fill runners with more dread than trying to figure out what and how much to drink for a workout. Should you consume an energy drink? If so, what kind? Or is plain water fine? Should you drink beforehand? Can you drink too much? When the weather warms up in the summer, the questions only multiply: Is it dangerous to train in heat? What about the humidity?

Unfortunately there are few solid answers. There’s no doubt that drinking too little over an extended period of time can be dangerous, but in fact it may be even more dangerous to drink too much. Tim Noakes reported that in 1985, of the 17 runners hospitalized after the 56-mile Comrades Marathon in South Africa, 9 of them were overhydrated: their blood sodium levels were dangerously low. In the 1987 race, three runners nearly died of the condition, and some runners in the U.S. have indeed died of overhydration.

You can get a sense of how much water you’re losing during a run by weighing yourself with no clothes on before and after a run and subtracting: 1 pound=16 ounces of water; 1 kilo=1 liter. It’s best to do it over several days because the amount you sweat can vary from day to day. Over three days where the temperature was in the low 70s and the weather was extremely humid, I lost between 3 and 4.8 pounds in a 1-hour run, averaging 4 pounds. If all of the weight loss was attributable to sweat, that would be nearly a half-gallon! Not all of that weight loss is necessarily sweat, though. You also can lose weight by metabolizing carbohydrates, but Noakes says scientists disagree on precisely how much is lost in this manner. Let’s suppose that 75 percent of my weight loss was indeed sweat: That’s about three pounds or 1.5 liters. Should I drink the full 1.5 liters of water during my run? If I run a marathon in 3.5 hours, should I try to drink 1.5 liters per hour, a total of 5.25 liters?

Unfortunately (or perhaps fortunately) the body can’t absorb water that quickly during a workout. Noakes estimates that the average runner can absorb somewhere between 400 and 800 milliliters per hour. If you are a heavy sweater and consume water faster than that, particularly during an extremely long race such as a full Ironman triathlon or ultramarathon, you risk bloating and severe discomfort. If you tried to consume 1.5 liters per hour during a 10-hour race, you conceivably could have drunk 10 liters more than your body can possibly use!

If you don’t sweat much and still drink a lot, then the problem is potentially worse; this is where potentially fatal overhydration comes into play. Women, because they tend to sweat less than men, are at more danger for overhydration. Slower runners also tend to sweat less and have more opportunities to consume liquids during a race or workout, so they should be careful not to blindly drink as much as possible.

The consensus among many researchers including nutritionist Matt Fitzgerald is that runners should simply drink when they are thirsty. I’m not sure this advice is ideal, particularly for beginning runners. It can be difficult to tell how thirsty you truly are when you are near exhaustion. If, like me, you sweat a lot, you’re going to be losing much more in sweat than you can possibly absorb by drinking during a long race or workout in the heat. That’s not necessarily devastating: Most runners finish races with some level of dehydration, and there isn’t much evidence suggesting it impairs their performance. Indeed, race winners tend to be more dehydrated than other competitors.

But just as beginning runners have more potential to over-hydrate because they are on the course longer than advanced runners, so too is it possible that they may become extremely dehydrated. It takes me almost twice as long as an elite runner to finish a marathon, and it takes me longer to run between each aid station. If I am offered 6 ounces of water at each station, every two miles, that adds up to about 22 ounces per hour at my 8-minute-mile pace. That’s about 650 milliliters, or somewhere in the middle of the 400-800 ml range my body can absorb. If I skip just one station, I’ve only had 16 ounces, or 470 ml. I might be drinking only a little over half the water my body can actually absorb, which is still well short of the 1.5 liters per hour I may be losing on a warm day. For me, it rarely makes sense to skip a water station. For others who sweat less during a run, different advice might apply.

What this means for you: Drinking more water than you sweat/urinate away per hour during a workout can be dangerous. Knowing how much you sweat can help you decide how much to drink, but you likely won’t benefit from drinking more than 800 ml per hour. While thirst is a good guide, beginning runners might want to establish a regular drinking pattern.

A tip for measuring your sweat loss: You don’t have to forgo drinking during a run to measure sweat loss. Just weigh yourself with your water bottle before and after the workout. Your spouse may be confused when he or she sees you naked on the scale with a water bottle, but at least you won’t be thirsty.

Noakes, T. (2003). Lore of Running (4th ed.). Champaign, IL: Human Kinetics.


17 Responses to “The Basics: Running, sweating, and drinking in warm weather”

  1. KC
    July 19th, 2011 @ 7:13 pm

    Thanks for this one, Dave, especially during this hot spell!

    I can see that drinking more than 800ml/h could exceed the body’s rate of absorption and cause bloating. Ugh.

    But if I understand you correctly, the *dangerous* rate of consumption may actually be well below 1 liter per hour.

    I can over-hydrate only by absorbing water successfully, and if I’m average (and I am), I need consume only 400-800 ml/h to absorb as much water as I possibly can.

    If I were actually trying kill myself by over-hydrating, it seems like drinking over 800ml would not be any more dangerous – just pointless and painful.

    I may be misunderstanding.

    Is it also possible that those who die of over-hydration are able to absorb water faster than most, e.g. at a rate much higher than 800ml/h?

    Thanks again!

  2. Dave
    July 19th, 2011 @ 7:26 pm

    Good point, KC — it’s probably not a good idea to recommend a specific amount; you can over-hydrate drinking at lower rates if your body doesn’t excrete the fluids either through sweat or urine. I’m going to revise the post to take this into account.

  3. Jess
    July 19th, 2011 @ 9:13 pm

    Isn’t something missing from this discussion? If the concern is a runner becoming overhydrated, as defined by too low a level of blood soduim, then it seems obvious that how much salt you take in will affect how much water is “too much”. Electrolytes also affect the rate at which fluid is absorbed from the stomach and small intestine, and reduce the amount of fluid that goes to the urinary tract. This is why during hot summers, running while drinking just water will pretty quickly result in an unpleasantly sloshy stomach and possibly nausea, but with persistent thirst because the body’s hydration needs aren’t being met. Anecdotally, I know many people who had hydration problems in warm weather until they upped their salt intake.

  4. Dave
    July 19th, 2011 @ 10:16 pm


    You’re right that sodium/electrolyte levels play a role in this, but because hydration and beverage choices can be so complicated, I chose to focus only on water in this article. We’ll tackle those issues in another post.

    One response to your point that “running while drinking just water will pretty quickly result in an unpleasantly sloshy stomach and possibly nausea.” If it occurs over a short period (less than 2 hours), it is probably an indication that a runner is drinking too much water, not that they need more salt.

  5. Derrick
    July 19th, 2011 @ 11:53 pm

    Of course, there is the tried and true “pee test” to determine your level of hydration. The darker it is, the more dehydrated you are. I make sure I pee clear or very light yellow before a race or long run which has served me well. No this isn’t higly scientific but it works for a lot of people besides myself.

  6. Mark
    July 20th, 2011 @ 12:12 am

    Another problem in hot weather is the assumption that adequate hydration automatically prevents heat casualties. You can be well hydrated AND hyperthermic. Best to listen to your body. Great post Dave. Keep them coming.

  7. Sean
    July 20th, 2011 @ 5:39 am

    Thanks for the post. Answered my questions!


  8. Josh
    July 20th, 2011 @ 10:56 am

    I’ve never been clear on what happens to the “unabsorbed” water. I definitely drink more water than that regularly. If my body were not absorbing it, wouldn’t I get a big case of the runs? But that’s not what happens…

  9. Jess
    July 20th, 2011 @ 1:17 pm

    @Dave, sure, but try running for more than two hours when it’s warm and humid. It is a real challenge to get the water/salt balance right, at least if you sweat as much as I do. Though I suppose it also depends on how salty a person’s sweat is, which seems to be highly variable.

  10. Dave
    July 20th, 2011 @ 4:36 pm

    Derrick: The pee-test is a good way to make sure you’ve pre-hydrated well. But of course it’s only useful if you can actually urinate. My understanding is that in cases of overhydration, part of the problem is a kidney failure which means you probably won’t be peeing at all. If you are peeing a lot, that could be a sign that your kidneys are functioning correctly and you’re not at risk of overhydration.

    Josh: Assuming you’re not overhydrated, the unabsorbed water will be absorbed eventually; in cases of overhydration what actually happens is that the water is absorbed and you’ve got too much water and not enough salt in the blood stream, which impairs the ability of your blood to do all the things it’s supposed to do, like deliver oxygen to your muscles and brain.

    For many people what happens is it just sloshes around in their stomach. If neither of these things is happening and you’re sweating and urinating normally, it’s possible that you can actually absorb more water than most people. For example, cyclists do seem to absorb much more water than runners, up to 2 liters per hour.

  11. Travis
    July 21st, 2011 @ 6:22 pm

    Just wanted to reiterate Mark’s comment RE hyperthermia. I’m not sure if you’ve gotten to Noakes’ views on the hydration and over-heating, but he argues that basically they’re not closely related (or at least not anywhere near as related as people often think), and that that can actually cause problems when people try to cool off by simply drinking cold water.

    Very much enjoying your walk through the book!

  12. Eric
    July 23rd, 2011 @ 3:38 pm

    I recently read Relentless Forward Progress where the author talks about measuring your sweat loss like you describe here. I’ve always been a sweat’er, but I didn’t realize how much until I measured it. I’m concerned that over longer distances I won’t be able to match the water replacement with the sweat loss, so I’m wondering if I will get more efficient as I build up my base.

    I’ve enjoyed your posts, Dave. I look forward to a post on salt replacement. :)

  13. zoon politikon
    July 24th, 2011 @ 12:26 pm


    english science Pharyngula Bad Astronomy Bad Science english IR/PoliSci The Monkey Cage Crooked Timber PolySigh Foreign Policy Passport Stephen M. Walt Daniel W. Drezner Marc Lynch The Cable The Multilateralist The Duck of Minerva IPE at the University…

  14. Colin Santos
    July 27th, 2011 @ 10:51 pm

    Hi, I’m working on a marathon this fall and this blog has become my favorite, for reals.

    I would also like to echo the previous commenter, as it seems dangerous to classify hyponatrimia as overhydration. Are these simply 2 sides of the same coin?

    Looking forward to your electrolyte post!

  15. Alex Hutchinson
    August 3rd, 2011 @ 2:46 am

    Jess wrote “If the concern is a runner becoming overhydrated, as defined by too low a level of blood soduim, then it seems obvious that how much salt you take in will affect how much water is “too much”.”

    Just wanted to point out that Noakes would very strongly disagree with this statement. The amount of sodium contained in sports drinks is still much lower than than the typical concentration in blood, so even drinking sports drinks rather than water lowers your sodium concentrations (and if you’re drinking more than thirst, can put you at risk of hyponatremia). Noakes published a pretty strongly worded paper about this earlier this year:

  16. JohnE
    August 10th, 2011 @ 7:42 pm

    Are keen runners into Hydration packs (e.g. camelbaks?) As a mountainbiker I find them a good way to stay hydrated because the pinch valve system encourages small but frequent sips of water, but I can imagine runners prefer not to be waering a pack?

  17. Mizunogirl
    February 26th, 2012 @ 4:01 pm

    I typically do not have any issue drinking too much/not enough while running. But I recently bonked out on a long run (at 3 miles to go I just lost it). I was about 5-6 pounds down after and was wanting to drink my entire water bottle at mile 5 (12 mile run). (usually I only drink about 3/4 of it on a run of that distance) trying to figure out if I was underhydrated before the run and if there is a way to be too overhydrated, I mean say drinking the day before, I assume any “water” my body does not need the day before a run will be gotten rid of by my normally functioning kidneys?