Wednesday, September 24, 2008

Hydration and Exercise, Part 3

Those cloistered within the ivy-covered walls of university human-performance labs aren’t always very good about giving us real-world answers to real-world questions. From their research we can often get suggestions, possibilities, and general directions. But they seldom go out on a limb and tell the athlete what to do in given situations. Straight answers to clear questions are rare in academia. That is left for those in the field – the athletes and their coaches – to figure out for themselves.

It seems that the more you know about something, the more possibilities you can see for how the question might be answered. I suffer from this affliction, also. With every passing year of studying sport science I find my answers to athletes’ questions are more likely to start with “it depends.” I try not to, but as soon as I say do such-and-such, someone is going to say, “Well, that doesn’t help in this unique situation.” So I hedge my bets knowing that there are just too many variables in studying humans to give sure-bet answers to even the simplest of questions.

I’m telling you this because I am about to answer some simple questions about hydration and exercise, or at least they appear simple on the surface. I’ll try hard not to say “it depends.” But no promises. Sometimes I just can’t help myself. The answers are some mix of what I’ve seen in research and my experiences as an athlete, coach and observer of sport. Here goes…

Q: How much should I drink during a race or workout?
A: This gets back to Part 2 of this series. You should drink enough to satisfy your thirst. I know this seems simplistic and athletes often being type A want something more complex and defined. How could anything so simple actually work? But how much you drink depends (oops) on the conditions of the race or workout and how much you are sweating. The bottom line is that you’ll need less fluid in cool weather than hot. How hot is hot and how cool is cool? (I just can’t control myself!) How much fluid you need also varies based on how hard you are working. Rather trying to come up with tables that you study before exercise with various situations there is a rather simple solution: drink when you are thirsty and drink until satisfied. If you do that there is no need to give it any deeper consideration. Thirst is a great mechanism and is quite accurate despite what we’re told. The challenge, of course, is paying attention to it. If you become singularly focused on other matters, such as your competition or pacing, then you are likely to forget to pay attention to your signs of thirst such as a dry mouth. During a workout or race you should be repeatedly running through a checklist in your head of many items including thirst.

Q: Should I drink enough during exercise to maintain my body weight?
A: No, you should not (how’s that for being definitive?). You will lose weight during exercise primarily due to dehydration and secondarily to the consumption of energy stores. The latter is quite small as a portion of your body weight. Trying to maintain body weight by drinking large amounts of fluid sets you up for hyponatremia as described in Part 2 of this series (below). Well before your sodium stores become significantly diluted you may well experience what Ironman triathletes call “stomach shutdown” with what I suspect is the gut's inability to process fluid or food due to having taken in too much water or sports drink. This may be accompanied by nausea which is good because vomiting would remove excess fluid in the stomach.

Q: How much weight loss due to dehydration is acceptable and can be well-tolerated by the body?
A: The standard answer used to be 2% but that keeps going up in the literature with most finding that a 3% loss of body weight due to dehydration has no significant effect on endurance performance [1,2,3]. But one recent study did find an 8% decline in cycling time trial performance with a 3% loss of body weight due to dehydration in a moderate temperature of 68F (20C) but not at 36F (2C) [4]. Around 4-5% losses do seem to have negative consequences, however [5]. For a 154-pound (70kg) athlete 4% of body weight is just over 6 pounds (2.8kg). Assuming that a pint of water weighs about a pound, this would be a loss of 3 quarts (nearly 3L) of fluid. That would be a gross error in hydration and would certainly indicate that the athlete was not paying attention to thirst. The bottom line is that you should lose water and therefore body weight during exercise, but if you drink until satisfied when thirsty then body weight losses and therefore dehydration will not be an issue. That’s “managed dehydration,” to again use coach Gordo Byrn’s apt phrase.

Q: How do I gauge my level of dehydration following exercise and then go about replacing lost fluid?
A: This one is easy and by now you should also know the answer: Drink according to your thirst. If thirsty, drink. When no longer thirsty, don’t drink. It’s pretty simple. There is no reason for elaborate drinking schedules or water volume goals. I recently read an article by a noted nutritionist who suggested that Ironman triathletes should drink 180 ounces (5400ml) of water daily. Wow! That’s really not necessary or even healthy. Thirst does indeed work. And also be aware that fluid comes not just from drinking but also from the food you eat. For example, a study of 14 elite Kenyan runners whose water losses and rehydration were tracked for 5 days supports this notion [6]. No instruction was given on how much to drink. During workouts they drank nothing and typically lost 2.7% of body weight daily. On average they took in 4 quarts (3.8L) of fluids daily based entirely on thirst. No changes were reported in daily hydration status, body weights or responses to training over the course of 5 days.

Let’s stop now and allow you to think about all of this. I’ll be away from the office the rest of the week. Next week I’ll answer more questions along this same line and get into the matter of how to replace fuel expended during exercise, also.

1. Cheuvront, S.N., R.I. Carter, M.N. Sawka. “Fluid Balance and Endurance Exercise Performance.” Current Sports Medicine Report 2 (2003): 202-208.
2. Oliver, S.J., S.J. Laing, S. Wilson, J.L. Bilzon, N. Walsh. “Endurance Running Performance After 48h of Restricted Fluid and/or Energy Intake.” Medicince and Science in Sports and Exercise 39, No. 2 (2007): 316-322.
3. Laursen, P.B., R. Suriano, M.J. Quod, H. Lee, C.R. Abbis, K. Nosaka, D.T. Martin, D. Bishop. “Core Temperature and Hydration Status During an Ironman Triathlon.” British Journal of Sports Medicine 40, No. 4 (2006): 320-325.
4. Cheuvront, S.N., R.I. Carter, J.W. Castellani, M.N. Sawka. “Hypohydration Impairs Endurance Exercise Performance in Temperate but not Cold Air. Journal of Applied Physiology 99, No. 5 (2005): 1972-1976.
5. Slater, G.J., A.J. Rice, K. Sharpe, R. Tanner, D. Jenkins, C.J. Gore, A.G. Hahn. “Impact of Acute Weight Loss and/or Thermal Stress on Rowing Ergometer Performance.” Medicince and Science in Sports and Exercise 37, No. 8 (2005): 1387-1394.
6. Fudge, B.W., C. Easton, D. Kingsmore, F.K. Kiplamai, V.O. Onywera, K.R. Westerterp, B. Kayser, T.D. Noakes, Y.P. Pitsiladis. “Elite Kenyan Endurance Runners Are Hydrated Day-to-Day with Ad Libitum Fluid Intake.” Medicince and Science in Sports and Exercise 40 (2008): 1171-1179.


At September 24, 2008 6:43 PM , Blogger Mama Simmons said...

Your articles on hydration are highly interesting to me. Thank you for writing them!

I've completed 8 Ironmans, most recently Kona last year, and almost always have issues with cramping toward the end of the bike. I always assumed it was b/c I wasn't taking in enough sodium? Salt tabs usually seemed to work. But last year in Kona I had a major issue that I have not yet figured out. I was drinking gatorade almost exclusively (that and GU for calories). But before the bike turn-around I was feeling twinges of cramps and felt thirsy. So naturally, I drank more gatorade. A lot more.

By the end of the bike I was really cramping and took like 8 minutes in transition before I could start running. Long story short, I managed to run maybe 11 miles but then my body pretty much shut down and I walked the majority of the rest of the day. I felt like I had all the classic signs of dehydration and low sodium (cramping, thirst, but peeing all the time), so I kept drinking as much as I could (again, mostly gatorade b/c I knew water would cause hyponatremia). I also took many more salt tabs over the course of the marathon.

Ends up at the end of the race I was so bloated that my mom didn't recognize me crossing the finish line. I had gained 13 lbs from start to finish as my body was just storing all the liquid rather than letting my body use it as needed. In the med tent, after a blood test, they told me I was low on potassium. I peed off 13 lbs of fluid in the next two days as my body composition returned back to normal.

Lesson? Drink less altogether? Clearly, after reading your articles, that's what it seems... just hard for me to comprehend b/c my issues started relatively early on and I felt so thirsty. What do you think?

At September 25, 2008 2:14 AM , Blogger Bahzob said...

Help. I would like to believe this and in fact my experience has been that I have often drunk much less than the recommended amounts on long (6+ hour) rides and not felt bad as a result.

But it is worrying that in the same week subscribers to your blog and that from Training Peaks (which you mentioned in passing) get such contradictory advice.

How can we tell which is right?

At September 26, 2008 7:45 PM , Anonymous Anonymous said...

Staying properly hydrated during multi-day events such as a bike tour is extremely important for your health, comfort and performance. The bicyclespokesman blog ( has a good post on tips to stay hydrated on multi-day or weekend events.

At September 27, 2008 11:18 AM , Anonymous Ansis said...

My name is Ansis, I am a doctor in physics, incl. colloidal chemistry. In your publication I paid attention that with given data it is possible to calculate how liters of pure water (x) one must drink to reach the treshold of hyponatremia (128 mmol/l), assuming the initial concentration to be 140 mmol/l. The equation to solve is:
note that keeping denominator constant means an athlete who compensates all water lost - as suggesting by ACSM, and I take "medium sweat", i.e. 40 mmol/l. This equation returns x=12 liters. I dare to say that nobody loses 12 l of water during exercise... even if we imagine n-times Ironman distance and the athletes do, it seems to be insane to compensate such a huge amount without taking any food.
With solving this simple equation I have to conclude that the threat of hyponatremia due to compensating all water lost with pure water in fact is negligible (?).

At September 29, 2008 2:46 PM , Blogger Joe Friel said...

Hi Mama S--That's a very interesting story. You didn't say what your body weight was at the start but 13 pounds is a lot regardless. It's remarkable that you even finished. You must have been highly motivated.

I don't believe that the localized cramping of the propulsion muscles for cycling and running are related to sodium concentration (cramping during hyponatremia is generalized). There is no research to support the theory that sodium is related to cramping I know of. You might see my post on cramping from last May (

How were you able to drink so much and yet remain thirsty? Good question. I don't really have an answer for you as I've never seen any research on such stuff nor have my athletes experienced it. It could be that you were taking in so much sodium that thirst was stimulated. Or perhaps it was the sweetness of the sports drink that kept you drinking while you were fighting off a total bonk. Either way I'd suggest not using sodium supplements the next time and drinking to thirst.

At September 29, 2008 2:54 PM , Blogger Joe Friel said...

Hi bahzob--I always recommnend that we should all be skeptical any time someone who is purported to be an authority (including me) tells us anything. But at the same time we must be open minded and be willing to accept new ways. When involved in such discussions I always have a little thought in the back of my mind -- "He may be right." This attitude has led me to accept rather uncommon points of view (for ex, midsole cleats, Paleo nutrition and now drinking to thirst rather than a schedule). So what should you do with conflicting ideas on such a topic as this? My mother used to say that "the proof of the pudding is in the eating." That means try it and see what happens. Training is the place to try such things. Good luck!

At September 29, 2008 3:02 PM , Blogger Joe Friel said...

Hi Ansis--The onset symptoms of hyponatremia start well before reaching 128mmol/l. And there are even those in the medical profession who say hyponatremia is achieved at some number above 128, such as 134mmol/l. The bottom line is that we can avoid overdrinking and its potential symptoms at whatever level of seriousness they may elicit by drinking to thirst rather than to a schedule.

At September 30, 2008 12:05 PM , Blogger Sketti Scramble said...

Drink to thirst. That seems easy enough. However, and i hate to contribute to the "it depends" mantra, i find that when i drink to thirst i will drink 8-16oz or more at one time. Of course this isn't possible or recommended while in a race. So a regimented sip or gulp before i feel the complete onset of thirst seems to make more sense to me. For instance, say i get thirsty on a tempo ride at 90min in, i'm not going to gulp down 16oz of water all at once. I'd rather sip 2oz every 12min to complete my 16oz every 90min. And we are back to the original scheduled drinking habits... my head spins.

At September 30, 2008 2:25 PM , Blogger Joe Friel said...

Hi sketti- I don't think it needs to be confusing at all. Just take a drink when thirsty. That doesn't mean emptying the bottle each time. If you're keeping up with your thirst it won't take all that much. But if you forget to check your thirst regularly, which is certainly possible in a race, and fall behind then you would be forced to take bigger swigs. But, again, remember that you will not replace all of the fluids lost. Don't even want to. "Managed dehydration."

At September 30, 2008 8:05 PM , Blogger Jot said...

So I am a very heat adapted individual. It is my understanding that one of the adaptions that happens is that you tend to sweat earlier, more profusely and less salty.

This summer (in the morning) I went on a 6 mile run. I weighed myself with all of my fluid (24 oz of water) and clothing. When I returned I weighed in again (so now we're talking net water loss since it includes the 24 oz I drank) and I was down 4.5 lbs, or 2.5%. For me this was very normal, and not extreme. (When I run in the heat of the AZ summer it's much more extreme).

In my mind this is an (N=1) indication that the "conventional wisdom" of 2% is horribly wrong. I've never done anything but drink to thirst, and this includes multi-hour runs at temps in excess of 100F.

After 3 hours I think the game could change, and I think the questions are far more interesting.

Thanks for the series. I enjoyed it. I haven't done the calculus to determine at what rate do you need to consume various concentrations of salt to deal with the loss of salt due to sweating at it's concentrations. That would be interesting.

I'll stop blabbing now.



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