Friday, September 5, 2008

Hydration and Exercise, Part 1

Until recently relatively small losses of body weight due to dehydration during exercise were thought to be detrimental to athletic performance. The threshold for drops in performance related to dehydration has long been considered to be around 2% of body weight [1]. So a 150-pound man who lost 3 pounds or a 125-pound woman who dropped 2.5 pounds during exercise were expected to slow down as a result. In 1996 the American College of Sports Medicine (ACSM) went a step further suggesting that the loss of any body weight due to dehydration could potentially result in poor performance or even health issues [2]. Their position stand was that an athlete should drink enough to replace all water lost through sweating and that this would likely be in the range of 18 to 36 ounces (600-1200mL) of fluid per hour. Last year the ACSM officially changed their stand by adopting the more common 2% of body weight as the upper end of what an athlete should lose due to dehydration [3]. This time they stopped short of making recommendations on how much to consume.

Two-percent is still too low according to Tim Noakes, PhD, a South African exercise physiologist who has led the fight to get the ACSM and other sports organizations to relax their standards on drinking during exercise [4]. Why would Noakes want us to drink less during racing and training? The reason is something called “hyponatremia” – the dilution of the body’s stores of sodium. Athletes who were encouraged to replace all fluids lost while exercising as per the 1996 ACSM position stand put themselves at risk for this condition which is worse than the dehydration they were trying to prevent, according to Noakes [5]. And he believes that 2% still encourages excessive drinking and encourages risk.

By over-hydrating while exercising, sodium concentrations drop and put you at risk for hyponatremia. Some early signs of hyponatremia are bloating around elastic bands in socks and the waistbands of shorts, disorientation, headache, nausea, muscle cramps, lethargy, confusion, reduced coordination and tunnel vision. In recent years there have been deaths in back-of-the-pack runners related to hyponatremia. Drinking an excessive amount of water for several hours during exercise (4 hours is considered the danger threshold) increases your risk. One study of Ironman-distance triathletes going way back to 1987 found that as few as 8% and as many as 30% of the race finishers experienced mild to severe hyponatremia [6].

It’s generally accepted that elite athletes have a lower risk of hyponatremia but a higher risk of dehydration since they are focused on performance and may take in less fluid than demanded by the body. Slower athletes simply have more time to drink and may well believe it is in their best interests to consume a lot of water. The mistaken notion that we should finish a workout or race with no loss of body weight further increases the risk, especially in long events.

So what does all of this mean for your training and racing? How much should you drink during a race or workout? What should you drink? How much weight loss due to dehydration is acceptable and can be well-tolerated by the body? Should you drink extra amounts in the days and hours leading up to a race in hot conditions so that you’re hyperhydrated? How do you gauge your level of dehydration following exercise and then go about replacing lost fluid? Does age have any implications for hydration? I’ll address these questions and what the research and my experience suggest are the answers in my next blog.

1. Naghii, M.R. “The Significance of Water in Sport and Weight Control.” Nutritional Health 14, no. 2 (2000): 127-132.
2. Convertino, V.A., L.E. Armstrong, E.F. Coyle, G.W. Mack, M.N. Sawka, L.C. Senay Jr., W.M. Sherman. “American College of Sports Medicine Position Stand. Exercise and Fluid Replacement.” Medicine and Science in Sport and Exercise 28, no. 1 (1996): i-vii.
3. American College of Sports Medicine, M.N. Sawka, L.M. Burke, E.R. Eichner, R.J. Maugnah, S.J. Montain, N.S. Stachenfeld. “American College of Sports Medicine Position Stand. Exercise and Fluid Replacement.” Medicine and Science in Sport and Exercise 39, no. 2 (2007): 377-390.
4. Noakes, T. “IMMDA-AIMS Advisory Statement on Guidelines for Fluid Replacement During Marathon Running.” New Studies in Athletics: IAAF Technical Quarterly 17, no. 1 (2003): 7-11.
5. Noakes, T. “Hyponatremia in Distance Runners: Fluid and Sodium Balance During Exercise.” Current Sports medicine report 1, no. 4 (2002): 197-207.
6. Hiller, W.D., M.L. O’Toole, E.E. Fortress, R.H. Laird, P.C. Imbert, T.D. Sisk. “Medical and Physiological Considerations in Triathlons.” American Journal of Sports Medicine 15, no. 2 (1987): 164-167.


At September 5, 2008 9:09 AM , Blogger Ross Tucker and Jonathan Dugas said...

Hi Joe,

Thanks for posting on this topic. There is so much dogma associated with fluid balance that it will take many years before we can expel it. Your post will help in the long run as many athletes read your blog.

Just one thing---you mention that depletion of sodium is a cause of hyponatremia. In fact this is a myth that is kept alive by the sports drinks companies, especially Gatorade. Hyponatremia is a disorder of fluid balance, not sodium balance.

Although we lose some sodium in sweat, there are two important aspects to remember. First, the amount of sodium is very small compared to what we have available in the body. Second, we are also losing volume (i.e. sweat) with the sodium, and as we remove fluid from our plasma, it will cause the sodium concentration to rise, not fall.

So we do lose both---sodium and volume, but the volume losses have a much more profound effect on the sodium concentration. If you do not drink water to thirst, you can expect the sodium concentration to rise. If you drink water to thirst, you can expect to maintain the sodium concentration just where your body likes it. Finally, if you drink water beyond your thirst, you can expect the sodium concentration to fall, most likely in direct proportion to the excessiveness of the ingested fluid volume---that is, the more you drink, the lower your sodium concentration will drop.

We did an entire series on dehydration on our Science of SportScience of Sport website, and some of your readers might be interested in reading that.

Well done again on the great post.

Kind Regards,

At September 5, 2008 9:20 AM , Anonymous Anonymous said...

Looking forward to this series! Also curious about your thoughts on hydration, leg cramps, and sodium.

At September 5, 2008 9:29 AM , Blogger Nate said...

What if I drink a high sodium drink during exercise. If I can stomach it, is that the best of both worlds?

What about sodium loading before an event? I know there is a study going on right now about drinking a high sodium drink 1 hour before exercise.

At September 5, 2008 10:38 AM , Anonymous Stefan said...

I also wonder whether I can drink as much as I want (within reasonable limits of course) as long as I add sodium to the water ?

At September 5, 2008 10:50 AM , Blogger peter g said...

What about athletes watching their sodium intake to control hypertension?

At September 5, 2008 11:41 AM , Blogger BFW said...

How about simply drinking an electrolyte replacement? Something like Gatorade, but without the sugar.

I use one, it has basically no calories, and just a faint citrus taste, but provides a little sodium, magnesium, potassium, and some other minerals.

At September 5, 2008 4:27 PM , Blogger Joe Friel said...

Ross and Jonathan--Good point. I stand corrected. BTW, you have an excellent website. Good info.

At September 6, 2008 10:40 AM , Blogger Andrew said...


I'm new to your blog, but have been using your bible for 5 yrs now and love it! I'm curious to what TSS/day is?

At September 6, 2008 1:30 PM , Blogger Joe Friel said...

Andrew--TSS/day means daily average Training Stress Score. It really woudn't mean anything to you unless you use WKO+ software (find at

At September 12, 2008 8:03 AM , Anonymous Anonymous said...

When will part 2 be posted?! I'm dying to read it. I've had stomach issues during and after races this summer and I suspect over-hydration and too highly-concentrated sports drinks are the culprit.

Is there any legitimacy to Hammer's claims that "complex carbohydrates," i.e. the maltodextrin they use in all their drinks can be absorbed well at a higher concentration than the monosaccharides that are commonly found in other sports drinks? If so what implications does that have for drinks with multiple sources of carbs (e.g. my drink mix that has fructose, maltodextrin, and glucose)?


At September 12, 2008 2:24 PM , Blogger Joe Friel said...

Hi Anon and everyone else--I apologize for the gap in postings after Hydration part 1. Our home was broken into and among the many items stolen were our computers. I'm writing this from phone. As soon as things settle down with polce, insurance, etc and I get a new computer I'll get back to it. Thanks for eading my blog.

At September 24, 2008 6:08 AM , Blogger The Sports Scientists said...

Several of the comments here are perfect examples of the many misconceptions around this issue. Fortunately, none of you are to blame. The sports drinks manufacturers utilize powerful marketing strategies seemingly based on science that are difficult to refute for the everyday athlete.

Nate said, "What about sodium loading before an event? I know there is a study going on right now about drinking a high sodium drink 1 hour before exercise.

Sorry folks, we cannot store or stock pile sodium for use later. Our physiology just does not work that way, and the same holds true for water. We cannot store it if we ingest a few Liters one hour before a race.

When you ingest excessive amounts of sodium or water, your kidneys will simply excrete the unnecessary amount. Yes, ingesting a sodium load will promote fluid retention, but eventually you will dump the excess sodium and the volume (your plasma volume) will return to its normal level.

Nate said, "I also wonder whether I can drink as much as I want (within reasonable limits of course) as long as I add sodium to the water ?

This holds true only if you ingest a fluid that is the same "tonicity" of your plasma---about 140 mmol Na/L. Sports drinks are in the range of 18-20 mmol Na/l, so you can do the math on how much saltier a sports drink would be if it were the same concentration of your bodily fluids. Suffice to say that not many people will be ingesting fluids of this sort.

However there is no need to, anyway, but this is a very common line of thinking brought on by Gatorade as they have punted their products as preventing hyponatremia. This leads people to think just what you have, Stefan---I can drink all I want if only I ingest some sodium with it.

Truth be told, however, is that even a sports drink will dilute your sodium concentration when ingested above the level of thirst.

So the take home message here is to drink to thirst. As humans we have a highly-evolved thirst mechanism that prevents us from drinking too much, but also prevents us from drinking too little.

Kind Regards,
Jonathan Dugas


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