sweat test

How a Sweat Test Changed This Coach’s View on Sodium Supplementation

By Adam St. Pierre,
CTS Ultrarunning Coach

For a few years now, I’ve been fairly critical of sweat sodium concentration testing. In my own personal experience, I’ve suffered more in races when I’ve taken in too much salt, rather than not enough. I’ve also seen, in both races and training, that the amount of salt residue on my shirt/pack/hat at the end of a run seems to correlate directly with the amount of sodium I take in while running. I thought this (along with some bad experiences taking in too much sodium at Western States 100 in 2011) was sufficient evidence to support the idea that sodium supplementation was “bunk”- a marketing ploy.

I planned my races and counseled athletes to: “Know your sweat rate in a variety of conditions, and ensure that you have sufficient fluids to prevent a large amount of dehydration as measured by body mass loss during training and racing. You will consume sufficient sodium in your running foods and sports drinks to replace enough of what you lose, so don’t worry about taking in any more or really tracking your sodium intake.”

This is the biggest problem with N=1 evidence, generalizing my experience to others.

I recently attended the TrainingPeaks Endurance Coaching Summit in Boulder, CO. Andy Blow, a former elite triathlete and Team Sports Scientist for Benetton and Renault F1 teams, gave one of the most useful talks I attended. I went to his talk, titled ‘Drink to Thirst or Drink to a Plan?’ expecting Andy – now the founder of Precision Hydration – to try to sell product or push drinking to a plan at all times. Instead, I was treated to a balanced discussion of the times/conditions when drinking to thirst is adequate and where having a plan is necessary, and the spectrum that exists between the 2 extreme positions.

When to Drink to Plan and When to Drink to Thirst

In general, the hotter/more humid the conditions, the longer the event, and the less experienced the athlete – the more necessary it is to have a plan. Shorter races, cooler conditions, and more experienced athletes can “wing it” with less chance for negative repercussions ­– these athletes can drink to thirst. Of course, the caveat here is that heat/humidity/duration/experience level are all relative based on what an athlete is accustomed to.

Sweat Sodium Testing

Andy and the crew were offering free sweat sodium testing to all Summit attendees, so I got tested. (Disclaimer: I have no financial or endorsement relationship with Andy Blow or his company.) The test starts with 15 minutes of relaxing while an electrode placed on the forearm stimulates sweat glands. Then a small disc is placed on the sweaty spot to absorb sweat via capillary action. When the capillary tube is full enough, the tester takes the sweat out and injects it into a sodium analyzer. The analyzer measures the concentration of sodium in mmols/L, which can be converted to mg/L for ease of planning.

It turns out I have a pretty low sweat rate and a moderate sweat sodium concentration. With this combination, I’ve been able to get through ultras without an extreme need to replenish sodium stores. My n=1 experiences made sense… for me and the way I sweat. If I was a heavier sweater OR had a higher sweat sodium concentration, then I likely would not have been successful in those events, as my sodium losses would have been too large for my minimal intake to cover for.

I’ve had some reasonable success as an ultrarunner, despite not intentionally supplementing sodium while running. However, another factor in my experiences is that I have done most of my ultras in moderate conditions. I’ve never raced a super hot or humid event. If any of my races had been on a hotter/more humid day, I may have had worse results from my method of planning fluid intake but not planning sodium intake.

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Sweat Concentration and Pre-Test Sodium Intake

In the past I’ve been reluctant to have my sweat sodium concentration tested, or to recommend testing to my athletes. My contention was that your sweat sodium concentration is impacted by the amount of sodium you take in; take in a lot, sweat out a lot; take in a little, sweat out a little. This view isn’t supported by anything more than anecdotal evidence, but also isn’t disproved by any available evidence.

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Being a bit of a skeptic… I did a second sweat sodium test later in the day… after consuming about 5000mg of sodium over the course of 2 hours. I thought for sure that I would sweat out more sodium in the second test because of the large dose. When I tested again, my sweat sodium concentration was about the same. I’d be interested in additional tests, during exercise, day to day… but for now I’m content that my own personal sweat sodium concentration is around 1000mg/L and will utilize this information for a more scientific approach to my sodium intake during training and racing.

Practical recommendations:

  • Measure your sweat rate. Weigh yourself nude before a run. Go for a 1-2 hour run. Don’t drink anything on the run, don’t urinate either. Weigh yourself nude after the run. Each pound of weight you lost is equivalent to 16oz of fluid (.47L). If you eat/drink or urinate while running you would have to factor that in, so it’s simpler not to.
  • Have your sweat sodium concentration tested. It is painless and simple and may be a beneficial starting point for dialing in your race day hydration plan (otherwise you may suffer a bit during trial and error). Knowing your sweat rate (the amount of sweat you lose) per hour in a variety of conditions is also crucial.
  • If you know that you lose 1000mg of Sodium per liter of fluid and that you lose about 1L of fluid per hour… then you should plan on consuming around 1000mg of Sodium per hour! You can get sodium from sports drinks, foods, and supplements as needed.
  • Use the above information to create a plan for training and racing to ensure that you stay within about 3-5% of your “normal” body weight and replace most of your sodium losses. It’s important to stick more closely to your plan during longer training sessions or events, hotter/more humid conditions, and when aid points are farther apart or infrequent. Less experienced athletes (or athletes who are inexperienced running in the conditions they will see on race day) may benefit from being more “to plan” while experienced athletes running in conditions they are accustomed to may be able to “wing it” successfully.

Comments 11

  1. It is worth noting, as I don’t believe it was mentioned in the article, that salt concentration in sweat is related to sweat rate, in an individual. This is because your body is trying to reabsorb the salt. So the faster the sweat is coming out of you, the less time you have to reabsorb the salt, and thus the more there is in the sweat. So the author’s quoted salt concentration ONLY holds for that sweat rate in them. There is plenty of research to support this, so it isn’t ground breaking. But it would be cool if there was a home test for athletes to do so they can check their rates at different intensities!

  2. Great article and I am happy to hear you enjoyed Andy’s presentation at the conference. I have been using Precision Hydration 1500 and the 500 levels for three years and have been Beverly happy with it on my long training miles and Ultras. I live in south Florida where we have very humid and hot conditions.

  3. Information very useful! nice blog and absolutely outstanding thank you for taking the time to publish this.

  4. In my experience, getting adequate hydration and electrolytes is fairly simple. I drink to pee. I feel that one needs to pee about every 3 hours or one is getting too dehydrated. So I make sure that I drink enough so that happens. If I don’t start to feel the urge in that last hour, I up my intake. I also drink to thirst. I should always be a little thirsty and have an urge to drink. If I don’t, I take more balanced electrolytes, which makes me more thirsty. Thus I take enough electrolytes to create thirst, a sufficient thirst that I will need to pee about every 3 hours. This is different from drinking or using electrolytes on a timetable, since it works no matter the weather or challenge. It has always kept me in balance.

    It’s convenient that we have a built-in mechanisms like thirst which keep us operational.

  5. I’m am quite sure that sodium is not the only electrolyte necessary to the hydration formulary when biking/running and that calcium, magnesium, potassium, to name a few; should also be an important part of keeping oneself properly hydrated. I have found from my experience as an athlete that an electrolyte/carbohydrate fluid really helps me when I am competing in long distance events. I have found various fluid formulary’s (not using proprietary names to remain objective) to work including coconut water with pulp.

  6. This article leaves one to question the ‘quality’ of information that comes out of the mouth/articles/books of such coaches. If he’s been doing N=1 (as in himself) as his information database, for how to coach athletes, then it very much leaves in question what other likely CTS coaches are doing as well.

    Let’s face it — for all the awesome endurance athletes who are lanky, tall, stick figures with VO2 max’s that are in the stratosphere, there are plenty of struggling wanna-be’s who are ~20+% body fat, pre/actual diabetic, and who have a VO2 max closer to 0, than that of a pro-Ironman.

    To have a ‘supposed’ coach admit his ‘lack of knowledge’ (or just narrow, lazy, close-mindedness) is somewhat refreshing — but should also call into question whether or not such coaches have your particular body/situation in mind.

    1. While this is probably harsher than I would have worded it, I do think this is a cautionary tale for coaches. The previous advice on sodium never seemed accurate for me, since I seem to need a great deal more to avoid nausea. But as a scientist, really it would be best to base advice on actual research studies, challenging as they are to perform. And at least recognize individual differences might play a larger role than you expect when advising other people. And everyone should always be trying to understand their own N= 1 selves and take a scientific approach to see what works best for yourself.

    2. JJ,

      I appreciate your comment. When working with athletes, I utilize a combination of my own athletic/life experience, my background knowledge (I have a masters degree in Exercise Science and did my masters research on the complex interaction between thirst, hydration status, and plasma sodium concentration), and the nuanced specifics of the particular athletes I work with (some are the high VO2max varieties, others have lower aerobic capacities). The art of coaching is taking what you know (or think you know- as best practices change on occasion) and applying it to unique individuals with whom you work. I provide my athletes with training recommendations geared toward their individual goals and lives. Similarly, I provide my athletes with race day plans geared towards their capacity and experience. Nothing is one size fits all.

      In this article, I meant to say that I now see the value in sweat sodium concentration testing, that I previously thought was not beneficial. My n=1 information database received new information, and has been modified. I would love to see further research into matters of hydration and electrolytes, and my database may change again. I don’t see that as narrow, lazy, or close-minded; quite the opposite in fact.


      Adam St.Pierre, MS
      CTS Coach

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