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How salty is your sweat? A home test kit

March 17th, 2010

I just noticed, a little belatedly, that an article I wrote for the March issue of Canadian Running magazine is now available online. It describes my experiences with a home sweat analysis kit from Medion Corporation, and compares the results to a laboratory sweat test I did with Lawrence Spriet of the University of Guelph and the Gatorade Sports Science Institute.

In the name of science, I dabbed some shaving cream on the back of my leg and scraped clear a fist-sized area of bare skin. I was about to undertake a new home sweat test to find out exactly how much salt my sweat contains, and I needed to make sure the absorbent patches would stay glued to my skin once the fluid started to flow…

It was an interesting experience, and the home test kit was pretty neat (though a little pricey at $250). It measured a lower sodium concentration than I got in the lab test, something that I think may be due to the fact that I was dehydrated before the lab test (I cycled for 40 minutes in hot sun just to get to the lab).

Overall, I’m not sure what to make of this information. I’ve just been reading The Runner’s Body, the book by the Science of Sport bloggers, and they argue that the theory linking electrolyte loss to muscle cramps is mistaken (a topic I’m looking forward to digging into a little more deeply). Personally, I’ve generally focused on (relatively) shorter distances, so my training runs don’t tend to be multi-hour affairs — which means I’ve never really worried about electrolytes. But for marathoners and triathletes out there, is knowing how salty your sweat is a piece of information that would help you plan your hydration strategy?

  1. MichaelMc
    March 31st, 2010 at 15:16 | #1

    Hi Alex, to answer your question before launching into my monologue: I don’t beleive knowing my sweat composition would be of any benefit in my marathons or Tris. I think it will vary based on my diet and prevailing conditions.

    I’m glad you found the “Science of Sport” guys. I read your article in Canadian running and was going to take issue with it. There are still some intriguing grey areas in the science, but I find it astonishing how frequently I hear advice given to runners that is downright dangerous. “Drink as much as possible” is truly bad advice, and sports drink companies were pushing it for a long time. The surprizing frequency of hyponatremia (see this study: http://content.nejm.org/cgi/content/full/352/15/1550 )has finally started to moderate this, but these companies seem to have simply found something else to sell us: electrolytes.

    The “everybody needs to take electrolytes” bandwagon is really getting rev’d up, and you’ll note most of the information out there is by companies that want to sell us electrolytes. They even want to change the definition of hyponatremia from overdrinking to “under electrolyting”. You’ll note from the study quoted in the Science of Sport that electrolyte supplementation didn’t alter the outcome for runners. As a point of interest I know a fair number of runners who have taken the (Gatorade supported) “sweat analysis” and ALL of them have been “salty sweaters”. What are the odds ALL of them are on one side of average; do you know of anyone they have told “you don’t need to supplement”?

    Humans evolved to run long distances in hot places, it would seem logical we’d have developed the neccessities to sense when we need water and salt. Drinking to thirst keeps electrolytes in perfect balance (since that is what drives thirst), and runners who do this in races NEVER develop hyponatremia, and seem to suffer no worse from dehydration or heat exhaustion.

    There are many uncertainties introduced once you try to outguess the body by taking in electrolytes in significant quantities. Bodies alter the saltiness of sweat in reaction to body chemistry, the absorption of ingested electrolytes is variable, and salt intake alters the body’s (otherwise reliable) thirst gauge. In so doing you are suggesting an educated guess on electrolyte and water intake will have a better outcome than using your body’s natural system. I am not so bold.

    I encourage everyone to do some research with a critical eye toward who is behind the studies and stands to profit from it. Electrolyte supplementation does NOT reduce cramps, and has not been shown to reduce incidence of hyponatremia. In very small quantities in sports drinks (and presumably gel/water) it seems to improve absorption of water and carbohydrates, but otherwise there seems no benefit and some real potential harm.

  2. alex
    March 31st, 2010 at 22:24 | #2

    Thanks for the comments, Michael. Indeed, I now wish my Canadian Running article had taken a little more space to discuss the uncertainties in the research.

    That being said, we should also take the Science of Sport guys in context. They both graduated from the University of Cape Town, which is where Martin Schwellnus is a professor — so it’s not surprising that they’ve had plenty of exposure to his alternate theory of cramp formation. I find the arguments quite persuasive, but when I’ve asked other exercise scientists, they’re not entirely convinced.

    For instance, one researcher I asked about it pointed me to one of the key Schwellnus papers (http://www.ncbi.nlm.nih.gov/pubmed/15273192), which is called “Serum electrolyte concentrations and hydration status are not associated with exercise associated muscle cramping (EAMC) in distance runners.” But if you look at the data in the paper, they actually found that the crampers had statistically significant lower levels of sodium than non-crampers after the race, even though both groups started out with the same levels. They dismiss these differences as “not clinically significant” — but still, it’s an indication that the science is by no means settled in either direction here.

    Overall, though, I agree with you, and share your skepticism about much of the “conventional wisdom” we’ve accepted as truth.

  3. MichaelMc
    April 6th, 2010 at 21:26 | #3

    Thanks for the thoughtful response. Professor Swellnus is certainly in the forefront of research on this, along with Professor Noakes, and the Science of Sport guys are alumni. Having said that, most studies I’ve seen support the concept that people have reserves of sodium which can be mobilized if required, and people cramp with sodium concentrations in the “normal” range. http://www.ncbi.nlm.nih.gov/pubmed/16344476 is more to do with hyponatremia, but applies. http://www.ncbi.nlm.nih.gov/pubmed/20351595 is fairly directly applicable.

    I understand companies with proposed solutions to issues which concern your readership are inherently newsworthy, but oftentimes the coverage is seen as confirmation of the validity of their claims. The anecdotal support is weak evidence, really, as most runners lose the tendency to cramp as they gain experience: many who were “cured” by electrolytes might have been cured with a placebo as well. If this WAS a placebo I wouldn’t be so worried about it, but IMHO messing with electrolytes can cause problems.

    You’re not in the “mythbuster” business, but personally I’m more comfortable with less credulous articles, especially in technical areas. The “Sports Science” companies pump out loads of “information” and few people have the desire or ability to pull the wheat from the chaff. I appreciate your openess to discussion.

  4. Matt
    May 6th, 2011 at 13:37 | #4

    Hey guys, I know this is a bit old but I feel that I can help out a bit here.

    Mike, the facts are: the more you sweat during exercise (without fluid replacement), the more your plasma volume decreases. As a result 1) heart rate increases, 2) stroke volume decreases, 3) cardiac output decreases, 4) skin blood flow decreases (the demand at the muscle wins over sending blood to the skin to thermoregulate), and 5) core temperature increases. All of these result in premature fatigue and the purpose of proper hydration during exercise is to minimize these side-effects.

    Hyponatremia is different, it results from fluid traveling out of your blood, into the cells of the body because of an ion imbalance. Where it become fatal is when your brain cells, which are limited by the size of your skull, swell too much because of this imbalance. This develops when the fluid a person invests is hypotonic compared to the sweat they are losing (ie the sodium concentration in their drink is less than in their sweat). The goal of proper electrolyte supplementation is to prevent drops in blood sodium during exercise, which in turn provides the osmotic pull to keep fluid in the vascular compartment.

    Alex, the biggest issue with your comparison is the sweat collection site for the lab and home tests. Did you use the same site? Sodium composition varies greatly over different sites on the body, and no one site is representative of whole body sweat sodium concentration. You can calculate based on readings from multiple sites, or apply a correction factor but nothing will be as precise as a whole body washdown where all of your sweat is collected and analyzed. Also, there is some work out to show that the patches themselves cause an overestimation of sweat sodium content, so this should be applied to your results before using them as well.

    In summary, proper fluid and electrolyte supplementation is definitely beneficial for longer duration (2+ hours) exercise, especially in the heat. As well, it will help you recover from that exercise by making it easier for you to get back into fluid and sodium balance sooner.

  5. Matt
    May 6th, 2011 at 13:40 | #5

    Invests = ingests in the hyponatremia section. Autocorrect caught me on that one.

  6. alex
    May 6th, 2011 at 14:18 | #6

    @Matt
    Thanks for the comments, Matt. It’s certainly true that the sweat was collected from different sites (a factor that was discussed in the Canadian Running article I linked to). Dehydration was also likely a factor, as confirmed by measurements of my urine’s specific gravity.

    One quick point about hypenatremia and electrolytes. You wrote:

    “This develops when the fluid a person ingests is hypotonic compared to the sweat they are losing (ie the sodium concentration in their drink is less than in their sweat). The goal of proper electrolyte supplementation is to prevent drops in blood sodium during exercise, which in turn provides the osmotic pull to keep fluid in the vascular compartment.”

    Whether the electrolyte concentration of a drink has any PRACTICAL effect on plasma sodium levels (and thus on risk of hyponatremia) is, of course, a hotly disputed topic these days, as discussed in this post.

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