Hooray for Science! Sweat Testing

Last year, a friend of mine told me about an at home sweat test she did where she got info on her sweat rate and electrolyte loss.  I've done a few sweat tests to figure out my sweat rate (weighing yourself before and after a workout) but those were informal tests that didn't give me any info about electrolyte loss.  I was intrigued, but didn't act on it....until I ended up in the med tent (again) after the Galveston 70.3.  It's time to figure out how to end this med tent streak.

So I ordered a test from Levelen.  A couple days later a package showed up in the mail containing my sweat test kit - an alcohol swab, cotton swab, a little plastic vile, tweezers and info card. 

The cotton swab on my arm....ready to start sweating.

You're supposed to replicate race conditions so I rode indoors and cranked the heat and humidity and got the temp up around 80 with 70% humidity.  I didn't use a fan (mistake) because I wanted to make sure I'd sweat enough to get good results.  I rode for an hour at Ironman power, but the heat, humidity and lack of a fan made for a pretty intense workout.  I was burning up and ready for the hour to end so I could drink something and cool down.

As soon as I finished I used the tweezers to remove the swab and put it in the vile and I weighed myself.  I had lost 3.8 pounds - 60 oz.  Ouch.

I dropped everything in the mail the next day and a few days later they emailed results...


 I was and wasn't happy to see these results.  This does help explain why I keep ending up in the med tent, especially the significant electrolyte losses.  This will definitely help me prepare for races, but losing a lot of fluid and electrolytes can definitely make long course races more challenging than they already are, especially hot races.  

But now I have some real numbers to work with.

I lose:
- 53 mmol of sodium per liter of sweat
- 8.1 mmol of potassium per liter of sweat
- 51 mmol of chloride per liter of sweat

That comes to about 1200 mg of sodium per liter of sweat (34 oz).  I've done a few sweat tests in the past and on hot days outdoors I tend to lose between 40-50 oz per hour.  That's about 1400-1750 mg of sodium lost per hour.  

They recommend replacing 60-70% of fluid and electrolytes lost.  So I need to drink around 30-36 oz per hour and take in about 850-1200 mg of sodium per hour for a hot race.  I'm probably getting about 300-400 mg of sodium per hour right now...maybe.

I asked them if electrolyte losses stay the same per liter of sweat lost regardless of sweat rate and they said yes, so now that I know how much I'm losing I can adjust my intake based on conditions and how much I think I'll sweat.  

I got my results back about a week ago, so about 3 weeks before IMTX.  It's a little late in the game to make changes, but what I'm currently doing isn't working so I'm taking a chance and more than doubling my electrolyte intake.    



Anonymous said...

Doesn't sweat rate change once we become more acclimated? I assume that you have not done that much heat training, if any.


Mike said...

Yes, but I've done a few sweat tests at times when I was heat acclimated so I think I have a good general idea what my sweat rate will be. I guess we'll see on race day. :)