Conducting a Sweat Test

Yesterday I ran into a bit of trouble with dehydration at the Evergreen Olympic Triathlon.  It was hot, I was low of fluids from the day before, I didn't take the race seriously enough, and it got the best of me. 

I'll cover more about that in my upcoming race report, but for now, it's sufficient to say I needed to start taking my hydration more seriously for my upcoming 70.3.  So, for this mornings bike ride, I included a sweat test, which was designed to measure how much fluid I lost during the course of exercise.  While am planning to repeat this test a few times, and use both runs and bikes, I have to admit the preliminary results are very surprising.

This is what I did, and could be used as a "how to" to do your own sweat test.

Step 1.  Wake up, eat normally, hydrate normally, and get ready for your workout.  Before you put your workout clothes on, weigh yourself.  I was 180.4 this am, fully hydrated, with some oatmeal in me, and ready to go.

Step 2.  Record the volume of fluids you consume.  To do this I weighed the bottles I was using before and after the test.  I did a 3 hour bike with Lana today and during that time I drank 1138g of water and 1297.4g of Gatorade.  I also "relieved" myself of about 250g of fluid.  After the ride I consumed 373g of recovery drink.

Step 3.  After your workout, shower, dry off very well, and weigh yourself again.  Post workout I was 177.6 pounds.

Step 4.  Tally the amount of fluid you lost.  I converted everything to grams and then added/subtracted it all.

Starting Weight	81827.64
Small Bottle	569
Small Bottle	569
Large Bottle	706
20 oz gatoraid	591.4
Bathroom Break	-250
Recovery Drink	373
Net Starting Weight	84386.04
Ending Weight	80557.58
Sweat Loss	3828.45	grams
	8.44	pounds
	129.47	fluid oz
Per Minute	0.63
Per Hour	38.08	fluid oz

Results

And there you have it. Biking at about 150W, in 82 degree weather, with 72% humidity I lost 38 oz of water per hour.  I was only taking in about 12 oz of water per hour for the first two hours of the race, which was done at a higher intensity.  Additionally the temperature was much higher yesterday.  It seems clear that my hydration was insufficient to say the least. 

It's probably not reasonable for me to take in 38 oz of water per hour during my upcoming race, however I think this shows the important of going into the event as hydrated as possible, taking in at least a bottle an hour, and not letting myself get behind on electrolytes as well, since I'm likely losing 546-2187mg of sodium per hour along with that 38 oz of fluid (460-1840 mg/L, I'm likely on the high end of that scale based on how much salt is on my clothes).