The Science of Sweating

A sweat test can help make the difference in your performance
Staff Writer

By Scott Jurek—It was July 2005, and temperatures hovered in the mid-120s during the infamous 135-mile Badwater Ultramarathon. I ran through the blowtorch heat of Death Valley feeling like an ant burning under a magnifying glass.

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It was a challenge to find the balance between being slightly and severely dehydrated as I drank  60 ounces (three average water bottles) of water per hour while running 9:00 to 10:00 per mile pace. To monitor my hydration levels, my crew weighed me every hour or two to determine how much water weight I was losing and if I was drinking enough fluids to replace the loss.

A race like Badwater enables runners to monitor their hydration, but the typical half-marathoner, marathoner or ultramarathoner doesn’t have access to a scale during a race.

Knowing how to perform a sweat test is an effective and reliable method for determining how much you should drink to stay appropriately hydrated. Here’s how you do it.

  • Take a naked body weight before a training run.
  • Follow with a very easy, five- to 10-minute warm-up. Then run at your average race pace effort for one hour in the conditions you expect on race day. Keep track of any water you drink (measure in ounces) and avoid urinating, or any other fluid loss, if possible.
  • After one hour, remove your clothes, wipe off all the sweat from your body and hair and take another naked body weight.
  • Subtract your post-run weight from your pre-run weight and convert to ounces. (1 pound = 1 pint or 16 oz. of water). Add the number of ounces of fluid you drank if applicable.
  • This number will be the amount of ounces you should drink per hour to stay fully hydrated in similar conditions.Since the human body can be 1 to 2.5 percent of our bodyweight dehydrated before blood volume and performance is affected, you can find the minimum amount of fluid you should drink with one more calculation.
  • Multiply your pre-run naked body weight by 0.025.
  • Convert to ounces by multiplying by 16.
  • Subtract this number from your “fully” hydrated ounces in No. 5 and you will have your minimum ounces per hour.

Practice drinking this range of fluids while training, and be sure to drink at regular intervals of 10 to 15 minutes. The stomach empties quicker when you take gulps every 10 to 15 minutes versus sips every five minutes.

To prepare for alternate race conditions, including heat, humidity or altitude, repeat the test during your training in the altered conditions. You can also use this test while training for different race paces and when your fitness level changes. Keep in mind medications and certain foods (i.e. caffeine) affect hydration, so note what might be circulating in your body when performing the test. Your numbers are only as accurate as your most recent sweat test and the setting in which it was performed.

Based in Boulder, Colo., Scott Jurek is a seven-time winner of the Western States 100-mile trail run. Have a question for Scott? Ask him here!

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