The Sweat Test

Tri Faster LLC Todd Jensen, PhD
Tri Faster LLC

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How much should I drink during training or racing? There are literally thousands of articles on the web and in magazines regarding this subject, and most recommend somewhere between 16-24 oz of fluid per hour during heavy exercising. Is this enough? Is it too much?

Most people will start to notice a degradation in their athletic performance when they lose 2-3% of their body weight during a bout of exercise due to sweat loss. For a 150 pound person, that translates to 3-4.5 pounds. While that seems like a lot, I personally can drop 6-8 pounds per hour on a sunny, hot, and humid day. If I drink the recommended amount of fluid and do the conversion of 16 ounces is one pound, I might limit my loss to “only” 4.5-6.5 pounds per hour. One can quickly guess that I would need to be transported to an emergency room if I stuck to that protocol.

So, how do you know how much you need to drink to stay hydrated? The Sweat Test! It is fairly simple and not as gross as it sounds. The protocol:

  1. Weigh yourself while dry before exercise
  2. Exercise, keeping track of how much you drink in ounces and how long you exercise.
  3. Weight yourself while dry after exercise.
  4. Use the calculations below to determine your sweat rate.

Your change in weight in ounces will be:

[ (Pre-exercise wt) – (Post-exercise wt) ] x 16 ] = (Wt Change in Ozs)

But you need to account for the weight “gained” back from drinking, so add the amount you drank to get your total sweat loss:

(Wt Change in Ozs) + (Fluid Consumed in Ozs) = (Sweat Loss in Ozs)

Now divide by the number of hours you exercised to get your sweat rate:

(Sweat Loss in Ozs) / (Hours Exercised) = (Sweat Rate Ozs/Hr)

Using my recent four-hour long ride on a 90F day as an example:

Wt Change in Ozs = [ ( 160# ) – ( 152# ) ] x 16 = 128 oz Sweat Loss in Ozs = 128 oz + ( 4 x 24 oz bottles ) = 224 oz Sweat Rate Ozs/Hr = 224 oz / 4 hr = 56 oz / hr

How do we use this number? To stay perfectly hydrated under similar conditions, I would need to consume at least 56 oz/hr of fluids – much higher than most recommended articles would every suggest. While this is on the high end, we suggest all our athletes do this test for all three sports and under various weather conditions to be better informed on their personal hydration needs.

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