Marathon Training Update: Sweat Rate Test

sweatWhile writing yesterday’s post about how I lost about 8 pounds during Saturday’s 12 mile run, the thing I realized I need to do first is make a more accurate assessment of how much fluid I’m actually losing by doing a sweat rate test.  So, that’s what I did this morning in conjunction with my short run.

A sweat rate test is not rocket science.  Anyone can do it.  You simply weigh yourself (nude) before the run, keep track of any fluids you consume during the run, and then weigh yourself (nude and toweled off) after the run.

This Morning’s Sweat Rate
So, this morning I weighed in at 164.5 pounds.  (BTW, it’s good idea to use the restroom before you weigh in.  If you do so between weighings you’ll mess up the results.)  Then I drank 32 ounces of water right before I left on my 6 mile run (more on that later)  I ran a 3 mile route, drank a 16 ounce bottle of Gatorade I had left in my driveway, and then ran another 3 miles.  Went back into the house, stripped off my sopping wet clothes, and then weighed myself.  The scale read 164.0 pounds.

Let’s do the math…

Take the 164.5 starting weight, subtract the 164.0 end weight, and I lost a half pound or 8 ounces.  Add to that the 48 ounces of fluid I took in between weighings and my total fluid loss was 56 ounces.  Divide that by 6 miles, and we’re talking about 9.3 ounces per mile.

On an interesting side note… I continued to sweat profusely after the second weighing.  In fact, I felt like I was sweating at a greater rate after the run than during it (though that could be just because I was annoyed by all the sweat dripping on my kitchen floor while weighing and getting drinks).    So, out of curiosity… after I drank a 12 ounce glass of Gatorade while stretching and drank a 12 ounce glass of chocolate milk while soaking my legs in a cold bath, I weighed myself again.  I was up to 165.0.  The scale is measures within a half pound, so precision here isn’t great, but if you figure 24 ounces injected minus 16 ounces gained, that means I sweated about another 8 ounces while stretching.

Sweat Rate Test Adjustments
So anyways, the 9.3 ounces/mile measured today isn’t quite the 12.3 ounces/mile estimated from Saturday’s run, but it’s still pretty high.  There are also a couple of possible factors that could bring those two numbers closer together.

First, after Saturday’s run I walked about a half mile to cool off before I weighed myself.  If I sweat 8 ounces while stretching after the run this morning, it’s like I sweat at least that much in the extra half mile Saturday.  If we subtract 8 ounces from the estimated 148 lost Saturday we get 140 ounces.

Second, perspiration doesn’t begin on the first step of a run.  In fact, I’m not sure I sweat at all in the first mile.  If that’s the case then for today we’re really talking about 56 ounces divided by 5 miles, which is 11.2 oz/mi, and for Saturday 140 ounces divided by 11 miles, which is 12.7 oz/mi.  That’s a difference of only 13 percent between the two.

Continued Tests
Saturday, I’m planning to do 13 miles and I’ll do another sweat rate test.  If I drink 32 ounces before leaving and a 16 ounce bottle between each of 4 loops, I’ll consume 80 ounces.  If the calculation of 11 ozs/mi over the last 12 miles is accurate, then I would lose 132 ozs, a net loss of 52 ozs or 3.3 pounds.  It will be interesting to see if that’s accurate.

BTW, drinking 32 ounces right before a run sounds crazy to me too, but what’s the better alternative?   Shorter loops?  Drinking more than 16 ozs at a time?  Becoming more dehydrated?

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