Dehydration impacts performance.  I know this, but I have never really figured out how much I should be drinking, to stay hydrated on my runs.  This thought has crossed my mind the past few days, now that the weather has really warmed up.

I had a one hour run workout to do this evening.  The run included 10 minutes of hard intervals, so I knew I would be sweating quite a lot.  This seemed to be the perfect workout to try and figure out how much fluid I am losing on hot days.  I decided to do a ‘sweat test’.

A sweat test is done by simply weighing yourself (naked) before and after your run.  Your fluid loss can then be calculated by the difference in weight plus the weight of any fluids or nutrition that you take in during the workout.

At the time of my run, the temperature was 78’F and the dew point was 62’F.  Not quite as hot as the past few days, but still pretty warm.  I guessed I would lose a couple of pounds during the run.

It turns out, that I underestimated how much weight I would lose.  I lost 3.5 pounds in an hour.  That is the equivalent of 56 fluid ounces of water and probably something like 1,750mg of sodium.  I was shocked it was that high, and clearly, I am not drinking enough to compensate for that loss on my runs.

This is obviously just one data point, but still an eye-opening one.  I will be doing a couple more tests in the next week or so.  That will allow me to plan my fluid intake for my runs, a little better moving forward.