When I ran my first marathon, at check in there were lots of those big blue water bottles, loads of paper cups, and signs everywhere stating “DRINK!”  I dutifully drank a cup of water then tossed my paper cup after using it all of 3 seconds.  Later I ate a pasta dinner sufficient to make a rhinoceros barf.  A marathon is such a long way, hadn’t people actually starved to death prior to mile 23?  I began reading magazines with lots of articles on hydration, the importance of knowing your sweat rate to maintain body weight and the importance of drinking before thirst.  So, being somewhat expirementally inclined and with an available laboratory (me) I began measuring sweat rate under a variety of conditions.  Even going so far as running 30 miles on a hot day, recording temperature, weight and fluid intake each mile.  What did I learn from all this?  That the only time I could maintain weight was with the temperature below 60 deg and drinking until I could feel the sloshing in my stomach.  I kept wondering if there was anyone that actually maintained weight or if it was all just a big farce.  I got my answer.  At the Leadville 100 prerace meeting last year the race doctor stood up and said that the consensus of an international group of physicians, coaches and sports medicine professionals was that you should drink when thirsty.  No further comment on that.

So here is how it looks.  I used an exponential fit because it just feels right, though I don’t really think this is the best fit.  I’ve looked at sr versus temperature, humidity and pace but it primarily seems to be a function of temperature.  Below 50 deg I dress warmer, so I think that skews the data at the low end.

Sometimes I go running with my doctor.  We talk about things like whether sweat rate changes over time with improvements in maximal oxygen uptake.  We decided that both should rise and fall together but don’t know if it is a direct relationship.  I think I am his only patient who shows up for an office visit with a lap top.

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