I don't think there's an easy way on this. For me it was really trial and error and the error was ending up in the med tent with 2 IVs. It took me many years to realize I have very salty sweat and that if I don't take 8-900 mg sodium and hour I'll cramp up. From what I've read and talking with people who have done many IMs is that you start out taking the higher level (in training of course) and dial it back from there---to find your optimum sodium intake. I also salt load in addition to carbo loading 2-3 days out. I only salt load if it's going to be a really hot race. What I've been told is that you'll just piss out what you don't need.

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http://www.mountainmettle.com

A blood test will give you a base to work from. They'll be able to tell you if all your electrolyte levels - sodium, calcium, potassium etc are in the normal range. Mine were actually below normal a bit so I supplement them on a daily basis and really bump it up in the few weeks before a long race.

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