Calcuating Sweat Loss for Optimal Hydration
As you can see, maintaining proper hydration levels and electrolyte balance is critical for optimal performance and safety when running in the summer heat. The most efficient way to rehydrate properly is to put back exactly how much fluid you've lost while running. The Internet is filled with general advice on how much you need to drink in order to accomplish this goal: drink to thirst, which doesn't keep up with the body's sweat loss rate; or drink 8-10oz per hour, which doesn't factor in temperature or individual sweat rates.
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Unfortunately, there are two major issues with generalized advice when it comes to hydration. First, different temperatures and humidity levels will drastically influence your sweat-loss rate. Second, every runner has their own unique sweat rate and will be impacted by weather in a different way.
Humidity has a major impact on sweat rates (as I am sure you've noticed if you've done any running in muggy weather) because sweat can't evaporate to cool the skin off, which raises internal body temperature and produces more sweat. Therefore, it's critical that you factor humidity levels into your hydration strategy and not rely on general advice.
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How to Calculate (Your Own) Sweat Rate
Likewise, runners can have drastically different sweat rates—some runners can sweat twice as much as a similarly-conditioned runner. So, applying generalized advice might be too much water for you, but far under what your training partner may need.
This sounds like a terrible predicament, but calculating your exact fluid loss in any given temperature and humidity is actually quite easy if you use this sweat-loss calculator. All you need to input is your weight before each run, your weight after each run, any fluid taken in during your run, and the distance/time you run. The calculator will do the hard work for you.
By recording the temperature and humidity for that particular run, you'll now have the exact sweat-loss rate in those conditions. Do this a few times in different temperatures, and you'll have an easy reference chart of exactly how much fluid you need to take in on any given run in any given temperature.
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Not only is this useful in training to keep you safe and balanced, but it's also an amazing tool when you're racing. You'll know exactly how much fluid you'll need to take at each water stop or how much you'll need to carry with you. You can eliminate sloshing stomach issues or wasted time at water stops.
By taking the time to make a chart and approaching your hydration strategy in a methodical, scientific manner, you can easily beat the heat this summer and remove all fears related to dehydration and hyponatremia.
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