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Hydration 101 and the Sweat Test

Running in the summer heat can be difficult. If you have a long run or race planned, hydration should be one of your primary focuses. It doesn’t even have to be an ultra—

anything over 90 minutes in extreme heat or humidity can become dangerous if hydration goes awry.


If you’re struggling with over/under hydrating, you may want to perform a sweat test to get a baseline of how much sweat you’re losing per hour.



How do you do this? It's pretty simple!

  1. Relieve your bladder, get naked and weigh yourself

  2. Go for a 60 minute run (preferably in the conditions you typically run in)

  3. When you get back from your run, dry off and weigh yourself again

  4. Convert the weight lost to ounces and subtract any water you might have consumed during the run

  5. The difference in your starting weight vs. the ending weight (minus any water consumed) is your sweat rate.


For example, if you perform a sweat test in hot weather, run for one hour, consume no liquids and you lose 1 pound, that means you have lost 16 oz of water weight. You need to replace AT LEAST 16 oz of water per hour to stay out of a deficit. Even a 1% deficit can affect heart rate, fatigue/effort levels and more.


Once you are running past the one hour mark, electrolytes become increasingly important in addition to just drinking water. Although this can vary widely from person to person, 250-500+ mg of salt per hour may be needed to replace what is lost through sweat. This takes some practice to get just right--so practice a lot before your race!



For many runners, hydrating on thirst cues alone become difficult in the summer. Knowing your sweat rate can be helpful for planning on how much water to drink for a long run. Then you can play around with the amount of salt/hour to nail hydration 100%.



To stay safe in the heat, stay hydrated!


-Dehydration not only increases the risk of heat stroke, but also impairs your ability to breakdown and utilize carbohydrates….causing you to hit the dreaded wall sooner than normal.


-Anything past 60 minutes in the heat — add in sodium through sports drinks, salt pills, or salty foods. Signs of electrolyte imbalances can manifest as swollen hands, nausea, fatigue, headaches and heat stroke.

-Other tips for running in the heat: slow down. Heat and humidity can raise your heart rate, so running in these conditions often feels much harder


-Use ice and cold water to help cool you down

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