Why Runners Need to Know Their Sweat Rate

Molly Hurford
by Molly Hurford
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Why Runners Need to Know Their Sweat Rate

Almost every state in the U.S. has been feeling the heat this summer, and no one feels it more than runners. A hot day can make even a short run feel almost unbearably difficult, and even worse, it can lead to potential dangers like heat stroke and dehydration. We all know we should be staying hydrated on the run, but there have been plenty of times where you forgot a water bottle or didn’t bother refilling it when you decided to add a few extra miles to your planned run. Bad news for those who aren’t the best at sipping while sprinting: A new meta-analysis has shown even minor dehydration can mean major cognitive fuzziness.

As we dehydrate, our concentration and ability to stay alert drops significantly, according to the leader of the study, Mindy Millard-Stafford, a professor in Georgia Tech’s School of Biological Sciences. She says things like focusing while driving, staying attentive during a snoozefest of a meeting at work or any kind of monotonous task can be made much harder when dehydration is introduced.

The study done at Georgia Tech noted that even two hours of yard work, done on a hot day without breaking to grab water, can blunt concentration. With so many of us sneaking out for a lunch run while at the office or getting in long runs on the weekend, the risk for minor dehydration is high.


The risks begin with 1–6% loss of body mass due to dehydration and become more severe when dehydration levels rise to 2%. For a 150-pound runner, 1% body mass loss would mean dropping 1.5 pounds during a run — think about the times you could wring out your shirt or shorts at the end of a run, and now consider how easy it is to sweat out 16 ounces of water, especially if you’re a heavy sweater.

“If you weigh 200 pounds and you work out for a few of hours, you drop four pounds, and that’s 2% body mass,” Millard-Stafford said in the study’s press release. “With an hour of moderately intense activity, with a temperature in the mid-80s, and moderate humidity, it’s not uncommon to lose a little over 2 pounds of water.”

(To find out how much you sweat during a run, especially when you know dehydration on that specific day may be an issue, hop on the scale before you head out, and get back on when you get home.)

The study originally points out potential problems for those working in industries that could be dangerous if cognitive function is impaired, like construction on a hot day or soldiers after a long training day. But Millard-Stafford says the impact of dehydration on cognitive function pertains to athletes as well, so runners are certainly at risk.


Brain fog on the run can lead to more than just not being able to find your house on your street — road runners know how important it is to stay alert to traffic, while trail runners know it’s key to focus hard on the technical terrain in front of them.

While you should be drinking during your run, either using a handheld water bottle or a hydration vest, drinking ahead of your run and re-hydrating post-run are equally key in bringing your hydration status back to normal — especially if you need to get back to work in the afternoon.

If you’re leaving a car at a trailhead, make sure you sip some cool water on the way there, and have cold, fresh water to come back to. The UA MVP Stainless Steel Tumbler is perfect for this, since water stays chilled in it for 14–24 hours, even if your car is sweltering.

About the Author

Molly Hurford
Molly Hurford

Molly is an outdoor adventurer and professional nomad obsessed with all things running, nutrition, cycling and movement-related. When not outside, she’s writing and podcasting about being outside, training and health. You can follow along with her adventures on Instagram at @mollyjhurford.


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