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  • Writer's pictureJackie K

Why runners need to know their sweat rate

Did you know that getting even a little bit dehydrated can severely impact your running performance?

It's true! Your effort may increase (even though you're not going faster), your heart rate may go up, you'll likely feel lousy - nausea, headaches, lethargy... YUCK. And it only gets worse if you get more dehydrated -- with side effects becoming downright dangerous if it gets bad enough.

As little as 2% dehydration (that's 3 lbs in a 150-lb athlete!) can make it harder to run the way you want. In muggy conditions, that can happen quickly, especially if you're running for longer than an hour.

When you know your sweat rate, you can make sure you're drinking enough fluids on the run to prevent dehydration. You can match (or nearly match) your fluid intake to your sweat rate so you don't have the side effects that come from being dehydrated.

What is sweat rate?

Sweat rate is basically the amount of fluids you lose via sweat during a one-hour run.

Everyone's sweat rate is different - a 110-lb female is going to sweat a lot differently than a 220-lb male, for example.

When should I figure out my sweat rate?

Your sweat rate will vary based on temperature, humidity, effort, and even how well you're acclimated to the heat. For that reason, I'd recommend repeating the test for different weather conditions at the very least - hot and muggy summer months vs cool fall months vs cold and blistery winter months. You may even want to repeat the test once your body is used to the conditions.

In addition, if your fitness level is quite a bit different (increase OR decrease in fitness), then you should repeat the test again.

Moral of the story here is that your sweat rate isn't static; it can change. Don't do the test just once and expect that to be your sweat rate forever.

How do I figure out my sweat rate?

Step #1: Weigh yourself before your run, after you've gone to the bathroom, and ideally in the nude. (Example: 150 lbs)

Step #2: Go for a 1 hour run.

Step #3: Measure any fluids you consume during your run. (Example: 8 oz)

Step #4: Weigh yourself after your run, before you've gone to the bathroom, and again ideally in the nude. (Example: 148.8 lbs)

Now we figure out the difference! We're going to do a little math.

Remember: You lose 16oz of sweat for every 1 pound you lose.

First, subtract your finish weight from your starting weight. (Example: 150 lbs - 148.8 lbs = 1.2 lbs)

Second, multiply that number by 16 to get ounces lost. (Example: 1.2 x16 oz = 19.2 oz)

Third, add any fluids you consumed during your run. (Example: 19.2 oz + 8 oz = 27.2 oz)

Now you know how much you're sweating per hour in conditions like these!

What do I do with this information??

You can use this information to plan out how much you need to drink while running in order to prevent getting dehydrated! Let's say you're losing ~27 oz sweat per hour, but you're only drinking 8 oz. You're drinking wayyyyy less than what you need! Now you know you need to try to drink more, especially if you're going to be running longer distances.

You can also use this info to figure out how to rehydrate yourself. I always recommend drinking much more than what you lost in sweat within a couple of hours. Doing so will make you feel better the rest of the day and help make sure that you don't show up ALREADY dehydrated on your next run.

Let's implement it!

Knowing your sweat rate is one thing, but actually figuring out how to drink enough fluids during the run and afterwards can be tricky. The best fluids (for example: water vs electrolytes only vs sports drink) can vary from run to run and athlete to athlete. The best way to carry them is highly personal (for example: bottle vs belt?) as well. Then there's the pesky business of figuring out how to train your gut to get used to taking in fluids while you're running!

This is where I come in. As a sports dietitian and fellow runner, I want to help you figure out how YOU can stay hydrated - your conditions, your body, your goals. Hydration is a major part of fueling as a runner and it'll be nearly impossible to reach your goals if you try to do it while you're dehydrated.

If you want to reach your goals but you're having a hard time achieving them, then let's chat. I offer a limited number of free strategy sessions each week where we can discuss what your hydration challenges are (or any nutrition challenges) and talk through the next steps you need to take to overcome them. In no time at all, dehydration woes will be a thing of the past and you'll feel better than ever on the run.

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