Why I Got My Sweat Tested and What I Learned

October 05, 2017, Updated March 2024

In order to perform our best, we need to replace exactly what we lose in our sweat. But no sweat is created equal. Skratch Athlete Abby Levene shares why she got her sweat tested and what she learned.

Photos: Mike Thurk

You have a headache, feel confused, exhausted. You’re dying of thirst. You fill up an entire water bottle with refreshingly icy cold water. You pour it all into your mouth. You feel better…

...for approximately 60 seconds.

Then you feel just as crappy as you did before.

And then you immediately have to pee.

Has this extremely aggravating sequence of events ever happened to you? Or am I the only high maintenance one with no bladder control?

Okay, the bladder control issue is probably just me. But I think I was like most people in assuming that hydrating is hydrating. Water in, water out.

I implemented this philosophy during my first 50k a year ago, drinking a lot of fluids-- almost all in the form of water. 20 miles in, I was crying to the aid station volunteers asking how much further until the next aid station. I felt like I had been spinning around in a washing machine for the past two hours. Which way was up? Forward? SALT! I need salt! The last 11 miles dragged by in a blur of trying to make myself eat a pretzel stick, which might as well have been a dog biscuit for how edible it seemed to my bleary mind and churning stomach. (In case you are wondering, most of the pretzel ended up in a sweaty, disintegrated ball in my Naked Band. Yummy!)

Turns out -- I was forced to learn through the consequences of my naivety-- sweat isn’t just water. It contains electrolytes, especially sodium. And sodium is pretty critical to athletic performance.**

Sodium helps your gut absorb nutrients, keeps your mind sharp, and is essential for nerve impulse transmission and muscle contraction. In other words, it’s critical not only for exercising but also for just existing. And when you lose too much of it, the sodium in your blood gets diluted, which can result in a very scary condition called hyponatremia. (Imagine washing machine status on steroids.)

That’s why it’s so important to drink fluids with electrolytes (shocking to read that on a blog for a hydration company, I know). The catch is that not everyone’s sweat contains the same amount of sodium/electrolytes. People lose anywhere from 350 mg of sodium per liter of sweat to 2,000 mg/l. That’s a huge range! Like yuge huge (okay, no bigger than that)! If you want to hydrate properly, you need to know both how much fluid you lose and how much sodium.  

Sweat Test
That’s why Skratch mastermind Allen performs sweat tests to measure the concentration of sodium in your sweat. Skratch recently bestowed me with the honor of guinea-pigging this test. It was quite simple, and relatively painless.

If I am not training/racing, I am quite lazy. So thank god for me, sweat can actually be induced without exercising (!!). It’s a super not sketchy procedure (promise!) in which this drug called Pilocarpine is applied to a 1.5-inch diameter section of your forearm along with electrical stimulation. This process causes that small section of your skin to sweat.

A little plastic disk called a Macroduct Collector, also attached to your skin, collects the sweat. The sweat enters what looks like a spiral. Thanks to blue food dye, a line of blue sweat swirls around and around. After the sweat is collected, it’s attached to a sweat sodium analyzer that determines how much sodium your sweat contains. Magic!

Turns out my sweat concentration is pretty standard: 900 mg/l. (The average is about 800 mg/l.) But Allen could tell I sweat, to use the technical term, a sh*t ton just based off of how quickly my sweat collected in the Macroduct.

“I can’t really impress upon you how drastic of a difference being properly salted makes. ”
What I Learned
The verdict: I need to preemptively hydrate with electrolytes before extreme exercise/racing because I am going to lose a lot of fluids, and hence a lot of electrolytes. (My go-to preemptive hydration saviors are Wellness and Hyper which can be recreated with Skratch Everyday Mix and Sport) But during activity, I’m fine replenishing with normal Skratch, whose sodium concentration correlates to the average person’s sweat sodium concentration.

The best part about this test: it justifies my copious Skratch consumption. Well actually before this test, I was pretty blase about my sodium intake. Don’t feel like drinking plain water today? Okay, I will treat myself to some coffee. Now I treat my sodium intake as seriously as my food intake (which is very seriously). Before big efforts, especially on hot days, I pound Hyper or Wellness to top off my salt stores and help prevent total depletion.

The other major change I’ve made post-test-enlightenment is to be diligent about drinking Skratch during long runs (aka runs that last two to six hours) instead of just plain water. Since my salt sweat rate is pretty standard, I usually just drink the standard Skratch ratio. And on easy days, I chug a bottle of Skratch after running.

I can’t really impress upon you how drastic of a difference being properly salted makes.

Fast forward to my next 50k, post sweat test. I slammed Hyper before the race and lucked out-- Skratch sponsored the event so there were at least five flavors of ice cold Skratch to choose from at every aid station. (Who are we kidding I didn’t luck out; a large reason I chose this race was because Skratch was sponsoring it.) Sure, I furiously had to pee at mile five from chugging 20oz of salty mango elixir of life, but I felt alert, fresh, and “fast” for the entire 31 miles-- despite being drenched in sweat.

**Other electrolytes like potassium, magnesium, and calcium are also critical to athletic performance/staying alive. Potassium also helps prevent post-exercise exhaustion, supports intense training, and helps replenish glycogen stores by aiding the conversion of glucose to glycogen. If you don’t get enough magnesium, you may experience inflammation, soreness, and sleep issues. And calcium is crucial for potentially vital physiological tasks like your heart contracting.

The DIY Until You Can Get Your Sweat Tested Properly
One of the easiest ways to get at your sweat rate is to use a scale to weigh yourself before and after a workout. Any weight loss (where 1 lb is equal to 16 oz of water) plus the amount of fluid consumed while working out is your total sweat loss. That all said, if you're gaining weight over the course of a workout, then you are drinking too much. Ideally, try not to lose more than 3% of your body weight from dehydration.

An important consideration, however, is that rehydration during exercise isn't just about water balance, it's also about sodium balance. What this means is that to adequately hydrate, you need to replace both the water and the sodium that you lose during exercise. Unfortunately, sweat sodium is highly individual and can vary anywhere from 400 mg to 2000 mg per liter of sweat, with an average somewhere between 700-900 mg per liter (Skratch Sport Hydration is 800 mg per liter). As a general rule of thumb, if you're not consuming enough salt relative to water, you'll find yourself needing to urinate frequently despite losing body weight due to sweat loss. On the other hand, if you're consuming too much salt relative to water, you'll find yourself wanting to drink more water and potentially gaining weight as the high salt intake might drive you to drink at a greater rate than your sweat loss.

Remember, hydration is about 2 needs:

1) Water Balance - Measure with a scale pre & post workout

2) Sodium Balance - Measure by listening to thirst

So....drink to thirst with Sport Hydration Mix while you are working out or racing and if you are underweight after your workout/race add more salt! Questions about it or ways to add more salt? Hit us up: info@skratchlabs.com.

Abby Levene is a pro "short course ultra runner" for adidas and a Skratch Labs Athlete. When she's not busy sweating or grocery shopping, you can find her working for Verde Brand Communications, writing, and hydrating. She ran cross country and track and field for Princeton University in undergrad and CU Boulder in grad school before dipping into triathlon, where she won four national championships. Now you can find her tripping over rocks in Boulder, CO. Follow her on Instagram @aplevene.

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