Given to Tri

Understanding My Sweat & Sodium Loss Rates

At some point during our drive to the medical tent after I dropped out of Ironman Coeur d’Alene, Mike, the race official who took me there, brushed off some of the salty residue on my trisuit and commented, “Looks like you’ve lost some salt there.” I was in such bad shape I don’t remember anything else about our conversation, but that one comment has stuck with me as I’ve tried to understand what went wrong and how to prevent it in the future.

I’m not a heavy sweater. I’m not one to come back from a run drenched in sweat or leave a puddle under my trainer after a workout, and since I’ve never had issues with muscle cramps or any other signs of sodium depletion before, I assumed that meant my sodium loss wasn’t high. In hindsight, Mike’s comment and the salt stains on my trisuit in some of my past race photos made me realize I was wrong to assume that—it’s possible I’m actually a salty sweater, and that sodium depletion contributed to my problems in Coeur d’Alene. To address that and make sure I can come up with a good hydration strategy, I’ve spent the past few weeks exploring ways to calculate my sweat and sodium loss rate.

After some research, here’s what I’ve learned.

Precision Fuel & Hydration’s sweat test

Precision Fuel & Hydration offers a sweat test, which takes only 45 minutes and doesn’t require exertion, and works like this:

A quick, non-invasive sweat test that doesn’t require a long workout is compelling. Unfortunately, it has to be done in person, and the nearest test locations are in Boulder, Colorado, and St. George, Utah, both about an eight-hour drive away, so that’s out of the question for now. (Although I am racing Ironman 70.3 St. George next year, so I might schedule a test while I’m there.)

That said, Precision Fuel & Hydration has a huge nutrition and hydration knowledge base, where I’ve learned a few things that I wasn’t aware of, like that you can, in fact, sweat while swimming. What! I hadn’t considered the possibility that I may have started my bike leg at Coeur d’Alene already depleted after swimming for an hour and a half.

Another great resource they have is this sweat loss rate calculation, which involves weighing yourself naked before and after a workout, measuring the fluids you consume during the activity, and entering those values in an Excel spreadsheet. (I don’t have Excel, so I recreated it in Google Sheets; feel free to make a copy for your own use.)

I’ve been measuring my sweat loss a couple of times a week, in various conditions, during multiple types of workouts, and collecting the results in that spreadsheet; so far, my sweat rate is averaging about 1.17 L/h. To put that into context, a study of the sweat rate of Ironman-distance triathletes showed a mean sweat rate of 1.4 L/h amongst the participants.

However, without knowing my sodium loss, this only tells me half the story, and since I won’t be able to do this sweat test for a while, I looked at a couple of alternatives.

Gatorade Gx Sweat Patch

Gatorade sells these Gx Sweat Patches, which they claim can be used to calculate sweat and sodium loss with the help of the Gatorade Gx app. It works by sticking one of the patches on your forearm and going for a workout. As you sweat into the patch, the dye-filled channels fill up, allowing the app to scan the pattern with the phone’s camera and estimate the sweat and sodium loss.

An animation of the Gatorade Gx Sweat Patch, showing a growing orange line indicating "your sweat rate", and a blue line indicating "your sodium concentration".
Image: Gatorade

I tried these patches last year and didn’t get any valid results: the first patch didn’t capture enough sweat for a valid reading, and the app refused to scan the second patch. They’re relatively cheap, at $24.99 for a two-pack, so for the sake of being thorough, I decided to give them another shot, hoping that the experience had gotten better since the last time I used them.

After creating an account in the app, I created a new “sweat profile” for a 60-minute indoor ride. With that set up, I placed one of the patches on my forearm and started a TrainerRoad workout called Black, “an hour of aerobic endurance riding ranging from 50–80% FTP with no recovery,” which I did indoors with my fans off to ensure I worked up enough sweat to get a reading. Roughly 30 minutes into the workout, sweat started filling up the orange line and it kept growing steadily until I finished. The blue line that measures the sodium concentration never filled up at all, though.

My left arm, with a Gatorade Gx Sweat Patch on the inside of my forearm. The patch is covered by a zigzagging channel, which is about a quarter of the way filled by an orange liquid.
The Gx Sweat Patch at the end of my workout.

Once I finished my workout, I fired up the app again to scan the patch. It took me a couple tries to get it to scan, and then the app asked me to confirm a few additional details about the workout (duration, intensity, temperature, fluids consumed) before showing me the results of my new sweat profile: a sweat rate of 28–31 oz/h (0.83–0.92 L/h) and a sodium concentration of 9–23 mg/oz (304–778 mg/L). The app describes this sweat rate and sodium concentration as “moderate” and “low,” respectively.

I had no way to confirm if the sodium concentration reading was accurate, but I weighed myself and my bottles before and after the workout to compare the sweat rate. Plugging the results in my spreadsheet returned a sweat rate of 0.89 L/h, within the range the Gx app estimated, so it seemed right on the money.

The next day, I created a new sweat profile for an indoor run and used my second patch while doing a 60-minute run on my treadmill at a fairly fast pace (for me) of 5:02 min/km, with the fans off. Like in the previous test, it took about 30 minutes to start seeing the orange line, but once again the blue one didn’t appear at all. After scanning the patch with the app, it returned a sweat rate of 28–31 oz/h (0.83–0.92 L/h) and a sodium concentration of… 0 mg/oz. Zero. Nothing. This is the one thing I want to know, and this app couldn’t give me a number.

Two screenshots of the Gx app. The first one says "Guillermo, here's your new sweat profile, save this to know how to hydrate after future workouts in similar conditions" and shows a colorful widget that reads "Indoor run, moderate, 76ºF, 28-31 oz/hr sweat rate, 0-0mg/oz sodium concentration" with a white button underneath that reads "see your sweat profiles." The second screenshot shows the same sweat rate and sodium concentration data displayed as ranges on colored bars, with the explanation "your Sweat Rate for this activity is moderate" and "your sweat has a low concentration of sodium."
According to the Gx app, my sweat is pure distilled water.

Was the sweat rate accurate, at least? Well… no. My own sweat rate calculation was 1.44 L/h, well outside the range estimated by the Gx app, and closer to my sweat rate on previous runs.

These faulty results from the second patch make me question the results from the first one, even though the sweat rate from that one matched my own measurement—for all I know that was just a coincidence. Considering this was the second set of patches I tried and I still couldn’t get reliable results, my conclusion is: don’t waste your time and money on these things.

LEVELEN Athlete Sweat Test Kit

Last but not least, I tried the LEVELEN Athlete Sweat Test Kit. It’s much pricier than the Gx sweat patches, at $189 for the multisport kit (which contains two tests, one for the bike and one for the run), and requires mailing the sweat samples to a lab for testing. They also sell cheaper single-test kits and starter kits that include a personalized electrolyte mix based on your test results. I ordered the multisport kit, which arrived in the mail a few days later and contained the following:

  • Two sweat patches, which consist of a small piece of gauze on a transparent adhesive film.
  • Two plastic test vials to put the gauze in after performing the test workout.
  • Plastic tweezers to put the gauze in the tubes without contaminating them.
  • Alcohol wipes to clean your arm before applying the patches.
  • Some #SweatTest stickers to place on top of the patches to promote it on social media (which is an actual step in the instructions).
  • Two information cards to write down the test results (weight before and after, fluids consumed, etc.)
  • A postage-paid padded envelope to mail the tests back.
The contents of the LEVELEN sweat testing kit, including two gauze patches, two LEVELEN stickers, two cards to be filled out with the data from the test, alcohol wipes, and a plastic ziploc bag with two plastic vials and a set of plastic tweezers.
The contents of the LEVELEN Athlete Sweat Test Kit.

The test works very similar to the Gx sweat patch: you wipe your arm with the alcohol swab, wait for it to dry, then stick one of the sweat patches on your forearm and go do a workout. Once you’re done, carefully remove the patch, use the tweezers to pull the gauze off, put it in the test vial, and seal it. Once you finish both tests, place the vials in the padded envelope with the information cards and drop them in a mailbox.

Unlike the Gatorade Gx app, which tries to estimate your sweat rate for you, the LEVELEN test requires you to calculate it manually, using the same method used by Precision Fuel & Hydration: you weigh yourself naked before and after the workout, measure any fluids consumed, and write that data down on the cards provided, along with things like workout duration, temperature, and humidity. (Tracking this information was pretty straightforward since I already had my sweat loss spreadsheet, so I recommend setting that up ahead of time.)

They also have a few guidelines for the test workouts themselves:

For the most representative results, you should perform the sweat test while training in the activity, intensity, and environmental conditions representative of the competition you are training for (e.g. run on a hot day for marathon training). Triathletes should test separately on the bike and run.

Tests should be done during the first sweat/workout of the day, and multiple test should be completed on separate days. Workout duration should be at least 45 minutes.

My left arm, with one of the patches for the Levelen sweat test on my forearm. The patch is a small square of gauze held in place by a transparent adhesive film.
Unlike the Gx Sweat Patch, the LEVELEN patch goes on top of the forearm.

For the run, I did a 60-minute run on my treadmill at a 5:09 min/km pace, roughly the same pace I did at Ironman 70.3 Arizona last year. Although they recommend trying to reproduce race conditions, I ran indoors to avoid contaminating the sample with sunscreen. It was reasonably warm inside, though, at 80ºF (27ºC), and I did the workout with my fans off, so I got a pretty good sweat going.

The following day, I repeated the process with Rendezvous, a fun TrainerRoad bike workout consisting of “50 minutes of continuous progression from 60% FTP all the way up to 100% FTP via 10% increases every 10 minutes.” With an intensity factor of 80%, it was more intense than the workout I did with the Gx patch and, like the run, closer to Ironman 70.3 intensity. As in the previous tests, I did it indoors with the fans off.

For both workouts, I weighed myself and my water bottles before and after and entered the numbers in my spreadsheet before writing them down on the information cards.

After dropping the envelope in my nearest mailbox, I emailed LEVELEN to let them know my tests were on the way, and they immediately replied that I would get the results by email within one or two weeks of receipt.

I received a detailed six-page PDF from LEVELEN in my email exactly two weeks later, as promised. The document outlines my sweat rate as follows:

Bike Run
% dehydrated 1.4% 1.3%
Gross sweat loss 1.5 L 1.3 L
Fluids replaced 430 mL
29% replaced
330 mL
26% replaced
Sweat rate 1.48 L/h
62nd percentile (moderate)
1.28 L/h
49th percentile (moderate)

Unsurprisingly, these numbers match the ones on my spreadsheet, but what I really wanted to know was my sweat composition. LEVELEN provided the following breakdown:

Bike Run
Sodium concentration 33 mmol/L
759 mg/L
30 mmol/L
690 mg/L
Sodium loss 1,129 mg/h 882 mg/h
Chloride concentration 40 mmol/L
1,420 mg/L
38 mmol/L
1,360 mg/L
Chloride loss 2,096 mg/h 1,747 mg/h
Potassium concentration 8.7 mmol/L 8.4 mmol/L
Salt (NaCl) loss ¾ teaspoon per hour
46th percentile
½ teaspoon per hour
34th percentile

That sodium concentration (759 mg/L for the bike and 690 mg/L for the run) is within the range given by that first Gx sweat patch, and the percentiles are interesting—perhaps I’m not that salty of a sweater after all. However, looking at these numbers, it’s clear I wasn’t adequately replenishing fluids and electrolytes at Coeur d’Alene. LEVELEN recommends replenishing up to 75% of the sweat loss and 100% of electrolyte losses; I only consumed about 500 mL of fluids and 200 mg of sodium per hour, so I was nowhere near close to hitting those numbers. They also recommend preloading fluids and salt in the 36 hours leading up to the race, which I didn’t do.

The rest of the report goes on to make recommendations for fluid and electrolyte replenishment based on my results, using their level 3 electrolyte drink mix (the levels indicate the amount of sodium in the mix, with level 3 having a moderate amount). LEVELEN recommends I drink 35 oz (1.035 L) of their drink mix per hour, containing 500 Calories and 925 mg of sodium. That would certainly accomplish the goal of replenishing 75% of sweat loss and 100% of sodium loss, but 500 Cal/h seems bonkers to me—that’s 125 grams of carbs per hour, far more than the 80 g/h I’ve been training and racing with, and more than I think I can stomach!

I might be able to train my gut over time to handle that many carbs, but for now I’d rather stick with the Maurten 320 drink mix I’ve been using. Thankfully, LEVELEN’s recommendations clearly explain how to preload and replenish fluids and sodium, and in which quantities, so I’m confident I can supplement Maurten with additional sodium and water as needed.

Although this sweat test kit is pricier than the Gatorade Gx sweat patches, and it takes a few weeks to do the tests and get the results, it was absolutely worth the investment. I got the exact information I needed to start making much-needed changes to my fueling and hydration strategy.

What’s next?

The workouts I did for my LEVELEN tests didn’t quite match race conditions, so my next step is to wait for an appropriately warm day, do some bike rides and runs at race pace, and get a more accurate sweat rate for those conditions. With that, and the sodium concentration range I got back from LEVELEN, I can figure out how much water and sodium I need to supplement my race hydration and nutrition.

I haven’t decided yet how I’ll do that, so I’ve ordered a few things to try out: Precision Fuel & Hydration’s PH 1500 tablets, which I could use for sodium preloading before the race, and I could put them in my Maurten bottles on the bike; their electrolyte capsules, which would be easy to carry and take during the run; and BOA Endure Blast sodium sprays, which I could easily carry in a top tube bag to use on the bike and have the advantage of being sold at the Ironman race expo, so I could stock up there if I have to.

I’m not sure these sweat test results fully answer the question of what went wrong at Coeur d’Alene, so I’ll keep researching that, but at least now I’m at a good starting point to experiment with sodium supplementation and see how my body reacts to it. If nothing else, I feel optimistic I can dial in my nutrition and hydration plan to prevent any sodium-related issues in future races.

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