Sweat Testing Revealed the Hydration Mistake Behind My Postrun Fatigue

As an ultrarunner, I’m always trying to figure out how to dial in my hydration and fueling for optimal performance, but I’ve never been someone who identifies as a “salty sweater.” That is to say, I’ve had the occasional white patch from dried sweat on the bottom of my tank top on a hot day after a long run, but in general, I’m not gritty with salt when I get home. I don’t feel like I sweat buckets, though sometimes I do finish a long run dripping. But it turns out, sweat testing isn’t just for the folks who can literally wring out their T-shirt or clean salt off their forehead after every run. Understanding your sweat profile has potential to benefit any runner. So, when I heard friends talking about sweat rate testing, I had to see what all the fuss was about. I slapped on a sweat rate sensor—I used the Nix Hydration Biosensor—and headed out for a few runs. The results surprised me. How Sweat Monitors Work I should be clear: I’m not a data-hungry person, nor am I someone who’s a major fan of wearable monitors. At the same time, I do believe that information is power. To that end, I’ve used different wearables, from heart rate monitors to continuous glucose monitors, on and off over my years as a serious endurance athlete and as someone who writes about running and cycling for a living. (I will add, though, on a daily basis, I complete most of my training by paying attention to the simplest metric: how I feel.) But I’m always down to optimize my performance and testing my sweat felt like an interesting next step. The Nix Hydration Biosensor has one-time-use patches that connect to the sensor. That meant I only had four runs to test my sweat before needing to reorder, which worked for me since I wasn’t planning on long-term use. If I were, the other popular sweat-measuring device, the hDrop, is a reusable sweat sensor and you can wear it on every run. For someone who wants this data daily, it’s definitely the more cost-effective option. There are a couple of ways you can use a sweat sensor as a wearable. The accompanying apps can record workouts and ping you with notifications regarding information during a run about when you need to hydrate, and how much sodium you should take in. If you’re a data lover and want to try to dial workouts in perfectly, you may choose that option. Both Nix and hDrop offer in-workout notifications alerting you to drink, or you can turn notifications off and just check the post-workout insights. Because I prefer no in-run data whatsoever, I was a fan of simply checking the information after my workout and then chatting with a dietitian about what the numbers actually meant for me. Luckily, my strategy aligns with the best practice that TrainingPeaks’ Education Program Manager Cody Stephenson recommends: Don’t try to act on the information at first. “Don’t do anything different,” he adds. “Just pair the sweat sensor, set up the device, and keep your training and nutrition and hydration normal for a few weeks. Then, look for patterns in that data: What did it do when you felt great? What did it do when you felt terrible? What happened during your highest volume and hardest weeks compared to your recovery weeks? Then you can start making changes in the related behavior, like hydration, to see if you can actually influence that data, or your performance relative to that data.” Putting My Sweat to the Test My first time running with the Nix sensor on was an extremely easy, low-key five-mile recovery run on a sunny but not overly hot day—it was around 70 degrees, with a hint of breeze. Basically, your ideal summer day. When I got home and pulled up my stats, I was shocked when the app informed me that I sweat out an estimated 17 ounces of fluid in my sub-50-minute easy run. That’s more than a pint! As someone who rarely (okay, never) brings water for any run 10 miles and under, this was an eye-opener. If I sweat that much on a relaxed run, what was I doing on a regular training day with a 10-miler, especially if it got a little warmer? I felt a little bit stressed about this new information. Because the data on the Nix app was a little difficult to parse—it suggests which sports drinks hit close to my electrolyte profile, but little else—I wanted to check in with a registered dietitian. Kylee Van Horn of Fly Nutrition is a sports dietitian and a runner, and she’s been experimenting with sensors and sweat rate testing with her athletes (primarily using the hDrop and Levelen sensors) in recent years. First, I wanted to understand the water volume better. If I sweat out 17 ounces of fluid in an hour-long run, does that mean: a) I should drink 17 ounces during my run, b) should drink 17 ounces right after my run, or c) should slam a pint of water right before my next run? “A good general rule of thumb is to aim to replace about 75 percent of your fluids lost per hour during the training session,” Van Horn explains. So ideally, even on that easy run, I should have had around 13 ounces of water. The ultimate goal, she says, is to figure out the minimum you need to consume to minimize losses to less than 2 percent of your bodyweight from fluids, which is the point where dehydration starts to affect performance. For me, that means avoiding losing more than 2.5 pounds from fluid. While the total volume loss in-run was relatively easy to understand, the other numbers like total electrolyte and sodium loss were a bit more confusing. Sweat testing isn’t just about the water volume—electrolytes also come into play, because they allow the body to absorb and actually utilize the water that you are drinking. In that easy run, I lost 915 milligrams of electrolytes, 430 of which were sodium. But did that make me a salty sweater? Was that normal? Should I be chugging an electrolyte beverage afterward to recover? Sodium is the most important electrolyte for athletes, and Van Horn explains that your goal should be to match your sodium intake with your fluid intake: If you’re replenishing 75 percent of your fluids lost, you should replenish 75 percent of your sodium lost. For me, after four runs, I found that on average, I lose around 430 milligrams per hour of sodium, plus or minus around 30 milligrams. So if I’m drinking 13 ounces of water, I’d want around 308 milligrams of sodium in that water to keep the ratio of hydration to sodium matched. And if I was drinking 17 ounces of water per hour, I’d up the sodium to 430 milligrams to keep the ratio matching. This is where my major breakthrough came: I’m pretty good about fueling my long runs, and in doing so, I usually take in an appropriate amount of sodium since I use Tailwind’s Endurance mix, which has 310 milligrams of sodium per 100 calories and 25 grams of carbs per single serving. But the bigger performance gains came in my “average” runs, those eight- to 10-mile endurance runs and the 90-minute workouts that I’d been doing without any hydration at all. For five out of six runs a week, I was training sans water bottle except on the hottest of days. After seeing the data from the sweat sensor, I realized I was potentially leaving performance gains on the table, and slowing my recovery time, because electrolyte imbalance and dehydration can impact everything from gut distress (an issue I definitely contend with), mental sharpness during and after runs, muscle cramps, and headaches. So, I made some changes: I’ve kept my Tailwind Endurance Fuel for long runs (over 60 minutes) and started adding it to my hard workouts. For those regular endurance runs, I’ve started simply putting a pinch of sea salt in a small water bottle and bringing it along. Now, if reading this has you panicking about testing your sweat and dialing in your numbers, pause. Van Horn reminded me that we’re not necessarily aiming for 100 percent “perfection” when it comes to our hydration. In fact, it’s almost impossible to calculate fluid and sodium loss perfectly, considering fluid loss will change depending on the heat and humidity, as well as your effort level. So my unmeasured “pinch of sea salt” is just fine, especially for easy runs—I don’t need to perfectly pair an electrolyte tab with every single run, Van Horn says. “I see too many athletes overdoing sodium and ending up with nausea and vomiting,” Van Horn says. “Just because you lose 567 milligrams of sodium per hour does not mean you need to take that in exactly. I usually have my athletes give themselves a range to make things easier and ensure they are not getting overwhelmed and confused.” For the average person, Van Horn recommends a range of 250 to 500 milligrams of sodium during a run—if you don’t have a sweat sensor, simply start to play with that range and see how you feel during a run with 250 milligrams per hour versus 400 milligrams per hour. The Takeaway on Hydration Status Speaking of potential for overwhelm and confusion, TrainingPeaks recently added the sensors from Nix and hDrop to their platform so that athletes can see their sweat rate data, alongside metrics like power and heart rate. That means a lot of numbers showing up on your dashboard after any training session. But Stephenson is quick to note that before you start to look at data from a sweat sensor, it’s helpful to have the basics mastered: Know your training zones, understand your heart rate and pace, and then, once you’re comfortable with those basic metrics, you can start looking deeper for what’s affecting performance. “If you’re working on finishing your first 5K then hydration might not be the thing keeping you from being your best, but three years later if you’re trying to set a new personal best in the marathon, then it might be really important, and you’ll have a long history of that data to review,” he adds. “Once you know what your training zones are, and how you should feel and perform at different intensities and different parts of the training cycle, you can start looking for more enriching data, like hydration status, to see when those things fall apart, and you can work to prevent it and give yourself an edge.” I can’t say that I’ve shaved time off of my usual pace by increasing my water intake on runs, but I can say that since I started paying attention to my hydration and sodium intake, I’m finishing runs feeling better and maintaining my pace the whole way through, which is the main goal in my long runs. But for someone like myself who trains and then has to go right back to work, the biggest benefit has been postrun, particularly after my everyday training sessions that are under 10 miles. I had been experiencing the occasional headache and near-constant postrun slump, especially when I had to get right back to a call or meeting as soon as I was done with my run. But now that I’m drinking something even on short runs, I’m feeling a lot peppier when I open up my laptop afterward. I don’t experience the same afternoon drag that I’d been having. And for the recreational athlete, that’s worth much more than a three-second improvement in a mile-split. Molly writes about cycling, nutrition and training with an emphasis on bringing more women into sport. She's the author of nine books including the Shred Girls series and is the founder of Strong Girl Publishing. She co-hosts The Consummate Athlete Podcast and spends most of her free time biking and running on trails, occasionally joined by her mini-dachshund.