Review: Nix Hydration Biosensor

In a world where unmanned spacecraft have landed on Mars and artificial intelligence can read your mind, one would think someone would have figured out a precise way to measure how much athletes should drink while exercising. Hydrating, or replacing body fluids lost through sweating, exhaling, and eliminating waste, is essential. When 2 or more percent of body mass is lost through dehydration, the body can go haywire, with elevated cardiovascular strain, reduced aerobic exercise performance, and impaired thermoregulatory function. After losing 12 percent of body mass to dehydration, a human will die.

It’s rare for an athlete to exercise to the point of death by dehydration. But it’s also odd to consider that, for such an important physiological necessity, many athletes rely on thirst as their definitive guide to how much they should hydrate during exercise. The trouble with that built-in system is twofold. By the time your brain registers that you need water, your body is often already dehydrated. Also, it’s easy to alleviate your thirst before you’ve completely rehydrated.

Outside a lab, the most accurate standard for determining an athlete’s fluid loss is to weigh the nude body before and after the activity. (For every pound of body mass lost, 16 ounces of water should be consumed.) But that method doesn’t help a runner determine how much fluid they’ve lost while on mile 16 of a marathon.

Cyclists can rely on a GPS computer with a drink alarm that flashes a reminder to take a swig from a water bottle every 15 minutes. Runners and gym rats can wear a smartwatch with a hydration sensor, like the Apple Watch, which uses electrodes placed against the skin to measure electrical conductance of the wearer’s sweat. This can determine the concentration of electrolytes (or lack thereof) in the sweat, which helps determine the user’s hydration level. There’s also a $25 gadget called the GX Sweat Patch, marketed by Gatorade, a one-time-use biosensor that, when applied to the inner left forearm, measures the user’s sweat rate, fluid loss, and sodium loss. When that data is transferred to the companion app—which runs on iOS or Android—it operates as a guideline for the athlete’s future performance.

Until recently, however, biosensing technology that can analyze the content of an athlete’s sweat to provide personalized, real-time hydration recommendations as they are exercising has been out of reach because the sensing tech hasn’t been affordable enough to build into a consumer product.

In December, a Boston startup founded by Harvard Business School graduate and marathoner Meridith Cass unveiled the Nix Hydration Biosensor, the first wearable sensor that promises to provide real-time sweat science to athletes. Cass, who is also a former collegiate basketball player, started thinking about biosensing technology to measure hydration after she struggled with her own body’s reaction to heat and humidity when training for marathons. “I was feeling very sluggish on some of those longer runs,” she says, “and I wondered, ‘could a hydration sensor be a thing? And would anybody find it useful other than me?’”

Nix works like this: When attached to the bicep (via a protective film on the underside of the patch, which is about the size of a round orange slice), the patch measures the sweat profile of the body locally, extrapolating it to the full area of the body via algorithmic computation. As sweat dribbles across the electrodes on the bicep patch, the patch measures the content of the sweat twice along its flow path. By comparing data on those two locations, the sensor can tell how quickly fluid is moving through the body. When connected via Bluetooth to the Nix app, the sensor relays hydration notifications to the phone in intervals customized by the user. The point is to keep the athlete, as they are working out, within 1 percent of their starting body mass (or 1 percent of dehydration) to avoid the nasty pitfalls that come with dehydration.

When the workout is complete and downloaded, the app divulges the user’s sweat composition, sweat rate, and electrolyte loss per hour. The app also displays a chart of drink choices—with water at the top, followed by a list of 12 sport hydration beverages on the market—ranked in order of their electrolyte composition. The user can then match their specific electrolyte loss to the hydration beverage that best matches their profile.

In addition to the customized data, the sensor also captures the day’s temperature, humidity, dew point, solar load, wind, and altitude, spitting out a Nix Index, a number from zero to 100 that is a measure of the user’s thermoregulatory potential in the current environment. If the Nix Index is 65, for example, your sweat rate for that workout is likely to be in the 65^th percentile of your personal sweat range.

In theory, this concept is genius. In reality, the product team still has a few quirks to iron out. The sensor itself is a reusable device that comes with its own charging pod and attaches to a single-use disposable patch. It was easy enough to click the pod into the disposable patch, but device registration and Bluetooth pairing were both glitchy. The initial kit comes with four patches, but a replacement pack of four more costs a hefty $25, which adds up if you're using the sensor a few times per week. 

During my first indoor training ride, a last-minute work call delayed me by 20 minutes, but I had already applied the patch to my bicep and pressed play. Not knowing whether I would waste the expensive disposable patch if I pressed stop, I kept it rolling. Nix literature says it takes your body up to 25 minutes for sweat data to begin to appear. In addition to the 20 minutes I wasted on a phone call, I had limited workout time that day, so my 45-minute ride (the minimum time Nix recommends for a workout when using the patch), plus the 20-minute phone call, yielded 4.8 ounces of fluid loss, 341 milligrams per hour of electrolyte loss, and 71 mg of electrolytes per ounce of fluid lost. When I compared that number to the hydration beverage chart, it pointed me toward LMNT, a product I had never used before.

Interestingly, my go-to hydration product, also listed, didn’t provide nearly enough electrolyte replenishment even for a short 45-minute workout. But sweat contains several different electrolytes, including but not limited to potassium, sodium, and magnesium. Each hydration product contains its own proprietary formula of different electrolytes. So, I wondered how Nix could sense which electrolytes had been lost in my sweat, and in turn, how it could accurately direct me to the appropriate cocktail of electrolytes in each product?

Because Nix is not yet set up for Android (the company promises it will be within months), I created a separate profile on my iPhone for my Android-using partner, Brian. I was curious to see the differences in our sweat profiles, considering our respective genders and weights, and an eight-year age difference. It turns out that after Brian had completed a 45-minute indoor cycling workout on his trainer, we had almost the same electrolyte loss per ounce of fluid, and he was also shown LMNT as his recommended hydration beverage.

Nix is testing in a wide variety of sporting arenas, from soccer to rowing to high-sweat jobs like firefighting. Currently, however, the app only prompts the user for cycling or running workouts, indoors or outside. Because I live in a cold climate and Nordic skiing is one of my go-to winter sports, I attached a pod to my bicep on a relatively balmy 30-degree day and went skate skiing, a sport that leaves me drenched in sweat. When I downloaded my 1:27-hour workout, however, the app had registered zero fluid loss, zero electrolyte loss, and zero sweat composition.

Cass from Nix assured me that the patches work in cold weather, so I may have been wearing too many layers between the patch on my arm and the iPhone in my hip pack. But it was a disappointing result, especially because that was the last of the four patches that came with the biosensor. It also led me to the realization that to check my hydration needs in real time, I had to stop skiing and open my pack to look at the app on my phone, a step I probably wouldn’t bother to take in normal circumstances. Theoretically, because the app is compatible with an Apple Watch (Series 3 or later), this step might have been much easier if I owned an Apple Watch. 

Since that test, Nix has added compatibility with Garmin devices, one of which I do have, which should have made it easier to check the data on the fly. I was able to download the Nix widget easily enough to my Garmin Edge 1030 Plus GPS device and pair it with Nix. But the Nix data field on my Garmin had only two measurements: fluid loss and electrolyte loss, which would require some mathematical skills in the field to determine how much and what I would need to drink in order to replenish them.

To work with the Garmin watch, however, Nix still needs a Bluetooth connection to the iPhone. I paired the sensor to my phone before I left the house, but by the time I reached the trailhead, the Bluetooth connection was lost. I tried again the next day and was successful at first. After I spent 30 minutes happily sweating on the trail, I checked the Nix app and got the message “Check Bluetooth connection.” Nothing had registered on my Garmin watch or my iPhone, and I had wasted two of the new batch of patches I’d received from Nix. 

Curious to hear what a scientist might say about Nix, I emailed Noé Brasier from ETH Zurich and University Hospital Basel in Switzerland, who had chaired an interdisciplinary session on biosensors during a virtual congress called Sensors in Medicine 2022. Brasier, whose focus is clinical applications of sweat analysis, told me, “I am convinced that noninvasive molecular diagnostics will help physicians to extend and improve remote patient-monitoring in the future.” He’s currently conducting a clinical study at ETH Zurich, where he will investigate the proteomic and partly metabolic sweat profiles in participants exposed to heat.

Upon examining Nix’s methodology, he noted that “extrapolating the local sweat rate to the whole body sweat loss is challenging, as we sweat differently along the body surface.” He explained that, among other issues, there are different local sweat rates and different sweat gland types throughout the body. He also added, after digging around a bit on Nix’s website, that “Nix writes that their platform is scientifically validated, but I have not found any studies or references to read about how they build up their knowledge.” Short of having the physical device to assess, Brasier concluded that Nix is “interesting” in that it includes specific fluid recommendations, like Nuun, Scratch, Gatorade, Tailwind, LMNT and others. The field of sweat-sensing is “extremely promising,” he says, “but the challenges remain big, and a lot of work needs to be invested to develop a solution with a clinical value.”

In response to Brasier’s concerns about scientific validation, Nix confirmed that there have so far been no external reviews of its in-house validation studies. Now that the product is launched, it says that’s where it will focus. “This product will be the core of Nix for a little bit to come,” says Cass, the founder. But she has bigger goals, not a bad gamble, considering the market for biosensing skin patches is expected to reach $18 billion by 2027. “Anything you can measure noninvasively through sweat, saliva, breast milk—there are tons of biomarkers you can pull,” Cass says. “That’s really the North Star of this company."