Julie and Zach Pastko are both competitive athletes with years of racing behind them, so when they started training for HYROX with the Nix Hydration Biosensor, the question wasn't whether they took their performance seriously. It was what the data would show about two well trained athletes preparing for the same event. The answer turned out to be a clear example of individual sweat variation: two athletes, same race, same training block, and sweat profiles that differed by a factor of 2-3x across nearly every metric.
HYROX is a hybrid fitness race that alternates eight 0.62-mile (1-kilometer) runs with eight functional strength stations, including sled pushes, sled pulls, farmers carries, wall balls, and rowing. It's designed to test endurance and strength in the same event, which is partly why the event has pulled together both lifelong runners and lifelong lifters.
HYROX forces athletes to manage both cardiovascular fatigue and muscular fatigue at the same time, often for well over an hour. The combination of the two makes fluid and electrolyte replacement far more complicated than training for a marathon, where one's hydration strategy only has to solve for one type of effort. A runner's usual hydration plan doesn't always hold up once heavy compression movements and grip intensive stations are added to the mix.
Zach's path ran through competitive team sports, including basketball, baseball, and volleyball, where his conditioning was built around repeated short bursts of speed and power rather than sustained distance work. That power base translates directly into HYROX, where explosive strength stations are interspersed with running. What pulled him into the sport was the combination of endurance and strength under competitive pressure, a format that rewards a broader athletic base than either of his prior sports required on its own.
Their training reflects two approaches to the same competitive goal. Julie runs three days a week, one easy run and two structured interval sessions, a volume she manages carefully because higher mileage increases her injury risk. She adds one to two HYROX specific training sessions weekly and several dedicated strength sessions split across lower body, upper body, and full body work, along with a Lagree style Pilates class to reinforce core stability and support recovery between hard efforts. Zach trains without a fixed weekly template, but his HYROX specific work is consistent: one to two sessions a week built around race simulation, either using gym equipment or dumbbells paired with running loops to replicate race stations under fatigue.
Before Nix, both athletes hydrated using the same approach most competitive endurance athletes rely on: experience and feel. Julie used an electrolyte drink on high sweat days and carb gels during longer training sessions, layered on top of a consistent pre-workout routine. Zach relied on water and an occasional sports drink, with a consistent pre-race meal, but no structured way to know whether his intake matched his actual losses.
Both athletes came to Nix from a performance optimization mindset. Julie already used multiple wearables to track her training load and recovery, but she wanted the one data point none of them captured: the actual composition of her sweat, rather than a generic hydration guideline. Zach's interest came from recognizing that the training and recovery approach that worked for him in his 20s and 30s needed to be re-evaluated as a competitive athlete in his 40s. Both were looking for objective data to replace assumptions they had been training on for years.
Once both athletes started training with the sensor, their data sets diverged in a way that neither expected, and the gap was large enough to matter for how each of them should actually be hydrating. Across her recorded sessions, Julie's average electrolyte loss rate came in at roughly 580 milligrams per hour, with a sweat composition of 36 milligrams per ounce. Zach's numbers ran substantially higher across the board: an average electrolyte loss rate of about 1,925 milligrams per hour, more than three times Julie's, and a sweat composition averaging 64 milligrams per ounce, nearly double hers. Both athletes are training at a comparable competitive level. The difference in their numbers isn't a difference in fitness or effort. It's a difference in individual sweat physiology, which is precisely the variable that generic hydration advice cannot account for.
That distinction matters in practice. A sweat composition in the mid 30s (mg/oz), like Julie's, sits toward the lower end of the range Nix typically sees across athletes, meaning her sweat is relatively dilute even when her total fluid loss is high. A composition in the mid 60s, like Zach's, sits well above that, meaning every ounce of sweat he loses is carrying meaningfully more electrolytes out with it. Applied to a standard hydration plan, that asymmetry cuts two ways: a generic electrolyte protocol sized for an average sweat rate would likely under-hydrate Zach during longer or hotter sessions, while the same protocol would probably over-hydrate Julie, which is exactly what her data showed.
The gap held up across every individual session, not just in the averages, which is what makes it a physiological pattern rather than a one-off result. Zach's electrolyte loss rate exceeded Julie's on every recorded session, and it trended upward across his most recent training block, climbing from roughly 800 milligrams per hour in late April to nearly 2,735 milligrams per hour by the end of May. Julie's numbers, by contrast, stayed in a tight band across the same stretch, ranging from about 425 to 700 milligrams per hour regardless of session type. Consistency like that is itself useful data: it told Julie she could rely on a stable replenishment plan, while Zach's upward trend signaled his needs were shifting as his training intensified, something a flat hydration routine would have missed entirely.
For Julie, the biggest surprise was learning she wasn't losing nearly as much electrolyte volume as she assumed. “I don't lose as much sodium and electrolytes as I thought,” she said. “I was over replenishing.” For Zach, the data pushed back on his own assumption in the other direction. He had believed he needed more aggressive rehydration after a hard workout than his numbers were actually showing him.
For Julie, the data led directly to a measurable change in her protocol. She cut her post workout electrolyte replenishment roughly in half, replacing a habit she had followed for years with an intake level backed by her own sweat data. The result was less bloating during training blocks and a hydration routine calibrated to her actual physiology rather than a generic assumption about what a serious endurance athlete needs. “I'm much more thoughtful about my hydration choices and the impact it has on my training now,” she said. Zach's numbers pointed him in the opposite direction: toward taking his hydration and electrolyte needs more seriously rather than less, especially as his training volume increases and his electrolyte loss rate climbs with it. He now treats hydration as a variable that shifts with conditions and training load, rather than something a standard routine can handle on its own, a shift he's applying directly to how he prepares for race day heat and longer efforts.
That data driven approach was put to the test under real race conditions. During a recent HYROX event in New York City, Julie's sweat rate rose well above her training baseline, driven by 90 degree heat and a rare outdoor course. Julie and Zach train for both indoor and outdoor conditions, but stepping into direct heat for one of the few times all year is exactly the kind of outlier a training routine built around a climate controlled norm isn't designed to anticipate. Julie developed a headache in the later stages of that race, a signal in hindsight that her electrolyte intake needed to scale up for the heat rather than mirror her standard indoor training routine. It's now a specific gap she's working to close: a race day hydration plan that adjusts for heat and outdoor conditions when they come up, separate from the baseline she has already dialed in for training. “I'd love to use Nix to train in a variety of environments to have insights for each type of workout, before and after, and on the days leading up to it,” she said. “That would be so helpful.”
The core finding from Julie and Zach's data is a reminder that applies to any competitive athlete: two people training for the exact same event, at a comparable level, can have sweat composition and electrolyte loss rates that differ by two to three times. Fitness level and effort don't predict that difference. Only measurement does. For athletes trying to optimize performance rather than just survive a race, that distinction is the difference between a hydration plan built on assumption and one built on evidence.
