Based in Australia, RunLab is an educational, performance-focused YouTube channel dedicated to helping endurance athletes train smarter through practical experiments and evidence based insights. By combining real-world testing with sports science principles, hosts Chris Beavon and Vlad Shatrov break down complex topics like fueling, pacing, and recovery into clear, actionable takeaways for runners of all levels.
Last year, RunLab set out to answer a big question: Why can elite runners go further without falling apart?
In that episode, Chris and elite trail runner Vlad ran side by side, comparing biomechanics, efficiency, and late-race fatigue. The differences in form were clear, but many viewers noted that hydration was one factor they didn’t examine closely.
In their latest episode, they revisited the question, but this time through the lens of sweat science. Using the Nix Hydration Biosensor, they measured exactly how much fluid and electrolytes each runner lost, built individualized hydration strategies from the data, and returned a week later to test a more intentional approach.
The results reinforced a critical message for endurance athletes: Hydration doesn’t have to be guesswork. It’s measurable, it’s actionable, and it deserves as much attention as fueling and carbohydrate intake during endurance training.
Chris and Vlad ran two 10 kilometer (6.2 mile) laps of Newcastle’s Croc Loop, totaling 20K (12.4 mi).
For the first run, they followed their typical hydration strategies, consuming roughly 50-80 grams of carbohydrates per hour, relying on electrolytes passively from carb mixes and gels, and setting no specific sodium or fluid targets. In other words, the approach many runners use: fuel well, drink when needed, trust experience.
At the same time, both athletes wore the Nix Hydration Biosensor, which continuously measured sweat rate, total fluid loss, electrolyte losses, and sodium concentration.
After reviewing the data, they built personalized hydration plans based on actual losses and returned a week later to repeat the effort with an updated strategy.
Visually, Chris appears to sweat far more heavily than Vlad during the first test, but the data told a different story.
Their overall sweat rates and total fluid losses were very similar at roughly 25–27 oz/hr, which is considered a moderate sweat rate, with Vlad losing slightly more electrolytes than Chris during the run.
This aligns with what we often observe in trained endurance athletes. Highly fit runners tend to produce efficient, evenly distributed sweat for evaporative cooling. They may not appear drenched, despite significant total fluid loss.
For Chris, the more important discovery was that he was regularly under-replenishing both fluid and electrolytes during long efforts.
He reflected that hydration deficits may not just affect race-day performance, but also recovery across the entire training block.
“You shouldn’t just think about your fueling strategy from a race perspective. You want to train how you race... Not just for practice, but for recovery. If you’re finishing a 30K long run significantly dehydrated, that’s going to cost you.” Chris states.
The Nix Solo app supports this directly with the Recovery Calculator, which provides a personalized two-hour post-workout recovery plan based on measured sweat losses and logged intake.
One of the most interesting findings from their initial test was the timing of sweat onset.
Chris began sweating 10 minutes earlier than Vlad (11 minutes into the run versus 21 minutes). This likely reflects differences in perceived exertion at a shared pace, which is a key metric for hydration planning. If the effort felt like an RPE 8 for Chris but only a 6 for Vlad, Chris’ body would generate more heat early, triggering earlier sweat production.
Nix data can be exported and viewed minute-by-minute, allowing athletes to see exactly when sweating begins, when sweat rate peaks, when it tapers, and how it compares across sessions or training partners.
An athlete’s sweat profile is highly sensitive to intensity, environmental conditions, perceived exertion, overall fitness level, and other external factors.
In separate testing, Chris completed a three-hour Zone 2 run and sweated roughly half as much as he did during the Croc Loop effort, again underscoring the importance of RPE in hydration planning.
The implication is clear: hydration strategies must be adjusted based on effort and environment. There is no universal number that one athlete should follow.
For the second run, Chris and Vlad built hydration targets based on their measured losses versus their previous assumptions. They planned to consume approximately 700–800 mL of fluid per hour and around 1,000 mg of sodium per hour.
Conditions were slightly cooler, but more humid than the first test. Chris cut his run short due to calf tightness, but Vlad completed the effort. Interestingly, Vlad ran slightly faster and lost more fluid and electrolytes, even though perceived effort was marginally lower.
While a 20K effort may not fully expose hydration-related performance decline, especially for a highly trained athlete like Vlad, extrapolating those losses over a 50K, 100K, or 100-mile race underscores the importance of proactive planning.
As Vlad noted, having a number to work from changes everything. Hydration becomes intentional rather than reactive.
The video also highlights how lab-based sweat testing provides a snapshot under controlled conditions, but doesn't capture overall sweat profiles over time. The Nix Hydration Biosensor allow repeated testing across varied environments, capturing how sweat rate and electrolyte losses shift with changes in temperature, humidity, intensity, and duration.
In this case alone, Chris and Vlad’s sweat rates across sessions ranged from 19 to 37 oz per hour, while electrolyte losses ranged from approximately 1,500 to nearly 4,000 mg per hour.
That variability reinforces a core principle: hydration is dynamic and evolves with conditions. For endurance athletes, ultra runners especially, repeated measurement and trend analysis offer a more practical and relevant approach versus a single lab snapshot.
While carbohydrate strategy has become central to endurance training and ultra athletes in particular, hydration deserves equal attention.
Fluid loss increases cardiovascular strain, while electrolyte loss compromises fluid balance and neuromuscular function. Deficits often accumulate quietly, until they don’t.
The RunLab experiment demonstrates what happens when hydration is measured instead of assumed. You replace uncertainty with data, guesswork with intention, and you train with a clearer understanding of what your body actually needs.
Watch the full RunLab episode to see the data, discussion, and performance insights unfold.