Ultramarathon participation has grown more than 300% over the last decade, even as marathon participation has largely leveled off, according to RunRepeat's State of Ultra Running report. As more runners push past the traditional 26.2 miles into 50Ks, 100-milers, and multi-day expedition races, a lot of the hydration advice built for shorter races comes with them, and it doesn't always hold up. Many ultra runners find themselves in trouble on race day simply because their hydration plan isn't as dialed in as they thought.
Ultramarathoners don't necessarily sweat harder than other runners. What separates them is duration, intensity, and how the body's fluid and electrolyte needs shift over many more hours than a typical race ever demands. Both the published research and the data collected from the Nix Hydration Biosensor point to the same conclusion: the biggest hydration risk in ultrarunning usually isn't losing too much, it's replacing too much without pinpointing actual needs or tracking how those needs changes over time.
Any race longer than the traditional marathon distance of 26.2 miles (42.2 km) qualifies as an ultramarathon. That includes 50Ks, 100-milers, and timed formats like 24-hour races or Backyard Ultras, where competitors run a 4.2-mile (6.7 km) loop every hour until only one runner is left standing. What connects these formats isn't one distance, it's pacing discipline stretched over many hours, spanning from 6 to over 70 hours, where fueling and hydration decisions compound over time in a way a 5K, or even a road marathon, never tests.
Sweat rate is driven far more by exercise intensity than by how many miles are on the clock. A 2016 trial published in the Annals of Sports Medicine and Research found that sweat rate nearly doubled between low-intensity and high-intensity effort, climbing from about 33.8 oz/hr (1.0 L/hr) to 64.3 oz/hr (1.9 L/hr), while hourly sodium losses roughly tripled. A research review comparing marathon and ultra-endurance heat stress in Frontiers in Sports and Active Living notes that because ultra-endurance events are typically run at a meaningfully lower relative intensity than a hard-effort marathon, absolute sweat rates in ultras tend to run lower too.
For years, hydration advice for runners has centered on a single fear: don't run dry. But research on both marathons and ultramarathons increasingly points the other way.
A study of 488 Boston Marathon runners published in the New England Journal of Medicine found that 13% finished with asymptomatic hyponatremia (low blood sodium). The single strongest predictor wasn't how little runners drank, it was how much weight they gained during the race, a sign of drinking more fluid than they were losing to sweat.
Ultra-distance research tells a similar story. A 2021 analysis of 266 competitors during an 80 km (50-mile) ultramarathon stage, published in Sports Medicine – Open, found that runners who finished dehydrated (having lost more than 3% of body weight) significantly outperformed those who finished overhydrated (having gained weight during the race), running about 5 minutes and 48 seconds faster per mile (3.6 minutes per km) and finishing roughly 4 and a half hours sooner. The study shows plainly that overhydration can carry a bigger performance cost than underhydration.
It's not just fluid intake that gets overcorrected. Electrolytes, and sodium specifically, are just as easy to get wrong. Part of the reason this has posed such a challenge for athletes is due to the enormous individual variation. In Nix's own data, for example, some athletes see their sweat composition come in below 20 mg/oz while others run over 100 mg/oz, a fivefold range that isn't explained well by sweat rate, body size, age, or training status alone. A generic electrolyte or sodium target, in other words, is likely to be wrong for any specific runner, sometimes by a wide margin.
That variability is exactly what caught up with endurance athlete and RunLab host Chris Beavon, who used Nix during a Backyard Ultra. He built his race plan around losing roughly 1,000–1,100 mg of sodium per hour, an estimate based on earlier training. The Nix Hydration Biosensor later showed his real sodium loss rate fell to less than half of that as the race wore on, but he kept replacing sodium at his original rate and added salty race food on top of it. The resulting surplus, layered onto a body already handling a large fluid load, likely contributed to the cramping, nausea, and GI distress that ended his race after 15 hours, a pattern consistent with research linking concentrated sodium intake and hypertonic drinks to nausea in endurance sport.
This is where Nix's own workout data adds something the research hasn't fully mapped out yet: hourly electrolyte loss rate isn't just different between individuals, it changes within the same athlete as an event goes on over many hours. Across Nix-logged ultra-length efforts (six-plus hours), hourly electrolyte loss averaged 24% lower than in runs of three hours or less. A fixed electrolyte plan, even a well-researched one, doesn't account for that decline.
Avoid overhydrating. Research on both marathons and ultramarathons ties some of the worst outcomes, in both safety and performance, to overhydration, not underhydration.
Don't assume more sodium is always safer. Generic targets can miss an individual's actual needs by a wide margin, and more sodium doesn’t necessarily equal improved performance.
Expect your numbers to be unique. Sweat composition varies enormously between athletes, so someone else's well-planned hydration strategy won’t be a reliable plan for you.
Expect your sweat to be nonlinear. Electrolyte loss rates can decline substantially over many hours, even when fluid loss rate doesn't fall nearly as much which is where hydration tracking becomes critical.
The research is fairly consistent: the runners who get into trouble in marathons and ultramarathons are more often the ones who overcorrected, drinking past their actual fluid losses or replacing electrolytes at a rate that no longer matches what they're losing.
The challenge is that “actual losses” aren't something a generic guideline can hand you. They're individual, and over the course of six, ten, or twenty-four hours, they change. That's the gap the Nix Hydration Biosensor is built to close: real, individualized fluid and electrolyte loss data to guide you, instead of a generic number.