Back-to-back heat domes moved across the US in July, at one point putting more than 160 million people under heat alerts at the same time, with record highs tied or broken from the Midwest to the Northeast (1,2). For athletes training through conditions like these, the generalized advice is to drink more. That guidance is not wrong, but it is incomplete, and incomplete guidance in extreme heat carries risk in both directions: too little fluid and electrolyte replacement leads to dehydration, while too much fluid without adequate electrolyte replacement introduces the lesser discussed risk of hyponatremia.
“Drink more” is not a target. It does not specify how much more to consume, nor does it account for how much fluid and how many electrolytes a given athlete is actually losing in the heated environment, and it says nothing about replacing electrolytes at all. Sweat rate alone varies enormously between athletes, with documented ranges spanning roughly half a liter to well over two liters per hour depending on body size, fitness level, heat acclimatization status, and genetics (3). Electrolyte concentration in sweat varies just as widely from one athlete to the next, and even in the same athlete from one day to the next as conditions shift.
Generalized hydration guidance exists to apply to everyone because it is calibrated to an average that describes no one athlete precisely. An athlete who follows population-level advice in the heat is, in effect, guessing at their fluid and electrolyte replacement needs and risking both safety and performance (4).
This is where hydration tracking changes the equation. Measuring an individual athlete’s sweat rate and electrolyte loss rate converts “drink more” into an actual number: a specific volume of fluid and electrolytes, calculated for that athlete under that day’s environmental conditions. Maintaining proper hydration status while training in the heat doesn’t depend solely on “drinking more.” It depends on having the number rather than approximating it, since both underestimating and overestimating fluid and electrolyte needs carry distinct physiological risks.
Hyponatremia is a drop in blood sodium concentration below normal range. In endurance sport, hyponatremia most often occurs when an athlete drinks plain water at a rate that exceeds fluid loss while electrolyte losses through sweat go under-replaced. The total body sodium may be roughly unchanged, but its concentration
falls because it is now dissolved in a larger volume of fluid.
Heat training raises the stakes on this specific mistake. As sweat rate and electrolyte losses climb with temperature and duration, the instinct to drink more water becomes both more common and more risky, precisely at the moments when electrolyte replacement should be increasing with fluid intake, not lagging behind it.
The danger is compounded by symptom overlap. Early hyponatremia presents with headache, nausea, confusion, and malaise, symptoms that closely resemble both dehydration and early heat exhaustion. An athlete or bystander who misreads hyponatremia as dehydration and responds with more plain water can worsen the underlying problem rather than correct it. Severe hyponatremia can progress to seizure, cerebral edema, and even death.
When training in the heat, it’s imperative that athletes stop treating fluid intake as the only variable. Electrolyte replacement needs to scale with electrolyte losses, which themselves scale with heat, humidity, and duration, the same conditions that drive fluid losses upward. Managing one without the other is how athletes end up correcting dehydration while inadvertently creating a hyponatremia risk, or the reverse (5).
The Priming Calculator in the Nix Solo app is built to identify the optimal amount of fluid and electrolytes an athlete should aim to consume in the two hours leading up to a workout. The calculator uses an athlete’s historic Nix data combined with the current environmental conditions in their geographical location to recommend a personalized hydration strategy in the two hours leading up to a workout rather than relying on population averages.
In the hours leading up to a training session in the heat, an athlete enters the planned start time, expected duration, and conditions into the app. The Priming Calculator then returns a specific pre-hydration plan rather than a single blanket number: how much fluid and how many electrolytes to consume at set intervals before the session begins, so the athlete starts the workout already at a stable hydration and electrolyte baseline instead of starting behind and trying to catch up mid-session.
This distinction matters physiologically. Pre-hydration status at the start of a workout affects heat tolerance, cardiovascular strain, and pacing capacity from the first minute of exercise.
On days when the forecast points to elevated heat and humidity, and therefore elevated sweat and electrolyte loss, the calculator adjusts the pre-hydration target accordingly, since the same workout on a hotter, more humid day demands a different starting point than it would on a milder one. The output at each stage is a specific fluid volume and a specific electrolyte amount, not a general instruction to drink more.
Both dehydration-driven heat illness and hyponatremia can present with headache, nausea, dizziness, and confusion, which is why field diagnosis is difficult and why treatment decisions should not be based on symptoms alone. A rapid or pounding pulse, a core body temperature of 103°F or higher, or loss of consciousness point toward heat stroke (5). Persistent nausea, confusion, or swelling that does not resolve with rest, and that developed after high fluid intake, warrants the same urgency and should be evaluated as a possible case of hyponatremia rather than treated with additional plain water.
Fluid loss and electrolyte loss are two separate quantities, driven by the same conditions but not interchangeable, and treating either one as a stand-in for the other is how athletes end up on the wrong side of dehydration or hyponatremia. Quantifying both, rather than estimating from thirst or a fixed daily number, is what allows hydration status to be managed with precision instead of guesswork, before a session in the heat begins and throughout it.
1. NBC News. Unpacking the U.S. heat wave ahead of July Fourth 2026, in 7 graphs and charts. July 2, 2026.
2. Axios. US heat wave death toll grows. July 5, 2026.
3. Armstrong LE, Stearns RL, Huggins RA, Sekiguchi Y, Mershon AJ, Casa DJ. Reference Values for Hydration Biomarkers: Optimizing Athletic Performance and Recovery. Open Access J Sports Med. 2025;16:31-50.
4. Bandiera D, et al. What do thermal indices lack to help predict heat-related risk in elite Athletics? Br J Sports Med. 2026.
5. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. Curr Sports Med Rep. 2021;20(9):470-484.