Mindful hydration

Ferenc Soma Kovács, Hungarian national record holder middle-distance runner, NCAA bronze medallist and Harvard-record holder

As summer is approaching, days are once again getting hotter, and hydration — along with its importance — is receiving increasing attention. Who hasn’t heard the recommendation of drinking at least two liters of fluids per day? Or  story about someone mixing table salt into their morning orange juice after a sweaty Sunday long run to replace lost sodium?

These examples clearly show that runners and endurance athletes today approach nutrition and recovery far more consciously than they did even a few decades ago. The question, however, still remains: how accurate and quantifiable are the recommendations attempting to define proper hydration and electrolyte replacement? It is certainly beneficial to restore fluid, electrolyte, and sodium balance after training or racing in hot conditions, but it is equally important to do so based on precise measurements rather than simply “reloading” the body by feel.

Fortunately, in the 2020s, sweat monitoring is no longer reserved exclusively for sports physiology laboratories. Even recreational athletes can now easily access modern biosensors that provide accurate and detailed data about fluid, electrolyte, and sodium loss. I personally first encountered the Nix Hydration Biosensor during a training camp in Houston in March 2026. Before a long run, American Olympic finalist Graham Blanks attached the sensor patch to his bicep in order to obtain precise statistics about his sweat loss.

And these statistics represent one of the greatest advancements in modern sports nutrition. The biosensor not only measures fluid loss through sweating, but also tracks electrolyte and sodium depletion. The sophistication of the device is demonstrated by the fact that if the athlete drinks fluids during exercise, the system detects it automatically and calculates net fluid loss based on the difference between sweat loss and replenishment. The same analysis is also performed for electrolytes and sodium.

Beyond the raw loss data, one of the most interesting metrics is sweat composition. In the mobile application, the value displayed next to “Sweat Composition” shows exactly how many milligrams of electrolytes are contained in each milliliter of sweat. The software also includes a detailed comparison table featuring the electrolyte concentrations of nearly forty commercially available sports drinks. This makes it extremely easy to select the isotonic drink whose composition most closely matches your own sweat electrolyte profile.

Another particularly important metric is the so-called Nix Index. This single aggregated value incorporates the environmental conditions of the workout: temperature, humidity, dew point, heat stress, wind speed, and altitude. Naturally, both the composition and quantity of sweat continuously change depending on these factors, which means the Nix Index also dynamically adapts to the conditions. The more workouts an athlete records, the larger the dataset becomes, allowing the recommendations to grow increasingly refined and personalized. Based on all collected data, the software also generates a general sweat profile that provides concrete recommendations regarding how much water, electrolytes, and sodium the athlete should consume per hour during exercise.

This is exactly why understanding the importance of sweat monitoring matters so much. Perhaps the most important factor is that not every runner sweats the same amount — and even the same athlete does not lose identical amounts of fluid during every session. Sweat rate is influenced by numerous variables: temperature, humidity, training intensity and duration, acclimatization level, body mass, clothing, sunlight exposure, and even wind conditions. For this reason, it is extremely difficult to develop an accurate hydration strategy purely based on feel.

Improper hydration does not only reduce comfort; it directly affects performance and recovery as well. In cases of dehydration, the phenomenon known as cardiovascular drift often appears, where heart rate gradually rises despite maintaining the same running pace. Performance may decline, the same intensity may feel harder, recovery and sleep quality may worsen, glycogen restoration may slow down, and injury risk may increase. It is no coincidence that sports physiology research suggests that even a 2% loss in body mass can measurably impair endurance performance.

In practice, of course, not every run requires a laboratory-level fueling and hydration plan. During shorter and easier sessions, drinking based on thirst is often perfectly sufficient. However, during longer runs or sessions performed in hot conditions, planned hydration strategies become highly beneficial — consciously determining hourly fluid intake as well as appropriate sodium and electrolyte replacement. Features such as Polar FuelWise™ can be especially helpful here, providing intelligent reminders that support regular fueling during longer training sessions.

Ultimately, hydration is a trainable strategy just like race pace or fueling practice itself. The more precisely an athlete understands their own sweat patterns and fluid needs, the more effectively they can adapt to different environmental conditions and racing situations.