The Truth About Sweat, Sodium, and Science

The Truth About Sweat, Sodium, and Science

Everywhere you look, there’s another rule about hydration. Or you’re being told to cut down on salt. Add electrolytes. Drink a glass of water first thing in the morning. Water is all you need. Drink before you’re thirsty.

It’s easy advice to follow, but hard to validate. Much of what’s shared online comes from generalisations rather than science. It’s well-intentioned, but often imprecise.

The truth is that hydration isn’t about copying what works for someone else. It’s about understanding your own physiology. And that starts with one question: Do you actually know what you’re losing when you sweat?

For a long time, hydration advice has been driven by convenience rather than accuracy. Numbers are easy to sell; nuance isn’t. But human physiology doesn’t work on averages.

Sports scientists, physiologists, and nutritionists have shown repeatedly that sodium concentration and sweat rate vary not only from person to person but also from session to session (Baker, 2017). Environmental conditions, exercise intensity, and heat acclimation all influence electrolyte losses in real time.

Research from institutions such as the Gatorade Sports Science Institute (Baker et al., 2016), the U.S Army Research Institute (Cheuvront & Sawka, Sports Medicine, 2005), and the University of Connecticut’s Korey Stringer Institute (Casa et al., 2012) has consistently demonstrated that environmental conditions, intensity, and heat adaptation all influence electrolyte loss in real time.

Leading practitioners in endurance performance emphasise the limitations of one-size-fits-all guidance. Dr. Asker Jeukendrup and Ronald J Maughan have both discussed the need for personalised sodium strategies based on individual sweat profiles. As other professionals have also reinforced that sex differences (Yanovich, Ketko and Charkoudian, 2020), heat acclimation (Péériard, Ejisvogels and Daanen, 2021), and training load all alter hydration needs.

Across the industry, there is a growing movement to put data before marketing. Companies like FLOWBIO are part of that shift, focusing on measurable, repeatable science rather than assumptions. The goal isn’t to make hydration more complicated; it’s to make it more accurate.

Because once you understand what you’re losing, the guesswork stops.

Salt isn’t bad. 

Salt has long been misunderstood. It’s often associated with high blood pressure, yet sodium plays a crucial role in maintaining fluid balance, nerve transmission, and muscle function. 

When sodium levels drop too low, the result can be reduced blood volume, delayed recovery, and, in extreme cases, hyponatremia -  a condition caused by excessive water intake relative to sodium loss (Montain, Cheuvront and Sawka, 2006; Hew-Butler, 2015).

Too little sodium can impair endurance performance just as much as too much can harm health. The key lies in balance - understanding individual sodium losses and matching replacement accordingly. That understanding begins with measurement, not assumption.

The myth of “take X amount of electrolytes”

Hydration advice often comes packaged in exact numbers. Take 1000 milligrams of sodium per hour. Drink one bottle every 30 minutes. These instructions sound definitive, but ignore the reality that no two athletes lose electrolytes at the same rate.

Studies show that sodium loss can range anywhere from 200 to over 2000 mg per litre of sweat, depending on the individual and conditions (Barnes et al., 2019). In other words, one athlete’s “ideal intake” could leave another overhydrated or depleted.

Hydration isn’t a fixed equation. It’s a variable process that shifts with temperature, humidity, altitude, and intensity. Training and acclimation enhance sodium reabsorption via upregulation of sweat gland Na⁺ transporters, reducing sweat sodium concentration (Buono et al., 2018).

A universal hydration strategy will always be flawed, because the body isn’t universal (ACSM, 2007).

Where’s the science?

Hydration is a crowded space. Claims are everywhere - faster recovery, better focus, improved endurance, yet very few are backed by independently validated data.

When researchers test hydration methods in controlled environments, results rarely mirror what happens in the field. Laboratory protocols may isolate single variables like temperature or workload, but real-world training layers them all at once.

Many sports nutritionists now argue that we should hold hydration claims to the same standard of evidence as performance supplements or training technologies. If a product claims to improve balance, recovery, or energy through hydration, it should be able to prove how and why -  with data, not slogans.

Sweat isn’t static.

Traditional sweat testing assumes consistency: one test, one number, one recommendation. But that model has always been oversimplified.

Research has shown that sweat composition changes with repeated exposure to heat, endurance training, and even changes in diet (Baker, 2019). A static reading taken in spring may no longer apply during peak summer training.

Sweat is dynamic because humans are dynamic. As the body acclimatises to heat, sweat rate may increase, but sodium concentration often decreases due to improved fluid regulation. This reflects increased sodium reabsorption in the eccrine gland duct during heat acclimation. Understanding hydration, therefore, means understanding that your body is constantly adjusting to the demands you place on it.

The evidence.

Over five years of research and development, FLOWBIO has continued to refine both hardware and algorithms to ensure accuracy in real training conditions, indoors and outdoors. Our latest independent validation study will be released in the coming weeks, showing how we compare to lab-based sodium standards in extreme environmental conditions.

We have already shown that the FLOWBIO Sensor has a typical (mean absolute) error of up to 160mL and 144mg/L for sweat loss and sodium concentration, respectively. Both were compared against current field standards (nude body mass, L’AquaTwin). For reference, this is roughly six times less than a teaspoon of salt, e.g. a pinch.

FLOWBIO stands at the forefront of sweat wearables, working with independent scientific partners across physiology and engineering. In a space filled with unverified claims, that decision speaks volumes and has allowed us to create a sensor that delivers and has been praised for its consistency and reliability as a potentially new scientific tool.

These findings align with established sports science literature showing that hydration measurement depends on individual, repeated, and context-specific data rather than one-off assessments for which reliability in performance, in a wide variety of environments, is a key factor.

It supports what researchers and nutritionists have long understood: you cannot improve what you do not measure.

Know what you’re losing.

Electrolytes matter. Water matters. But neither matters as much as understanding the balance between them.

Hydration isn’t about trends or routines. It’s about evidence. The more you know about your own body, the less you have to guess. Because when you know what you’re losing, you finally know how to put it back.

We’ve covered why science matters more than slogans. Soon, we’ll get into the details of how to replenish accurately - and how to make hydration work for you, not against you.

References:

American College of Sports Medicine, Sawka, M. N., Burke, L. M., Eichner, E. R., Maughan, R. J., Montain, S. J., & Stachenfeld, N. S. (2007). American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 39(2), 377–390.

Baker L. B. (2017). Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine (Auckland, N.Z.), 47(Suppl 1), 111–128.

Baker L. B. (2019). Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature (Austin, Tex.), 6(3), 211–259. 

Baker, L. B., Barnes, K. A., Anderson, M. L., Passe, D. H., & Stofan, J. R. (2016). Normative data for regional sweat sodium concentration and whole-body sweating rate in athletes. Journal of Sports Sciences, 34(4), 358–368. 

Baker, L. B., De Chavez, P. J. D., Ungaro, C. T., Sopeña, B. C., Nuccio, R. P., Reimel, A. J., & Barnes, K. A. (2019). Exercise intensity effects on total sweat electrolyte losses and regional vs. whole-body sweat [Na+], [Cl-], and [K+]. European Journal of Applied Physiology, 119(2), 361–375.

Barnes, K. A., Anderson, M. L., Stofan, J. R., Dalrymple, K. J., Reimel, A. J., Roberts, T. J., Randell, R. K., Ungaro, C. T., & Baker, L. B. (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Sciences, 37(20), 2356–2366

Buono, M. J., Kolding, M., Leslie, E., Moreno, D., Norwood, S., Ordille, A., & Weller, R. (2018). Heat acclimation causes a linear decrease in sweat sodium ion concentration. Journal of Thermal Biology, 71, 237–240.

Casa, D. J., Armstrong, L. E., Kenny, G. P., O'Connor, F. G., & Huggins, R. A. (2012). Exertional heat stroke: new concepts regarding cause and care. Current Sports Medicine Reports, 11(3), 115–123.

Cheuvront, S. N., Sawka, M. N. (2005). Hydration Assessment of Athletes. Sports Science Exchange, 18(2), 1-6. 

Hew-Butler T. (2019). Exercise-Associated Hyponatremia. Frontiers of Hormone Research, 52, 178–189.

Maughan, R. J., & Shirreffs, S. M. (2008). Development of individual hydration strategies for athletes. International Journal of Sport Nutrition and Exercise Metabolism, 18(5), 457–472. 

Montain, S. J., Cheuvront, S. N., & Sawka, M. N. (2006). Exercise-associated hyponatraemia: quantitative analysis to understand the aetiology. British Journal of Sports Medicine, 40(2), 98–105.

Périard, J. D., Eijsvogels, T. M. H., & Daanen, H. A. M. (2021). Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 101(4), 1873–1979. 

Yanovich, R., Ketko, I., & Charkoudian, N. (2020). Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond. Physiology (Bethesda, Md.), 35(3), 177–184.