Sport, Science

The role of sodium
in endurance sport.

By Fluyd, 5 min read

If you train hard, run long, or push through anything that makes you sweat, sodium is one of the most important things your body needs. Here's what the research actually says about why, and what happens when you don't replace it.

You lose more sodium than you think.

When you sweat, sodium is the primary electrolyte your body loses. Research published in the International Journal of Sports Science found that athletes lose between 280mg and 900mg of sodium per hour during endurance exercise, depending on intensity, conditions, and individual variation. For longer efforts, those numbers add up fast.

A 2001 review on fluid and electrolyte balance in ultra-endurance sport noted that sweat rates can reach over 3 litres per hour in hot conditions, and that sodium intake becomes increasingly critical as exercise duration extends beyond 3 hours. At those levels, water alone simply isn't enough.

What happens when sodium drops too low.

Exercise-associated hyponatremia (EAH) is a condition where blood sodium falls too low, most commonly caused by drinking large amounts of plain water without replacing sodium. A 2022 systematic review published in the International Journal of Environmental Research and Public Health, which examined literature from 1900 to 2021, found that EAH is most likely to occur in endurance athletes who over-hydrate with low-sodium fluids.

Symptoms range from nausea and headaches to, in severe cases, seizures and loss of consciousness. The research is clear, replacing sodium alongside fluid is essential for anyone exercising for extended periods.

Sodium and plasma volume.

One of sodium's key roles during exercise is maintaining plasma volume, the liquid portion of your blood. As you sweat, plasma volume drops, which reduces the amount of oxygen your heart can deliver to working muscles. Sodium helps your body retain fluid and keep plasma volume stable, which directly supports endurance performance.

A 2018 study published in the Journal of the International Society of Sports Nutrition found that athletes following an individualised hydration plan based on their sweat rate and sodium loss showed markedly improved performance compared to those drinking ad libitum. The key variable wasn't just fluid intake, it was sodium-matched fluid intake.

Sodium and muscle cramps.

The link between sodium and muscle cramps is well documented in endurance sport. The 2022 systematic review referenced above found that electrolyte imbalance, specifically sodium, is associated with exercise-associated muscle cramps (EAMC), particularly during long-duration, high-intensity efforts.

Sodium works in conjunction with potassium to regulate nerve signals to muscles. When sodium levels fall, that signalling breaks down, leading to the involuntary contractions most endurance athletes know all too well.

How much do you actually need?

The research doesn't point to a single universal number, because sodium needs vary based on sweat rate, exercise duration, heat, and individual physiology. What the studies consistently show is that for efforts under an hour in moderate conditions, dietary sodium is usually sufficient. For anything longer, or in heat, active sodium replacement makes a meaningful difference.

The key is replacing sodium proportionally to your sweat loss, not just drinking more water. Plain water without sodium can actually dilute your blood sodium concentration, making the problem worse rather than better.

The bottom line.

Sodium isn't just a performance supplement. It's a physiological requirement for anyone training hard. Replace it alongside your fluids, and your body can actually use the water you're drinking. Don't, and you risk reduced performance, cramping, and in extreme cases, something much more serious.

That's the science behind the 500mg in every serve of Fluyd.

References

McCubbin, A.J., Costa, R.J.S. (2018). Impact of Sodium Ingestion During Exercise on Endurance Performance: A Systematic Review. International Journal of Sports Science.

Maughan, R.J., Shirreffs, S.M. (2001). Fluid and Electrolyte Balance in Ultra-Endurance Sport. Public Health Nutrition.

Tsoutsoubi, L. et al. (2022). Effects of Sodium Intake on Health and Performance in Endurance and Ultra-Endurance Sports. International Journal of Environmental Research and Public Health.

Raizel, R. et al. (2018). Individualized hydration plans improve performance outcomes for collegiate athletes. Journal of the International Society of Sports Nutrition.