Sodium and the Myth of Salt in Ultras

Electrolytes are one of the most discussed and least understood topics in ultra-running nutrition. Advice ranges from aggressive hourly sodium supplementation to avoiding all electrolyte products. The scientific picture is more nuanced than either extreme. This post discusses what electrolytes actually do, what the evidence says about supplementation in ultra-events, and what a practical approach looks like.

What Electrolytes Do

Electrolytes are minerals that carry an electric charge when dissolved in fluid. The most important ones for endurance performance are sodium, potassium, magnesium, and chloride.

Sodium is the main extracellular electrolyte. It regulates fluid balance between cells and the bloodstream and is lost in sweat at a rate of two hundred to over two thousand milligrams per liter, depending on individual sweat rate and sweat sodium concentration. It is crucial for maintaining plasma volume and blood pressure during prolonged exertion.

Potassium is the main intracellular electrolyte. It is involved in muscle contractions and nerve function and is lost in sweat much less than sodium. It is rarely an acute limiting factor but becomes more relevant in multi-stage events.

Magnesium is involved in over three hundred enzymatic reactions, including energy metabolism and muscle function. It is depleted more slowly than sodium. Supplementation during competition is of uncertain benefit, but its deficiency during training can impair recovery.

Chloride follows sodium in sweat and is usually not supplemented separately.

The Story of Sodium in Ultras

Sodium is the electrolyte that matters most acutely in ultra-racing, for two reasons.

The first is hyponatremia. Hyponatremia—abnormally low blood sodium—is a potentially life-threatening condition that occurs in ultra-runners who consume large amounts of plain water without adequate sodium replacement. Symptoms include nausea, confusion, swelling, and in severe cases, seizures and brain edema. Counter-intuitively, hyponatremia is more common in slower runners who spend more time on course and often overdrink plain water. Aggressive intake of plain water without sodium replacement is the primary cause.

The second is performance. Even subclinical sodium depletion—below the threshold for hyponatremia symptoms—impairs plasma volume maintenance and increases perceived exertion. Adequate sodium intake supports fluid retention, blood pressure regulation, and sustained performance.

How Much Sodium Do You Need

Sweat sodium concentration varies hugely between individuals—from around two hundred milligrams per liter to over two thousand. This means that generalized recommendations are inherently imprecise. For most ultra-runners, a practical starting point is three hundred to six hundred milligrams of sodium per hour for long events in moderate conditions, and five hundred to a thousand milligrams per hour in hot conditions or for heavy sweaters.

The simplest practical approach is to incorporate a sodium-containing electrolyte product into your hydration strategy—an electrolyte drink, an electrolyte tablet dissolved in water, or salty real food at aid stations—and monitor for signs of both dehydration and overhydration.

Keeping Fuel and Electrolytes Separate

A pure carbohydrate-based fueling system does not need to contain electrolytes. Separating fuel delivery from hydration and electrolyte delivery allows for independent control over each—and makes both easier to adjust during a race.

A practical consequence is that you must incorporate a source of sodium into your hydration strategy. Options include electrolyte tablets dissolved in water in soft flasks, electrolyte powder added to a water bottle at an aid station, or aid station food containing salt, such as salted potatoes, broth, or saltine crackers.

Do not mix electrolytes with concentrated carbohydrate gel. This changes the osmolality of the gel and can affect absorption and gut tolerance.

Cramps and Electrolytes

Exercise-associated muscle cramps are commonly attributed to electrolyte depletion. The evidence for this mechanism is weaker than popular belief suggests. Current research indicates that exercise-associated muscle cramps are primarily a neuromuscular phenomenon—a consequence of fatigue and altered neuromuscular control—rather than merely electrolyte deficiency in most cases.

This does not mean that electrolytes are irrelevant. Severe sodium depletion does increase cramp susceptibility. But electrolyte supplementation alone does not reliably prevent cramps in all athletes, and the primary intervention for avoiding cramps in ultras is pacing, managing training load, and avoiding extreme fatigue.

Summary

Sodium is the electrolyte that matters most in ultra-racing. Incorporate a consistent source of sodium into your hydration strategy, separate from your fuel. Avoid aggressive consumption of plain water without sodium replacement. Sweat rate and sweat sodium concentration are highly individual—heavy or salty sweaters will need significantly more sodium than average and benefit from personalized planning.

References

Hew-Butler T, Rosner MH, Fowkes-Godek S, Dugas JP, Hoffman MD, Lewis DP, et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine. 25(4), 303–320.

Maughan RJ, Shirreffs SM. (2010). Development of hydration strategies to optimise performance for athletes in high-intensity sports and in sports with repeated intense efforts. Scandinavian Journal of Medicine and Science in Sports. 20(Suppl 2), 59–69.

Latzka WA, Montain SJ. (1999). Water and electrolyte requirements for exercise. Clinics in Sports Medicine. 18(3), 513–524.