Exercise-associated hyponatremia (EAH) is the most dangerous mistake in endurance racing — more dangerous than dehydration, and far more common than most runners think. Up to 15% of marathon finishers in published studies test hyponatremic at the line. The cause is almost always over-drinking water alone. Prevention is straightforward once you know the rule.
The one-sentence answer
Drink to thirst, not to a schedule. Pair every litre of fluid with 500–1000 mg of sodium. Never gain weight during a race.
What hyponatremia actually is
Hyponatremia is blood sodium below 135 mmol/L. It happens when you replace sweat losses with hypotonic fluid (water or low-sodium sports drinks) faster than your kidneys can excrete the excess. Sodium gets diluted, water shifts into cells, and the brain swells inside a rigid skull. Mild cases get a headache; severe cases seize and die.
The symptoms — mild to severe
Early:nausea, headache, bloating, puffy hands, feeling “thick”.
Moderate: confusion, disorientation, vomiting, worsening headache.
Severe: altered consciousness, seizure, coma. Medical emergency.
Early symptoms overlap with dehydration. The wrong response — drinking more — kills people. Until you know which one you have, do not gulp water.
Who is at risk
- Slower finishers (more time on course, more drinks at aid stations)
- Female runners (smaller body mass, lower fluid tolerance)
- NSAID users (impaired renal water excretion)
- First-timers (“drink at every aid station” advice taken too seriously)
- Hot-weather races (high fluid availability)
- Ultra distances (cumulative dilution over hours)
The single rule that prevents it
Do not gain weight during a race. Drinking enough to gain weight means you are diluting your sodium. Weigh yourself before and after long training runs at race intensity — 1% body-mass loss is fine, 0% is marginal, any gain is dangerous. See our sweat-rate calculation guide to set personalised replacement targets.
Sodium is not optional
Sweat sodium varies fourfold — 200 to 2000 mg/L. Average runners lose 600–1200 mg per litre of sweat. Replacing 70–80% — 500–1000 mg/hr in moderate conditions, 700–1500 mg/hr in heat — protects against EAH. Full breakdown in our electrolytes guide and a hot-weather-specific adjustment in our hot-weather marathon fueling guide.
What to do at the first sign
- Stop drinking water.
- Find sodium — pretzels, broth, salt tablet, salty gel.
- Walk or stop. Do not push through.
- Find medical. Do not accept IV fluids unless they have ruled out hyponatremia first — IV in EAH can kill.
Race-week prep
Salt your food during race week. Aim for ~6 g sodium per day instead of the usual 2–3 g. This builds a small reserve that protects against a single bad fueling decision. Pair with adequate water — daily urine should stay pale yellow.
Aid-station discipline
Sip; do not gulp. Pass the water station if you do not need it. Sports drink at every other station — water plus food bar at the in-betweens. The “drink at every station” rule is the single largest contributor to EAH cases at major marathons.
Frequently asked questions
Are sports drinks enough? Most commercial drinks contain 100–200 mg sodium per 500 mL — not enough alone for a hot-weather race. Pair with salt tablets or salty food.
Can a half marathon cause it? Rarely — short races and high effort tend to limit fluid intake. EAH risk rises sharply past 4-hour finish times.
What about hydration packs? They make over-drinking easier. Pre-mix electrolytes into the pack rather than carrying plain water.
If I bonk, is it sodium or sugar? Different fixes — see the differential diagnosis in our bonk rescue protocol.