Exercise-Associated Hyponatremia
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Hyponatremia is dangerously low blood sodium, usually caused by drinking too much plain fluid during long events; prevent it by not overdrinking.
Detail
Exercise-associated hyponatremia (EAH) is a fall in blood sodium below normal (typically <135 mmol/L) that occurs during or after prolonged exercise. The main cause is drinking more fluid than is lost in sweat — voluntary over-hydration with water or hypotonic drinks — which dilutes the blood; this is compounded by exercise-induced retention of water (non-osmotic vasopressin secretion). Sweat sodium loss plays a smaller role. It is most often seen in endurance events lasting many hours (marathons, ultras, long triathlons), particularly in slower finishers who drink at every aid station.
Symptoms range from nausea, headache, bloating, and confusion to, in severe cases, seizures, brain swelling, and death. Critically, mild EAH symptoms can resemble dehydration — but the treatment is opposite, so suspected hyponatremia (especially with weight gain during an event) should not be treated by drinking more water.
Prevention is straightforward: do not drink beyond thirst/sweat losses, allow modest body-weight loss (a few percent) over a long event rather than aiming to fully replace fluid, and include sodium in fluids/foods during prolonged exercise (see Hydration Strategy During Exercise). Drinking to thirst is a safe default for most recreational athletes.
Not exclusive to marathons/ultras. Case reports also document EAH after ~2 hours of weightlifting-plus-tennis, and asymptomatic EAH on routine screening in ~33% of rugby players after an 80-minute match — but the common thread in every documented non-endurance-event case is either very long total session duration (~2+ hours), team-sport practices with heavy sweat losses compounded by overcompensating on fluid, or deliberate extreme overdrinking behavior (water-drinking contests, hazing/fraternity calisthenics rituals, internet "drink as much water as possible" challenges) — not the fluid intake pattern of a normal ~45-75 minute lifting session where someone sips water to thirst. A typical gym workout under an hour, drinking to thirst, is not a realistic EAH risk context (Hew-Butler et al. 2017).
Key facts
- Blood sodium <135 mmol/L; caused mainly by overdrinking (dilution).
- Risk highest in multi-hour endurance events, especially slower finishers.
- Also documented in very long (~2h+) training sessions, team-sport practices, and deliberate extreme-overdrinking behavior (contests/hazing) — not typical <60-90min gym sessions drinking to thirst.
- Symptoms: nausea, headache, confusion → seizures, brain swelling, death.
- Can mimic dehydration but treatment is opposite — don't push more water.
- Prevent by drinking to thirst, allowing slight weight loss, including sodium.
Connections
- Hydration Strategy During Exercise — avoiding both dehydration and overdrinking.
- Electrolytes — Sodium, Potassium, and Fluid Balance — sodium balance.
- Daily Fluid Needs and Hydration Targets — appropriate fluid targets.
- Exercising in Heat — Illness Prevention and Acclimatization — related endurance medical risks.
Sources
See how this applies in practice, with honest limits on what the plan changes.
Educational content only — not medical advice. Always consult a qualified professional for individualized guidance, especially around injury, pregnancy, or medical conditions.