Exercise-Associated Hyponatremia

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 exercise-hydration-strategy). Drinking to thirst is a safe default for most recreational athletes.

• Blood sodium <135 mmol/L; caused mainly by overdrinking (dilution).
• Risk highest in multi-hour endurance events, especially slower finishers.
• 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.

• exercise-hydration-strategy — avoiding both dehydration and overdrinking.
• electrolytes — sodium balance.
• fluid-needs — appropriate fluid targets.
• exercise-in-heat — related endurance medical risks.