Exercise-Associated Hyponatremia: The Fatal Mistake of Drinking Until You Poison Yourself

A Problem More Deadly Than Dehydration

Exercise-Associated Hyponatremia (EAH) refers to a blood sodium level below 135 mmol/L during or after exercise. The primary cause is overhydration (free water intake exceeding the kidneys’ excretory capacity), which dilutes blood sodium; heavy sweat sodium loss without adequate replacement can worsen it. It has repeatedly caused hospitalizations and even deaths in marathons, ultramarathons, long-distance triathlons, and open-water swimming. Historically, it has often been mistaken for dehydration and treated with “more fluids,” leading to fatal deterioration.

Why It Happens

• Excessive water intake: Worried about dehydration, athletes chug water at every aid station, and the rate of intake exceeds the kidneys’ maximum water excretion (during intense exercise, antidiuretic hormone secretion may be inappropriately elevated, making it even harder to excrete water).
• Slower finishers are at higher risk: Slower runners pass more aid stations and continue drinking while sweating less for heat dissipation, making them the most susceptible.
• Smaller build, female, low body weight, extremely long events: These carry relatively higher risk.

The Dangerous Diagnostic Trap

EAH and dehydration/heatstroke share overlapping symptoms (dizziness, nausea, weakness), but the treatment is completely opposite: dehydration requires fluid/sodium replacement, while EAH must never be treated with large volumes of hypotonic fluids. Key clue: body weight that rises rather than falls after a race (intake exceeding losses) strongly points to EAH. Severe cases require medical treatment with hypertonic saline; do not self-administer large amounts of plain water on site.

Prevention: Drink to Thirst

Core recommendations from the international consensus (sports medicine societies and the hyponatremia consensus conference):

• Drink to thirst—do not force fixed volumes of fluid to “prevent dehydration.” Thirst is a highly reliable physiological signal.
• Estimate your individual sweat rate and sweat sodium (see the sweat sodium test article); for long events, replace sodium appropriately rather than water alone.
• Do not overhydrate before a race (excessive pre-hydration carries the same risk).
• Slower finishers in particular should limit fluid intake at each aid station.

The Role of Sodium Replacement

Sodium replacement alone cannot fully prevent EAH (the primary cause remains excessive free water), but pairing it with moderate sodium intake can reduce risk and maintain plasma volume. Ultra-long events and those with high sweat sodium loss require a sodium strategy, and sodium and water must be coordinated.

Education Matters More Than Tools

EAH is one of the few counterintuitive problems where “the more diligently you hydrate, the more dangerous it becomes.” Race medical staff, team leaders, and athletes should all recognize the red flag of “weight that rises instead of falls plus altered mental status” to avoid the fatal mistake of treating EAH as dehydration.

On the long-distance racecourse, desperately chugging water out of fear of dehydration can kill you. Remember two things: drink to thirst, and don’t let your weight rise instead of fall—these will save your life more than any electrolyte product.

Related Reading

• Exercise-Associated Hyponatremia (EAH): Hydration Mistakes and Prevention in Endurance Events Over 4 Hours
• Exercise and Hyponatremia: Drinking Too Much Water Is Actually More Deadly Than Dehydration
• Hyponatremia in Ultramarathons: The Dangers of Overhydration and the Importance of Electrolyte Balance
• Water Intoxication and Overhydration in Athletes: The Hidden Killer of Hyponatremia—How to Hydrate Correctly