What is hyponatremia and can it happen to endurance athletes?

Hyponatremia (serum sodium below 135 mEq/L) in endurance athletes occurs when fluid intake substantially exceeds fluid losses, diluting plasma sodium, it is the primary cause of exercise-associated collapse in marathon and ultra-endurance events, more common than dehydration as a collapse cause, and potentially fatal at severe levels (below 120 mEq/L), driven largely by the misconception that more water is always better during prolonged exercise (Almond et al., NEJM, 2005).

The Almond et al. 2005 Boston Marathon study found that 13% of runners completing a marathon had hyponatremia, and the primary risk factor was drinking beyond thirst (overdrinking). Most training recommendations from the 1990s and 2000s encouraged drinking ahead of thirst during endurance events. The current evidence-based recommendation for exercise hydration is drink to thirst, your thirst mechanism is a highly calibrated osmosensor that, in healthy men without extreme heat stress, accurately reflects your fluid needs. Drinking beyond thirst in endurance events is genuinely dangerous.

Honesty Scale: Solid (1) for exercise-associated hyponatremia and the drink-to-thirst principle for endurance athletes.

What to do: For events longer than 2 hours, include sodium in your fluid replacement (sports drink, salt tabs, electrolyte mix) to prevent the dilutional hyponatremia risk from drinking large volumes of plain water. Drink to thirst, not to a fixed schedule.

For the full picture, read The Hydration Deep Dive