> Hyponatremia occurs in a substantial fraction of nonelite marathon runners and can be severe.

> As marathon running has surged in popularity during the past quarter-century, reports have emerged of serious illness and death from hyponatremia, as in the case of a 28-year-old woman who died after the 2002 Boston Marathon.

> Participants in the 2002 Boston Marathon were recruited … 488 runners (64 percent) provided a usable blood sample at the finish line. Thirteen percent had hyponatremia (a serum sodium concentration of 135 mmol per liter or less); 0.6 percent had critical hyponatremia (120 mmol per liter or less).

[1]: Hyponatremia among Runners in the Boston Marathon. https://www.nejm.org/doi/full/10.1056/NEJMoa043901

However, a 6-year study reported that exercise-associated hyponatremia, dehydration, and heat exhaustion occurred at nearly the same rate, prompting re-examination of emergency protocols. Most exercise-associated hyponatremia cases at the Grand Canyon occur among summer hikers to the inner Canyon and are often misdiagnosed as exertional heat illness due to overlapping signs and symptoms.

https://journals.lww.com/cjasn/fulltext/2024/05000/exercise_...

Hyponatremia is especially dangerous because a common response is to drink even more water, which makes the condition even worse. The National Park Service changed all their hiking advice to explicitly recommend "salty snacks" and "sports drinks".