Even for something as essential as water, there really can be too much of a good thing — that's the lesson that Dr. Thomas Myers and Dr. Martin Hoffman want people to take to heart.

In this month's Wilderness & Environmental Medicine, the pair have detailed the case study of an otherwise perfectly fit woman who died from water intoxication after a five-hour-long hike at the Grand Canyon National Park in 2008. As they explain, her death may represent the first reported "fatality due to acute hyponatremia (low sodium in the blood) associated with hiking in a wilderness setting." And they're hoping that it remains the last.

According to their report, the healthy 47-year-old British woman from London spent the morning hiking 6 miles across the South Kaibab Trail in Grand Canyon National Park on a clear and sunny September day with her husband. No sooner than an hour after their hike ended at 1 p.m., the woman fainted but quickly recovered, prompting emergency medical service (EMS) responders to take her to the nearest hospital. Sluggish and complaining of a headache, the worst was unfortunately yet to come for the woman.

At approximately 2:30 p.m., while the responders were arranging to transport her, “she abruptly sat upright, pulling out her IV line in the process. She then vomited a large amount of clear fluid and immediately became unresponsive,” the authors wrote. Though hospital staff would stabilize her physical condition eventually, the woman never woke up again, having become brain dead. She was pronounced fully dead the next morning, less than 19 hours after she fainted. Her brain had swelled to the point where blood flow had stopped entirely, a condition known as cerebral edema.

According to her husband, the woman “drank a large amount of water and ate very little” during the hike. The high level of water consumption, coupled with her strenuous physical activity, had forced her body’s levels of blood sodium to swing dramatically low, causing exercise-associated hyponatremia (EAH).

Long bouts of exercise, mainly through the act of sweating, can make the body lose sodium, but it also causes the body to retain excess amounts of fluid. That perfect storm can lead to the body becoming hypotonic, such that our cells contain more sodium than the surrounding blood outside. Water then rushes into the cells in a desperate and ultimately self-destructive act to restore balance. Without proper treatment, these bloated cells wreak havoc across the body and brain, clogging up the infrastructure of our bloodstream like a stopped pipe that eventually bursts and floods the basement, only with far more lasting side effects like coma, seizure, and death.

Though certainly rare, EAH is known to happen among marathon runners and other endurance sports enthusiasts, with the authors noting that hikers had previously been theorized to be at risk of it as well — a risk tragically confirmed.

Yet the woman’s death might have been preventable, they concluded. Had she been immediately given a hypertonic IV solution that diverted water away from her cells, instead of the standard (isotonic) saline treatment intended for dehydration, which may have only worsened her condition, things could have turned out differently.

Similarly, if EMS had had the opportunity to test her sodium levels prior to her arrival at the hospital almost three hours later, they may have been able to diagnose EAH much earlier. Even there, though, her tested sodium levels may not have been considered low enough to conclude EAH, a mistake that “should serve as further evidence that treatment should be based on the extent of symptoms rather than the absolute sodium concentration,” the authors cautioned.

They added: “It is likely that symptom severity is related more to the rapidity at which the serum sodium changes than to the absolute value.”

In the wake of the hiker’s death, the Grand Canyon National Park has implemented changes to their emergency care, including the stocking of hypertonic IV fluids and immediate blood sodium testing, strategies the authors advise any establishment or sporting event that regularly deals with endurance athletes to adopt (though water intoxication is only one of the unique problems that marathon runners have to deal with).

“We hope that this report of a fatal outcome from EAH and the discussion about the importance of rapid recognition and treatment will stimulate others to develop more appropriate systems for the recognition and treatment of EAH,” they concluded.

In the meantime, it seems that taking an extra nibble or two more during a long hike may go a long way.

Source: Myers T, Hoffman M. Hiker Fatality From Severe Hyponatremia in Grand Canyon National Park. Wilderness & Environmental Medicine. 2015.