In 1986, a research group published a paper in the Journal of the American Medical Association describing the experience of a medical student and a physician who’d become stuporous and disoriented during an ultramarathon. The men were diagnosed with hyponatremia, and they concluded that they’d developed the condition by drinking too much [water].

There’s never been a case of a runner dying of dehydration on a marathon course, but since 1993, at least five marathoners have died from hyponatremia they developed during a race. At the 2002 Boston Marathon, researchers from Harvard Medical School took blood samples from 488 marathoners after the finish. The samples showed that 13 percent of the runners had diagnosable hyponatremia, and three had critical cases of the condition. German researchers similarly took blood samples from more than a thousand finishers of the Ironman European Championship over multiple years and found that 10.6 percent of them had hyponatremia. Most of the instances were mild, but nearly 2 percent of the finishers had severe or critical cases. Although the findings indicate that hyponatremia is still a rare condition, what makes them especially concerning is that the early symptoms of hyponatremia are very easily confused with those of dehydration — weakness, headache, nausea, dizziness and lightheadedness.

FiveThirtyEight