This morning, almost 27,000 entrants ran in the 115th annual Boston Marathon. Geoffrey Mutai of Kenya won the men's race in a world-record time of 2:03:01, denying American Ryan Hall, who finished 4th, the chance to become the first American to win the race in almost 30 years. (Mutai's time equates to around an astonishing 4:45 per mile.) On the women's side, New Zealand's Kim Smith led for much of the race, but had to fall back towards the back half of the course, and Kenya's Caroline Kilel won in 2:22:36. Yet while the attention was on the elite runners, thousands of entrants were far more ordinary: despite the marathon's qualifying standards, in 2010 almost 6,000 runners were nonqualifying entrants, "who either bought entry numbers from foreign tour operators or were granted invitations by organizers, sponsors, vendors, licensees, consultants, municipal officials, or marketers peddling entries for profit." Of course, even most of these inexperienced entrants know to stay hydrated during the run. But is there a limit to hydration? Can you drink too many fluids?
Well, the danger of over-hydrating became all too clear in the 2002 Boston Marathon, when a 28-year-old runner collapsed during the race and died two days afterwards from hyponatremia, "a condition often resulting from over hydration, which depletes essential minerals from the body, causing disorientation, illness and in rare cases, death." Three years later, doctors from Harvard Medical School and Harvard School of Public Health reported in the New England Journal of Medicine on an analysis of runners in the same marathon. The participants "were recruited one or two days before the race. Subjects completed a survey describing demographic information and training history. After the race, [about 500 of the 766 participants] provided a blood sample and completed a questionnaire detailing their fluid consumption and urine output during the race." Disturbingly, "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)...[T]he strongest single predictor of hyponatremia was considerable weight gain during the race, which correlated with excessive fluid intake." In addition, hyponatremia was more common runners who had slower times and "body-mass index extremes." (The kind of fluid intake--water, sports drinks, etc.--did not correlate with hyponatremia.) The authors concluded, "hyponatremia occurs in a substantial fraction of nonelite marathon runners and can be severe." With the marathon organizers tightening qualifying standards for next year, hopefully the new standards will go some way towards lessening this problem.