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Posted: April 14, 2005

Sportsmedicine: New Study Finds 13 % Hyponatremia At Boston Marathon

The April 14 issue of the New England Journal of Medicine includes results from the largest-yet study of hyponatremia among marathon runners. These results, taken at the 2002 Boston Marathon, found that 13 percent of subjects finished with hyponatremia, a low concentration of sodium in the blood. Excessive fluid intake was found to be the most important factor contributing to this condition, which in rare cases can lead to seizures, coma or even death.

The study singled out three factors: substantial weight gain while running, longer race duration (more than 4 hours) and a lower body-mass-index as the primary risk factors associated with hyponatremia in runners. Surprisingly, the type of fluid--water or sports drink--ingested by runners during the race was not found to be a factor. Researchers suggest that the best way to reduce the frequency and severity of hyponatremia is to inform the greater public about the risk factors identified in this study that are linked to this potentially life-threatening condition.

"Our findings showed that it was the amount rather than the type of fluid a runner consumed that put them at greatest risk for hyponatremia," said Dr. Christopher Almond, a lead investigator of the study, and cardiac fellow at Children's Hospital Boston. "Specifically, runners who drank enough fluids to actually gain weight while running were at highest risk for hyponatremia; whereas runners who lost weight--as one might expect to see with running--were at much lower risk of hyponatremia. Ultimately, the observed weight changes correlated well with the fluid intake reported by runners, suggesting that hyponatremia may be a largely preventable condition if runners avoid over-drinking."

Prior to the marathon, subjects filled out a survey including demographic information, information on their training regimen for the marathon, their medical history and details about their planned hydration strategy for the race. Upon finishing the marathon, participants provided a blood sample and completed a questionnaire regarding their fluid consumption during the race. Researchers expected to find that runners who consumed large amounts of water, as opposed to electrolyte containing solutions, would become hyponatremic. What the researchers learned after analyzing the data, however, was that the type of beverage consumed had no significant effect on this condition. In other words, whether a runner drank water or a sports drink during the race, they were equally as susceptible to hyponatremia if they over-drank either of these fluids.

The research study, which was led by resident physicians from the Department of Medicine at Children's Hospital Boston, is the largest study to date looking at the problem of hyponatremia in marathon runners. While previous studies have implicated excessive fluid intake, earlier studies have been small or restricted to highly elite runners who appear to be at much lower risk of hyponatremia, limiting the ability of these studies to discern with any confidence major risk factors associated with the condition. The broad population of runners examined, which included both elite and non-elite runners, allowed researchers to estimate the scope of the problem in a population more representative of the diverse group of individuals entering marathons today. According to co-author Dr. David Greenes, if the study's results were representative of the entire field of runners, approximately 1,900 of the 14,573 finishers of the 2002 Boston Marathon had some degree of hyponatremia, 90 of those having critical levels of hyponatremia. This suggests the problem of over-drinking may be more common than previously believed, especially in non-elite runners.

In addition to these findings, while earlier studies have suggested female sex is an important risk factor for hyponatremia, the Children's report suggests that gender per se is not the essential factor. Children's researchers suspect that differences in body size, race time, and drinking patterns between male and female runners are key factors.

"This is a very important study which demonstrates the need for runners to be careful about their fluid intake both during and after the race," said Dr. Marvin Adner, medical director for the Boston Marathon. "Significant hyponatremia may develop in runners who are feeling well enough not to seek medical attention from medical staff during the race or at the finish line of a marathon. Despite feeling well, runners still run the risk of developing more serious illness if they presume they are dehydrated from running 26 miles and try to 'compensate' by drinking large amounts of fluid. It is difficult to make one recommendation for all circumstances, but typically one pint of fluid taken after the race may be enough to restore an adequate level of hydration."

For more information about recommended fluid intake and hydration strategies:

USATF Training Library - Hydration.

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