Claire Squires had been a healthy 30 year-old woman when she died less than a mile from the finish of the London Marathon two weeks ago. Her death has caused an outpouring of grief around the world and prompted more than £1m in donations to charity. This is laudable but what is unusual, is that even at this point after the tragedy, no cause of death has been announced. She is the 11th person to die in the London Marathon since 1990 - the others have all been male, ranging in age from 22 to 59. Most deaths in marathoners are due to cardiovascular causes, primarily in middle age men with preexisting heart diease. Claire was likely an outlier, having previously run a marathon in under four hours and successfully climbed Mount Kilimanjaro last year. Heart disease as the cause of her death, while possible, would be quite unusual.

Although an autopsy has reportedly been done, no results have been made public. Young women like Claire, who collapse or die during marathons tend to be susceptible to a form of water intoxication that is caused by drinking excessive water during the race. The excessive water concentrations in the blood may cause fatal brain swelling and can be diagnosed either by simple blood tests measuring sodium concentrations in the blood or by post-mortem examination of the brain. However, simply drinking too much water by itself may not be the only cause of the phenomenon of overhydration (known scientificially as exercise-associated hypontremia). Other factors as yet unknown, perhaps hormonal and genetic, are believed to contribute, especially in young women (although that may be attributable to their body mass index rather than their sex). While death is unusual from exercise-associated hyponatremia, confusion or disorientation, thought to be caused by less severe temporary brain swelling, are surprisingly common.

This is not an academic concern as we have discovered from marathon deaths in the United States. Runners should drink only when thirsty rather than, as is commonly believed, drinking some predetermined amount. Conventional wisdom up until several years ago was that marathon runners should drink copiously during the race. This was called into question several years ago when exercise assocaited hyponatremia in Boston Marathon runners was described in the New England Journal of Medicine.

Certainly dehydration is a concern during marathon runs and adequate water intake is essential. But we are learning that the problem is more complicated than previously thought. Authorities in the United Kingdom have been slow to address this issue, which carries major implications for thousands of runners worldwide. The inquest into the death of Claire Squires may reveal that it was overhydration rather than underhydration or heart disease that caused her tragic death which should prompt a reevaluation of advice given to all runners during marathons regarding how much water to drink. When will British officials make this information available? Future deaths like that of Claire Squires may be preventable.

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