I was reading: Extreme exercise: an expert opinion
One of the most concerning life threatening conditions seen by sports physicians at endurance events is exercise associated hyponatraemia. This is caused by overhydration with fluids during the race, with dilution of serum sodium that results in encephalopathy and cerebral oedema. Sadly, this preventable condition has caused notable deaths at marathons and ultraendurance events around the world, including that of a 22 year old man who completed the London Marathon in April 2007. The key message for participants is to avoid overhydrating by drinking only according to thirst. Reluctance to accept this evidence in the past 20 years may be because it conflicted with the prevalent message of the sports drink industry.
And by the way:
Four important instruments a doctor should have ready access to when covering a marathon or ultraendurance event are a point of care analyser for checking blood sodium, a rectal thermometer to check for hypothermia and evidence of heat stroke, a glucometer to check for hypoglycaemia, and a defibrillator.