Exercise is healthy! But a study of participants in the 2002 Boston Marathon has revealed that drinking too much during long-distance endurance events can result in a life-threatening condition called hyponatremia. During long-distance events, drinking water is usually considered harmless –even necessary – but some participants may consume excessive amounts and thereby dilute their blood plasma. Some drink so much that their blood serum sodium concentration falls below 135 mmol l-1 (hyponatremia), and the falling salt concentrations in their plasma may be further exacerbated by the salts they lose through sweating. The symptoms of hyponatremia, which is also called water intoxication, are curiously quite similar to those of dehydration (apathy, confusion, nausea and fatigue). Ultimately, the lowered plasma electrolyte concentrations can lead to impairment of muscle and nervous function, which, in rare cases, can be fatal.

Christopher Almond and his colleagues investigated 488 runners before and after the 2002 Boston Marathon to identify the occurrence and principal risk factors of hyponatremia. To measure weight gain as a result of drinking during the race, Almond and his colleagues weighed participants before and after the race. At the end of the race, they took blood samples from the participants to measure sodium levels and asked them to complete a questionnaire detailing their fluid intake during the race. They investigated the role of participants' race, sex, use of anti-inflammatory drugs, water intake, race time and body mass index on the prevalence of hyponatremia.

They found that approximately 13% of the 488 participants showed signs of hyponatremia, and 0.6% had severe hyponatremia (less than 120 mmol l-1 sodium in their blood serum). Considering that, in some years,over 30 000 people have participated in the Boston Marathon, this would mean that more than 150 people would experience severe hyponatremia following such events. The study concluded that the principal risk factors for hyponatremia were weight gain, consumption of more than 3 litres of fluid, consumption of fluids every mile and a race time above 4 hours. Of these risk factors, the authors concluded that excessive drinking is the single most important factor associated with hyponatremia. Since this is easily monitored from a substantial weight gain during the race, the authors suggest a preventative strategy where runners monitor their weight and fluid intake during training and use these observations to regulate their hydration strategy during high-endurance exercise.

Historically, exercising athletes were advised not to drink, as this was believed to impair performance. However, since the late 1960s there has been an increasing focus on preventing dehydration (partially driven by the sports drink industry), which has led to recommendations of large fluid intakes during endurance events. This may not be such sensible advice; while there are no recorded fatalities related to dehydration, hyponatremia has claimed victims. Based on the findings of their study, Almond and colleagues emphasise that drinking too much while exercising is a bad idea. Their study also shows that hyponatremia is not exclusive to elite athletes but may be a more relevant concern for `normal' marathon runners who tend to race longer, sweat less and drink more than professional athletes.

Incidentally, the male and female winners of the 2002 Boston Marathon were Hailu Negussie (2 h:11 min:45 s) and Catherine Ndereba (2 h:25 min:13 s). Both were presumably too busy to drink excessively!

Almond, C. S. D., Shin, A. Y., Fortescue, E. B., Mannix, R. C.,Wypij, D., Binstadt, B. A., Duncan, C. N., Olson, D. P., Salerno, A. E.,Newburger, J. W. et al. (
). Hyponatremia among runners in the Boston Marathon.
New Engl. J. Med.