Excessive fluid intake during exercise can be fatal
19 July 2003
Overconsumption of fluid (either water or sports drinks) before, during or after exercise is unnecessary and can have a potentially fatal outcome, warns a sports medicine expert in this week's issue of the British Medical Journal (BMJ).
The warning, written by Timothy Noakes of the Sports Science Institute of South Africa, explains that drinking excess fluids can lead to hyponatraemic encephalopathy (brain damage arising from severely low sodium levels in the blood), and provides the recent example of the death of a female athlete in the 2002 Boston marathon due to hyponatraemic encephalopathy.
According to the article, the athlete most likely to develop hyponatraemic encephalopathy is a female marathon runner who runs a 42 km race at a speed of less than 8-9 km/hour, sometimes consuming in excess of 100 cups of fluid during the race (about 15 litres of fluid during 5-6 hours of exercise).
Until the late 1960s, athletes were advised not to drink during exercise since it was believed that fluid ingestion impaired athletic performance.
'The publication in 1969 of an incorrectly titled article, "The danger of an inadequate water intake during marathon running", provided the impetus for change to the new 'drink the maximal amount that can be tolerated' dictum ', wrote Dr Noakes.
The medical literature since that time includes 250 reports of hyponatraemic encephalopathy in athletes, military personnel, and hikers, including 7 deaths.
Dr Noakes advises athletes to drink according to their thirst, as this seems to be safe and effective. Such fluid intake typically ranges between 400 mL and 800 mL per hour in most forms of recreational and competitive exercise. Slower, smaller athletes exercising in mild environmental conditions would need to consume less fluid than superior athletes competing at higher intensities in warmer environments.
The fluid consumption guidelines outlined in the BMJ article have been recently adopted by USA Track and Field.
Last Reviewed: 21 July 2003
