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How much should athletes drink during exercise?

July 16, 2003

Exercisers must be warned that the overconsumption of fluid (either water or sports drinks) before, during, or after exercise is unnecessary and can have a potentially fatal outcome, says a sports medicine expert in this week’s BMJ.

The warning follows reports of several deaths from a severe lack of salt in the blood due to excessive drinking (hyponatraemic encephalopathy).

Until the late 1960s, athletes were advised not to drink during exercise since it was believed that fluid ingestion impaired athletic performance, writes Timothy Noakes of the Sports Science Institute of South Africa. The publication in 1969 of an incorrectly titled article, ‘The danger of an inadequate water intake during marathon running’ provided the impetus for change, and led to guidelines for ingestion of fluids during exercise.

These guidelines make several assumptions that are not backed up by evidence. Nor were trials undertaken to ensure that these guidelines are always safe, says the author. The first reports of hyponatraemic encephalopathy in athletes, military personnel, and hikers appeared shortly after.

Since the cause of the condition is now known, absolute prevention is possible. Perhaps the best advice is that drinking according to the personal dictates of thirst seems to be safe and effective, he adds. Such fluid intake typically ranges between 400 ml and 800 ml per hour in most forms of recreational and competitive exercise.

The recent adoption of these guidelines by USA Track and Field provides the hope that this sad aberration has finally run its tragic course, he concludes.

British Medical Journal (BMJ)