From antiquity to the late 1960s, athletes were advised not to drink during exercise since it was believed that fluid ingestion impaired athletic performance.² The publication in 1969 of an incorrectly titled article, “The danger of an inadequate water intake during marathon running,”⁴ provided the impetus for change, even though the study neither examined a 42 km marathon race nor did it identify any dangers.² Rather, the most dehydrated athletes won those 32 km races, as is usually the case.² This article's incorrect title provided the intellectual incentive for numerous studies, many funded by a fledgling sports drinks industry, culminating in specific guidelines for ingestion of fluids during exercise.^3,5

These guidelines make four assumptions. Firstly, that all the weight lost during exercise must be replaced if health is to be protected and performance is to be optimised, since, as the guidelines state, the greatest threat to health and wellbeing during prolonged exercise, especially when performed in the heat, is dehydration.⁶ Secondly, that the sensations of thirst underestimate the real fluid requirements during exercise. Thus athletes must be told how much to drink during exercise. Thirdly, that the fluid requirements of all athletes are always similar so that a universal guideline is possible. Fourthly, high rates of fluid intake can do no harm. Thus athletes are now advised to replace all the water lost through sweating (that is, loss of body weight), or consume the maximal amount that can be tolerated or drink 600-1200 ml per hour.⁵

But none of these ideas is evidence based.^2,3 In particular, there is no evidence that athletes must drink “the maximal amount that is tolerable” to optimise performance and prevent medical consequences. Thus the hyperbolic statement, “If strenuous exercise is undertaken by hypohydrated subjects, the medical consequences can be devastating,” has no factual basis.⁷ Nor is it proved that all the weight lost during exercise must be replaced immediately, since the resting human may carry a fluid reserve of about 2 litres.^2,3 Nor were prospective trials undertaken to ensure that these guidelines are always safe. Thus it was not then appreciated that unrestrained drinking, either at rest⁸ or during exercise⁹ can have fatal consequences.^1,2,8–12

The first reports of hyponatraemic encephalopathy in athletes, army personnel, and hikers appeared shortly after the change to this new “drink the maximal amount that can be tolerated” dictum.^2,3,9,10 To date at least seven fatalities and more than 250 cases of this condition have been described in the medical literature.^9–12 Presumably reported cases represent a small proportion of all such cases.

Aside from military personnel, the athlete most likely to develop hyponatraemic encephalopathy is a female marathon runner, who runs those 42 km races at speeds slower than 8-9 km/h (about 5 mph). She gains weight during exercise because she drinks excessively both before and during exercise, sometimes in excess of 100 cups of fluid during the race (about 15 litres of fluid during 5-6 hours of exercise.)¹² She does not develop a marked sodium deficit, nor does she have evidence of inappropriate secretion of antidiuretic hormone, although antidiuretic agents are clearly active.¹⁰ Since the cause of the condition is now known, prevention is possible. Thus Gardner has concluded that further deaths from hyponatraemic encephalopathy in the United States army will reflect the failure of the system to protect adequately its personnel through policy, procedures, and implementation.⁹

To protect all exercisers from this preventable condition, rational and evidence based advice must be provided.³ In particular, 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. Perhaps the best advice is that drinking according to the personal dictates of thirst seems to be safe and effective.^2,3 Such fluid intake typically ranges between 400 ml and 800 ml per hour in most forms of recreational and competitive exercise; less for slower, smaller athletes exercising in mild environmental conditions, more for superior athletes competing at higher intensities in warmer environments.³