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Exercise-Associated Hyponatremia

The notion that one could run a fast marathon without consuming an ounce of fluid sounds like a ridiculous notion in this day and age. But that’s the exact paradigm shared by many elite marathon runners right up until the 1970’s1. In this sense, the widely promoted idea that athletes need to actively stay hydrated so as to minimise body water loss is still a relatively new idea in sport. But to the disappointment of sport drinks companies, new research is starting to question the validity and safety of the current hydration guidelines2.

What Is Exercise-Associated Hyponatremia?

Exercise-associated hyponatremia (EAH) is the name given to the condition of critically low plasma sodium levels that arise principally from excessive fluid consumption associated with exercise. Low sodium levels ultimately impair organ and brain function leading to impaired exercise performance. Symptoms usually consist of nausea, vomiting, headache, bloating, and swelling of hands, legs, and feet.

With record participation numbers in popular events such as the City to Surf, one almost has to “have their head in the sand” to miss the messages that are beamed constantly about the need to stay hydrated during fun runs and the like. Water and electrolyte aid stations are a common feature in any mass participation fun runs or marathons. To omit them would be considered downright dangerous and potentially harmful to competitors.

Incidence of Exercise-Associated Hyponatremia

But with an increase in the number of reports of EAH in recent times, authorities and scientists alike are increasingly questioning the validity of the current guidelines on fluid consumption during exercise. A recent study of participants in the London Marathon found that of the 88 study volunteers that were recruited,  11 (12.5%) developed asymptomatic hyponatraemia (serum sodium 128-134 mmol/l)3. Compared with participants who did not develop EAH, they consumed more fluid and gained more weight.

The table below is taken from a recent 2014 review on EAH in the Journal of Clinical Medicine. It shows the reported incidence of EAH across a range of popular endurance and ultra-endurance sports4. Prevalence of EAH tends to be higher in longer events and in females.

prevalence of exercise-associated hyponatremia

Original Fluid Consumption Patterns & Beliefs

Ron Hill, a famous distance runner from the 1960s and 70s (the second man to break 2:10 for the marathon) did not take in any fluid during his winning run at the 1970 Commonwealth Games Marathon5, 6. Accordingly he lost 3.9% of his bodyweight, but having set a world record with a time of 2:09:28, it’s obvious the lack of fluid intake and associated increased body water loss did little to affect his performance.

Ron represents a number of successful runners who operated on the belief that fluid intake during running was to be kept to a minimum and totally restricted if possible. American Joseph Forshaw who finished fourth in the 1908 Olympic Marathon and tenth in the 1912 Olympic Marathon wrote: ‘I know from actual experience that the full race can be covered in creditable time without so much as a single drop of water being taken or even sponging of the head’6.

Renowned Englishman marathon runner Jim Peter (who set the world marathon record of 4 occasions) expressed a similar belief: ‘[In the marathon race] there is no need to take any solid food at all and every effort should also be made to do without liquid, as the moment food or drink is taken, the body has to start dealing with its digestion, and in so doing some discomfort will almost invariably be felt’6.

Evolution of Fluid Consumption Guidelines

Up until 1969, the American College of Sports Medicine’s (ACSM) official advice was to avoid drinking during exercise. In fact, up until 1975, race organisers were not permitted to provide runners with access to fluid until completion of 16km1. However, this changed gradually over the next two decades; starting in 1975 with the recommendation to consume fluid regularly both before and during exercise7. As shown in the table below, the increase in the recommended frequency of fluid intake during exercise culminated in 1996 with the recommendation to consume “the maximal amount that can be tolerated”8. It was only in 2007 that a more individualised recommendation was made to consume enough fluid to prevent greater than 2% body weight drop9


changing guidelines on fluid consumption during exercise

Is Dehydration Detrimental to Performance?

The whole premise of the above hydration guidelines is that any loss of body water (and associated weight) is detrimental to performance. A landmark study in 1969 linking >3% loss of bodyweight during exercise with unsafe increases in body temperature was the catalyst to the change in hydration guidelines10. But the study has since been widely criticised for its flawed design and conclusion, with many more recent studies suggesting that loss of body weight during exercise does not necessarily infer poorer performance and in fact may predict better performance1

The key assumptions that underpin the belief that >3% loss of bodyweight are as follows11:

  • All the weight lost during exercise must be replaced if health is to be protected and performance is to be optimised
  • The sensations of thirst underestimate the real fluid requirements during exercise
  • The fluid requirements of all athletes are always similar so that a universal guideline is possible
  • High rates of fluid intake can do no harm

But as we will see below, there is ample evidence showing that each of the above assumptions are misplaced.

Running Speed & Body Weight Loss

Challenging the notion that a loss of body weight is detrimental to performance are studies showing a trend for faster runners to lose more body weight when exercising. For example, studies in the late 1960's showed that race winners of marathons were those who lost the most weight and usually had the highest postrace core body temperatures10, 12. A 2011 study found an inverse correlation between percentage body weight change and finishing time in 643 marathon runners.

Perhaps one of the most fascinating studies concerns the sweat and fluid consumption rates of elite marathon runners. The graph below shows the actual fluid consumption rates of elite marathon runners (all with sub 2:10 marathon times) in comparison to the existing ACSM guidelines. It’s evident that most runners fall well below14.

rates of fluid consumption in different marathon races

What’s even more intriguing is the average body weight loss when fluid consumption and sweat rates were taken into account. On average, the body weight loss of the 5 elite marathoners was found to be 8.8% during a marathon. This took into account average measured sweat rates of 2.3 litres per hour.

Sweat Rate & Fluid Consumption of Haile Gebrselassie

One case in point is that of popular Ethiopian runner Haile Gebrselassie (multiple world record holder and Olympic Gold Medalist). In winning the 2009 Dubai marathon (run at 16.0oC and 53.5% relative humidity) in a time of 2:05:29, Selassie was found to have a reduction in body mass of 5.7kg (58.2-52.5kg); equivalent to a 9.8% body mass reduction. Haile was measured to have ingested 1.7L during the race, which gave him an estimated sweat rate of approximately 3.6 L/h14 as shown below.

Measured Body Mass Change for Haile Gebrselassie During 2009 Dubar Marathon14

Body Mass Loss (Kg/Hr)

Total Body Mass Loss (Kg)

Body Mass Loss (%)





The brief but powerful sample of data cited in this article seemingly provide a compelling case that >3% body mass loss is not detrimental to performance, particularly when concerning marathon running.

Another angle to approach the issue of the effect of minor dehydration on performance is to measure the relative performance of athletes when starting in a hydrated versus a dehydrated state.

Researchers from Amsterdam and Cape Town were the latest to examine this when they tested 40-km time trial performance in the heat in different states of hydration. One trial had the cyclists’ start hydrated but were not allowed water during the time trial. The other two trial has the cyclists’ starting dehydrated, with one allowing them to drink water ad-libitum and the other allowing no fluid ingestion. Body mass loss was as expected with the dehydrated trial + no water resulting in the greatest decrease. However, interestingly, power output, heart rate, gastrointestinal temperature, average skin temperature, rating of perceived exertion, thermal sensation and thermal comfort.

Together with other literature, these findings suggest the body holds a reservoir of fluid that is ample to meet reasonable exercise demands (1-2hr), without the need for additional fluid intake, even in hot conditions (>30oC).

Reconciling Practice with Recommendations

Given, the findings above on the relatively high rates of sweat and body mass loss and accompanying low fluid intakes in elite runners, the question must be asked why fluid replacement guidelines from leading organisations such as the ACSM encourage such liberal fluid consumption, which has undoubtedly played a role in the increased rates of EAH2.

In a nutshell, many of the assumptions and conclusions drawn from initial studies (that formed the basis of ACSM guidelines) were flawed. The points below provide an overview of the key issues.

  • There is no credible evidence that athletes must drink “the maximal amount that is tolerable”8 to optimise performance and prevent medical consequences11
  • There is no proof that all the weight lost during exercise must be replaced immediately, since the resting human may carry a fluid reserve of about 2 litres6, 16
  • Initial studies on fluid loss during exercise do not account for the practices of elite marathoners, who run at speeds of ~19km/hr compared with 8.5 and 15.0 km/h17
  • Moreover, initial studies on fluid loss during exercise do not account for the lower environmental temperatures typically experienced in many major city marathons18 or indeed the ambient conditions encountered where the world’s fastest marathons are run19.
  • Sweat rate is not determined by skin blood flow (i.e. cardiovascular response)20, change in body temperature, or body water loss. As such, elevated body temperature during prolonged exercise in the heat without fluid consumption is not caused by reduced sweating21.
  • Classical studies show that sweat rates are maintained in dehydration20, 23-25 and therefore, sweating can be reasoned as a unique human trait that protects them from developing heatstroke during exercise in the heat26, rather than something that is detrimental to optimum performance.


In conclusion, it is clear that there is a complex and fascinating history concerning the original beliefs and practices of fluid intake during exercise and the dramatic changes to these beliefs as reflected in the current ACSM guidelines.

With ACSM being one of the key international body’s influencing fluid consumption patterns during exercise, it’s important to highlight ACSM principle sponsors in the form of Gatorade, the Gatorade Sports Science Institute (GSSI) and Coca-Cola. Annual turnover for the US sports drink industry grew from a respectable USD 217 million in 1985 to a massive USD 2.69 billion in 2003. This growth  in sales occurred at precisely the time that athletes were being advised by the US Military and the ACSM to change their drinking behaviours from avoiding drinking (up to 1969) to drinking ‘as much as tolerable’ during exercise (1987 to 2007).

One of the best ways of explaining the disparity between existing ACSM hydration guidelines and the practices of elite athletes comes from the concluding statement in a study entitled: “Drinking behaviours of elite male runners during marathon competition”.

“there exists a tolerable range for dehydration that may not negatively impact on running performance in elite runners but may even confer an advantage by preventing a significant increase in body mass due to the “overconsumption” of large volumes of fluid14


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  2. Cotter et al. Are we being drowned in hydration advice? Thirsty for more? Extreme Physiology & Medicine. 2014;3:18.
  3. Kipps C, et al. The incidence of exercise-associated hyponatraemia in the London marathon. Br J Sports Med. 2011 Jan;45(1):14-9.
  4. Urso C, et al. Physiopathological, Epidemiological, Clinical and Therapeutic Aspects of Exercise-Associated Hyponatremia. J Clin Med. 2014;3(4):1258-75.
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