Exercise-Associated Hyponatraemia

In endurance sport, high levels of physical fatigue are often inevitable. The body undergoes extreme physical stress, where the accumulation of physical bi-products can cause exhaustion. As athletes, this is often where our primary prevention and race planning lies. However, it may be necessary to give adequate attention to other possible complications during both training and competition to ensure we can still perform at our physical best.

In sport, and particularly endurance sport, maintaining adequate levels of hydration is also a primary focus for most athletes. Typically, athletes will begin their hydration plan in the days before their race, as well as adhering to strict hydration strategies during the event. As the event and race durations increase so should fluid intake, and over the course of prolonged, sustained exercise, e.g. Ironman, considerable fluid intake will be needed to maintain adequate hydration. However, as important as it is to stay adequately hydrated, is it possible to overhydrate, meaning consume too much fluid? The answer is yes, and this should form part of our race planning and practising our hydration strategies. The potential to consume higher volumes of fluid is higher in endurance sport, so appropriate hydration is increasingly essential.

Exercise Associated Hyponatraemia (EAH) is a condition where one’s sodium levels drop below that of the safe and normal range (135-145mmol/L). The greater the discrepancy from this normal range the greater severity of EAH. EAH in sport is most commonly associated with sustained physical excursion and mostly seen in exercise durations exceeding four hours, e.g. Ironman 70.3 and Ironman triathletes, and ultra-distance runners.

In more recent times, less severe incidents of EAH have also been seen in sports of lesser durations such as marathon runners and shorter distance triathlon. EAH can occur at any time during events or even up to 24 hours after sustained physical activity. Thus, carefully monitoring fluid intake post-exercise is just as important as fluid intake before and during exercise.

Sodium plays an important role when exercising, including the regulation of bodily fluids, conduction of nerve impulses and maintaining regular heart rhythm, just to name a few. With the physiological changes that these elements undergo with exercise, maintaining safe sodium levels becomes highly important. As we sweat we lose both body fluids and sodium, which can disturb the sodium concentration in the bloodstream, and increase the risk of EAH. The main cause of EAH is where fluid consumption, of both water and sports drink, exceed the individual’s sweat rate. This can dilute the sodium concentration in the blood causing decreased sodium levels. As the individual’s sweat rate may vary depending on their size, clothing and environmental factors such as temperature, these elements must also be considered when planning hydration for both training and racing.

In mild cases of EAH, signs and symptoms may include light-headedness, dizziness, nausea, and high breathing rate. In more severe cases the athlete may experience vomiting, headaches, altered mental state (confusion, agitation and delirium), potentially even resulting in seizures and altered levels of consciousness. Athletes and coaches should be aware of these signs and take appropriate action given its presence, particularly in conditions where the risk of EAH may be higher such as exercising in warmer environments.

For optimal performance, effective hydration management should be trialled in training, and should, if possible, mimic likely race conditions. Various ways to monitor the balance of fluid and hydration exist in sporting populations, and some effective measures may include documenting the athlete’s body weight both pre and post training session – any increase in body weight post training may indicate excessive hydration volumes. Additionally, estimating hourly sweat rates and tailoring fluid intake from this can be useful to ensure fluid replacement does not exceed sweat volume. Other simple training and race recommendations encourage athletes to ‘drink to thirst’ meaning only consuming fluids when they feel they need to, rather than sticking to regimented hydration plans. Staying adequately hydrated throughout exercise is extremely important, however, using these guidelines may help ensure it is not done in excessive amounts.

With the primary cause of EAH being lower than normal sodium levels, the temptation for athletes is to supplement this with sodium replacement supplements. However, previous research findings have found sodium replacement supplements to be relatively ineffective in the prevention of EAH in endurance sport. Although not successful for the prevention of EAH, such supplements may be beneficial in other aspects of training and race performance.

EAH can have serious consequences in endurance sport if not adequately addressed, and sticking to strict hydration procedures in training and racing may have the potential to increase the likelihood of EAH. Despite the risks associated with EAH the importance of staying adequately hydrated throughout exercise should not be compromised. However, finding appropriate fluid balance, and practising this in training environments can be effective in maximising race day performance and minimise the risks of overhydrating and EAH.



Rosner, M. H., & Kirven, J. (2007). Exercise-Associated Hyponatremia. Clinical Journal of the American Society of Nephrology, 151-161.

Hew-Butler, T., Rosner, M. H., Fowkes-Godek, S., Dugas, J. P., Hoffman, M. D., Lewis, D. P., … & Roberts, W. O. (2015). Statement of the third international exercise-associated hyponatremia consensus development conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine,25(4), 303-320.


Zac Turner

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