Exercise induced hyponatremia occurs relatively often and has caused deaths in otherwise healthy individuals. It is associated to a wide variety of sports besides endurance events, and its main reason is fluid overload. This overload causes lung or brain edema, many times with fatal consequences.

Focusing on running events, as the number of endurance and ultra-endurance events increases, so it does the number of runners taking part in them. Therefore, the importance of studying the conditions that could be risk factors in developing hyponatremia, such as discipline, environmental conditions and effort duration.

Hyponatremia usually describes a condition developed during prolonged exercise, usually longer than 4 hours, when sodium concentration in plasma goes below a certain level (135mmol/L). During exercise there is a loss of sodium, mainly through sweat and urine. When drinking a sodium-free solution or a hypotonic fluid like water the sodium lost is not compensated. The remaining sodium is diluted, consequently decreasing its level. If the fluid intake is excessive, as happens with most cases of hyponatremia, the increase in total body water is determinant in lowering sodium plasma levels.

The first cases of hyponatremia were reported in the Comrades Marathon during the 1980s. It is a popular ultra-running event, taking place over 90k in South Africa. At that time, endurance athletes were recommended to avoid fluid intake during competition. Shortly afterwards the American College of Sports Medicine changed its view and recommended to drink as much as possible during competition to avoid dehydration and hypernatremia (a rise in the levels of sodium in plasma). This advice caused a rise in the hyponatremia cases.

Besides excessive fluid intake there is also another reason for hyponatremia: the syndrome of inappropriate antidiuretic hormone secretion, or SIADH. It is an existing condition that is not induced by exercise. The failure in suppressing the antidiuretic hormone (ADH) causes water retention, that ultimately leads to hyponatremia.

When comparing water against carbohydrate drinks, it was found that water tends to stay longer in the intestinal tract because of its slower absorption rate. This would contribute to the transference of sodium from the blood to the gut. This disadvantage is not present in most sport drinks.

The symptoms of hyponatremia change depending of the severity of sodium deficit. Early symptoms such as headache, vomiting and fatigue are not very specific and may be overlooked. When the sodium level decreases further appear confusion, coordination problem and even unconsciousness, as a cerebral edema develops because of brain swelling. If untreated if can lead to respiratory failure and ultimately death in severe cases.

Hyponatremia occurrence can be as high as 30% of participants in Ironman or ultra-running events, and especially among the slowest participants. In US marathons the prevalence is around 8%, with lower percentages in 100k events, and higher in races longer than 100 miles, although if this last case majority of runners didn´t suffer any consequence. It is important to notice that it happens in other disciplines besides the ultra-running and Ironman events.

It also appears more often in women. Physiology of men and women is different despite similar drinking guidelines for everyone. Women usually have higher water retention levels, but also tends to drink more than men. Nevertheless, it is not clear if the higher prevalence is due to their lower body weight or slower finishing times.

Other risk factors associated are: weight gain during the race, low competition experience, intake of non-steroid anti-inflammatories, high fluid availability during the race, and extreme hot/cold conditions, among others.

The best way to prevent hyponatremia is “drinking to thirst” instead of “drinking as much as possible”. Doing so you will certainly avoid fluid overload. The symptoms of untreated hyponatremia are so severe that is important to inform properly coaches, athletes and staff attending in races, to spread proper hydration practices and recognise the symptoms.

Run or practice sports, but always using your head to hydrate: being cautious can save your life.

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Exercise-Associated Hyponatremia in Endurance and Ultra-Endurance Performance-Aspects of Sex, Race Location, Ambient Temperature, Sports Discipline, and Length of Performance: A Narrative Review.

Knechtle B, Chlíbková D, Papadopoulou S, Mantzorou M, Rosemann T, Nikolaidis PT

Medicina (Kaunas). 2019 Aug 26;55(9). pii: E537. doi: 10.3390/medicina55090537.

Photo by Tobias Seidl on Unsplash

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