Hyponatremia in the Long Distance Athlete

author : AMSSM

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Exercise-related hyponatremia (low sodium in the blood) is a rare cause of death in endurance athletes. Symptoms include headache, nausea, vomiting, confusion, difficulty breathing and seizure.

Exercise-related hyponatremia (low sodium in the blood) is a rare cause of death in endurance athletes. Symptoms include headache, nausea, vomiting, confusion, difficulty breathing and seizure. The most common cause of exercise-related hyponatremia is drinking too much fluid, but salt losses in sweat can play a role, especially in longer duration events like the Ironman. For most Ironman triathletes, the problem occurs when more fluid is ingested than is lost through sweating and urination. The greatest volume of fluid intake occurs during the cycling portion of the event. A recent study of Boston marathon participants shows that hyponatremia occurs in those who drink too much fluid be it water, sports drink, or both. Another recent study of intermittent exercise with fluid replacement breaks shows that in a person susceptible to hyponatremia, sports drinks can delay the drop in sodium, but does not prevent hyponatremia. Exercising in hot and humid conditions seems to increase the incidence of hyponatremia.

While women seem to be more frequently affected during marathons, the gender distribution seems to be more equal in triathlon competitions, especially at the Ironman level. There are many variables that affect the final sodium level, but small size and longer time on the course seem to shift the incidence toward women in marathons.

Some athletes’ sweat contains more salt than the average person. It is possible for these athletes to lose enough sodium during an Ironman triathlon to develop hyponatremia and be dehydrated at the same time. You see these athletes who have salt caked on their clothing and skin during and after the race. This small group has special needs for salt and water replacement that differ from the group who develop hyponatremia from drinking too much fluid.

By understanding the risk factors for exertional hyponatremia, too much fluid intake and, in less common circumstances, excessively high sweat sodium losses, you should reduce your risk of developing this potentially life threatening condition. Salt losers combat hyponatremia by including oral salt supplements and drinking commercial carbohydrate-electrolyte drinks during competition. Those who drink too much fluid must learn to replace sweat losses without over consuming hypotonic fluid, including the sports drinks.

Ultimately, dehydration is a much more common problem than hyponatremia, so it is critical to learn your individual fluid and salt needs to match, but not exceed, sweat losses. Rather than trying to drink as much as possible, either drink whenever you are thirsty or calculate your fluid losses with before and after workout body weights and replace that amount on an hourly basis. There is no replacement volume recommendation that fits every triathlete.


The above article and answers are the collective work of:

Kristine Karlson, M.D.
William Roberts, M.D.
John Su, M.D., M.P.H.
Balu Natarajan, M.D.

We representing the American Medical Society for Sports Medicine

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date: May 1, 2006