Hyponatremia Causes and Treatment

Yes, you can drink too much water

Runner drinking water outdoors
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Hyponatremia, also called water intoxication, is generally the result of drinking excessive amounts of plain water which causes a low concentration of sodium in the blood. Once a rare occurrence at sporting events, it is becoming more prevalent as participation increases and more novice exercisers are entering endurance events.

Causes of Hyponatremia

During high intensity exercise, sodium is lost along with sweat.

An athlete who only replaces the lost fluid with water will have a decreased blood-sodium concentration. As an example, consider a full glass of salt-water. If you dump out half of the contents of the glass (as is lost in sweat), and replace that with water only, the sodium concentration of in the glass is far less and the water is more dilute. This can occur in the bloodstream of an athlete who only hydrates with water during excessive sweating. The result is hyponatremia.

Adequate sodium balance is necessary for transmitting nerve impulses and proper muscle function, and even a slight depletion of this concentration can cause problems. Studies have shown that high intensity athletes can lose up to 2 grams of salt per liter of sweat. Replacing this during the event is critical to performance and safety.

Endurance Events and Hyponatremia

Prolonged and excessive sweating increases the risk that an athlete will alter the delicate balance of blood-sodium concentration.

Because sodium is lost in sweat it is important for those exercising at high intensities for long periods of time to replace any loses.

Research has found that long duration endurance events, such as the Ironman distance triathlons, often have finishers with low blood sodium concentrations. Those at most risk are those who are on the course the longest, because they tend to drink the most water during the event.

Runners who drink extra fluids in the days before the race or those who stop at water stop during the race are also at increased risk of hyponatremia.

In fact, a study published in the New England Journal of Medicine found that 13 percent of Boston Marathon runners developed hyponatremia from drinking too much water. While another study published in the same journal found that 10.6 percent of Ironman European Championship participants developed water intoxication. Specifically, the female triathletes with a racing time of 9 hours or more appeared to be the most susceptible to hyponatremia.

Symptoms of Hyponatremia

The early warning signs are often subtle and may be similar to dehydration and include nausea, muscle cramps, disorientation, slurred speech, and confusion. At this point, many athletes drink more water because they think they are dehydrated. Unfortunately, water alone will increase the problem of hyponatremia. At the most extreme an athlete may experience seizures, coma, or death.

Treatment of Hyponatremia

At the first sign of symptoms an athlete should drink a sodium containing sports drink or eat salty foods. Ideally, an athlete should plan ahead and estimate his or her fluid loss and need for sodium replacement during the event, and stay on a hydration schedule during the race. If the symptoms are extreme, a medical professional should be seen.

Preventing Hyponatremia

The best way for an athlete to avoid such problems is to plan ahead by training in the same conditions you will encounter during race day. Hydration recommendations include:

  • Calculate your water intake with this simple formula.
  • Use a sodium containing sports drinks during long distance, high intensity events that are longer than 60-90 minutes in duration.
  • Increase salt intake per day several days prior to competition, unless you have hypertension. Foods that provide additional sodium include chicken noodle soup, a dill pickle, cheese, pretzels, and tomato juice.
  • Try not to drink more then you sweat.
  • During a marathon a good rule of thumb is to drink about 1 cup of fluid every 20 minutes.
  • Avoid use of nonsteroidal anti-inflammatory (NSAIDS) medicines that contain sodium. Research suggests that these drugs may predispose runners to hyponatremia.

Keep in mind that all athletes respond differently to exercise; which also means that fluid and sodium needs are individual. 

As always, it is important to consult your physician for special considerations if you have a history of any health problems or are taking any medication for a health condition.


Almond CS, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. N Engl J Med 2005;352:1550-1556.

Consensus Statement of the 1st International Exercise-Associated Hyponatremia, Consensus Development Conference, Cape Town, South Africa 2005. Clinical Journal of Sport Medicine. 15(4):208-213, July 2005.

Danz M, Pöttgen K, Tönjes PM, Hinkelbein J, Braunecker S. Hyponatremia among Triathletes in the Ironman European Championship. N Engl J Med. 2016 Mar 10;374(10):997-8. doi: 10.1056/NEJMc1510409.

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