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The One Thing Every Endurance Athlete Needs to Know

If you’re an endurance or ultra-endurance athlete, your ability to recognize – and prevent – exercise-associated hyponatremia (EAH) could not only improve your performance, it could save your life. 

EAH is the flipside of dehydration. It’s overhydration born out of misguided recommendations from the 1980s that were more marketing than science. That’s when the sensible (and safe) advice to drink enough water to quench thirst was pushed aside to make way for a new battle cry, “Drink as much as possible during exercise.”

Trouble is, the advice was wrong, dead wrong. And since then, more than a few athletes have died as a result.

Why? With prolonged exercise, you lose both fluid and sodium in urine and sweat. When you replenish this loss by drinking too much water, you risk developing EAH. Essentially, you have too much fluid sloshing around in the extracellular space and not enough sodium to keep things in balance.

University researchers from Switzerland, Czech Republic and Greece recently uncovered new findings about EAH unique to endurance and ultra-endurance athletes. Their exhaustive review of 40 years of research is published in the August 2019 issue of the peer-reviewed, scientific journal Medicina.1

You can check out the review online. For now, here are a few highlights:

EAH can be potentially life-threatening

Biochemically, EAH occurs when the plasma sodium level falls below 135 mmol/L. It can occur during and after prolonged physical exertion lasting 4-6 hours or longer and can be detected up to 24 hours after the end of activity.

With mild EAH, you may experience no symptoms or vague, nonspecific symptoms like general malaise, mild headache, vomiting and fatigue.

With pronounced EAH, which occurs when the plasma sodium level falls below 120 mmol/L, you can experience more serious symptoms like brain swelling. This can lead to seizures, sleepiness, coma and other central nervous system symptoms. When the plasma sodium level falls to less than 110-115 mmol/L, respiratory failure can lead to death.  

One symptom that occurs in virtually all reported cases of EAH is weight gain or inadequate weight loss during a race.2

EAH occurs more when activities are longer

As a general rule-of-thumb, EAH is more likely to occur in endurance activities lasting 8 hours or more, and less so in activities lasting less than 4 hours.

EAH occurs more in certain sports

The occurrence of EAH varies depending on the sport:

  • Swimmers, especially women, have the highest risk of developing EAH. In one study involving swimmers at the 26.4-km long Lake Zurich Marathon Swim in Switzerland, researchers found EAH to be about four times more prevalent among women than men.
  • Runners. Runners are at risk for EAH, and the longer the event, the greater the risk. Among those who participated in recent Boston, Houston and Zurich Marathons, anywhere from 1% to 22% developed EAH. For ultra-marathon races shorter than 100 km, the EAH occurrence ranges from 0% to 5%, and for longer runs of 100 miles, it’s in the range of 6% to over 50% of athletes.
  • Road and mountain bike cyclists rarely develop EAH. This may be because, unlike swimmers or runners, cyclists carry their drink bottles on their bicycles so they can cater to their individual needs.
  • Triathletes. About 20% of the athletes in Ironman and other long-distance triathlons develop EAH (similar to ultra-marathoners). In fact, EAH is reported to be the most important electrolyte disorder in the Ironman Hawaii.
  • Multi-Stage Events. For the most part, studies suggest that EAH can be corrected during multi-stage races before the finish or during intense training camps. But not always. In one study, 43% of rowers in a 4-week training camp experienced EAH after about 3 weeks.

EAH (may) occur more often in women

Some studies suggest that EAH is more common in women than men, but the main reason is likely their lower body weight. In one study involving Boston Marathon runners, any apparent difference disappeared when body mass index (BMI) and racing times were taken into account. On the other hand, women tend to drink more water than men (at least under laboratory conditions) and retain more water during exercise, which may increase their EAH risk.

EAH occurs more when temperatures are hot (or cold)

Hot temperatures increase EAH risk for endurance athletes. In one data analysis from 5 years of runners who participated in a 100-mile ultra-marathon in California, researchers found a significant link to ambient temperature. Humidity may also play a role, making hot, humid conditions a double whammy.  

Some, but not all studies, also point to cold temperatures as a potential risk factor for developing EAH. In a 100-mile, ultra-marathon held in Alaska, 44% of runners developed EAH. (Although, they also consumed more fluid and less sodium than those without EAH).

EAH occurs more in U.S.-based competitions

It may be surprising to know the country where a competition takes place matters. EAH occurrence is common for events in the United States, less so for events in Europe, and practically non-existent for events in Africa, Asia and Oceania. U.S.-based races are often completed in hotter (or colder) conditions, and this thermal stress could play a role.

EAH can be avoided with simple actions

If you want to prevent EAH during training or competition, it starts by preventing fluid overload. To do this, the experts in EAH research recommend that you drink according to thirst, reduce the availability of fluids along your routes of exercise, and monitor changes in your weight during exercise.

For more details about how much to drink before, during and after exercise to help you achieve optimal hydration (and avoid EAH), check out our fluid intake guidelines here.

To help maintain sodium balance, take a balanced electrolyte tablet with sodium. We recommend Replace® Electrolyte tablets because it’s formulated to meet the special needs of endurance athletes. First, it’s an easy-to-swallow (and carry) tablet, so it helps you avoid excess fluid that can increase your risk of overhydration. Second, it’s a balanced formula, so it not only provides sodium, but also chloride, magnesium and other electrolytes lost in sweat. Finally, the sustained-release tablet delivery slowly releases electrolytes over a period of 4 to 6 hours as your body needs.

You simply drink water according to thirst and take Replace® tablets as recommended to help you absorb and retain the water you drink and maintain proper fluid and electrolyte balance.*


  1. Knechtle B, Chlíbková D, Papadopoulou S, Mantzorou M, Rosemann T, Nikolaidis PT. 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. Medicina. 2019;55(9):537. https://pubmed.ncbi.nlm.nih.gov/31455034.
  2. Hoffman MD, Stellingwerff T, Costa RJS. Considerations for ultra-endurance activities: part 2 - hydration. Res Sports Med. 2019;27(2):182-94. PMID: 30056755. https://www.ncbi.nlm.nih.gov/pubmed/30056755.