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What You Need to Know About Hyponatremia

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There was a time when dehydration was thought to be one of the biggest risks of long distance running. As a result, runners were encouraged to drink specified amounts of fluid per unit of time, regardless of whether they felt thirsty. Which, in hindsight, had disastrous consequences.

In the early 1980s, a few years after recommendations had been published that encouraged runners to consume plenty of fluids on the run, Dr Tim Noakes M.D. started receiving reports of overhydration. Intrigued by these reports, Dr Noakes published a paper called “Water Intoxication: A Possible Complication During Endurance Exercise” in 1985. Few took note of the paper, however, and reports of hyponatremia kept pouring in. As a result, interest in the field steadily grew, with extensive studies conducted on the subject since the early 2000s.

What is Hyponatremia?

So what exactly is hyponatremia? In a nutshell, Exercise Associated Hyponatremia (EAH) (also referred to as low blood sodium or overdrinking) occurs when too much drinking during exercise dilutes the sodium (or salt) levels in your blood serum. This may manifest as one or more of the following symptoms:

  • Nausea and vomiting
  • Fatigue
  • Headaches
  • A decrease in or loss of appetite
  • Muscle spasms
  • Seizures

But it doesn’t end there. Acute hyponatremia, where sodium levels drop below 130 mE/L, or even 125 mE/L, can have a number of serious consequences. These include:

  • Brain disease
  • Cerebral edema or swelling of the brain
  • Cardiac arrest
  • A prolonged period of unconsciousness
  • Death

It is therefore clear that hyponatremia is not something to be taken lightly. In fact, as a long-distance runner, it might be one of the biggest health risks you’ll face during your running career.

It’s important to note, though, that not all cases of hyponatremia leads to death. It is possible to have modest hyponatremia (with sodium levels <135 mE/L) without feeling sick or being in any immediate danger. Amby Burfoot, Editor at Large of Runner’s World magazine, likens having modest hyponatremia to having modest high blood pressure. Says Burfoot: “It’s not good, but you’re still alive and probably don’t even know you have it”.

Who Is Most at Risk?

So are all runners at equal risk of developing hyponatremia? Not according to the experts. Slower runners are regarded as being at a higher risk of developing the condition. Why? Well, runners who move at a slower speed tend to perspire less and also spend a longer time out on the course. Which gives them more time to drink on the run, plus it gives the effects of hyponatremia time to start manifesting. It’s also physically easier to drink while running at a slower pace, which means that slower runners generally tend to take in more fluids.

Runners with a very small or large build are also regarded as being at a greater risk of developing hyponatremia.

Are Sodium Supplements and Sports Drinks a Possible Solution?

And if you think that popping a sodium supplement will prevent hyponatremia, think again. In a study published in Medicine & Sciene in Sports & Exercise in 2015, researchers found that there was no difference in the rate of sodium intake between hyponatremic and other study participants. This study focused on participants of the 2014 Western States Endurance Run, a 100-mile trail race held annually in California. Just under 7% of race participants were found to be clinically hyponatremic after the race, regardless of having taken sodium supplements. The study concluded that sodium supplements are not required to prevent hyponatremia during continuous exercise in moderate heat for up to 30 hours. It also confirmed that avoiding overhydration appears to be critical in preventing hyponatremia.

And as far as sports drinks go, Dr Noakes is the first to acknowledge the effectiveness of these products. They taste good, they’re rapidly absorbed, and they provide water, energy and electrolytes to athletes. Dr Noakes does not, however, agree with the notion that if a little sports drink is good for you, a lot must be better. This notion is not backed by science and its spreading can only be attributed to clever marketing. Remember that the concentration of sodium in the blood (approximately 140 mmol/L) is far higher than that found in sports drinks (around 10-38 mmol/L). It’s therefore only logical that you’ll be diluting the sodium in your bloodstream no matter what you drink.

So Exactly How Much Should You Drink While Running?

So just how much should you be drinking on the run? At present the best guideline appears to be to drink to thirst. Drink only when you’re thirsty, plain and simple.

It’s worth noting, however, that some experts feel that this one-size-fits-all running hydration recommendation is over-simplified. They argue that, in situations where unusually high temperatures are expected on race day, or where water points are spaced relatively far apart, runners might end up drinking too little. Having said that, it’s good to keep in mind that hyponatremia is a classified medical diagnosis, while dehydration is not. And while being sufficiently hydrated over the long term is vital, short-term partial dehydration (think post-run thirst) is easily rectifiable without huge risk.

The Crux

So while the dangers of consuming too much fluids on the run is clear, don’t let it cause unnecessary stress. Learn to listen to your body and drink only when you’re thirsty. And, if at any time you start getting that uncomfortable, sloshy feeling in your stomach, combined with swelling in your fingers, stop drinking (and running) and seek help.

In short: Take note and be careful not to drink too much, but keep on running!


  1. Runner's World Staff, Hyponatremia, Online publication
  2. Alex Hutchinson, How to avoid overdrinking, Online publication
  3. Scott Douglas, Overdrinking, not low sodium intake, leads to hyponatremia, Online publication
  4. Amby Burfoot, How much should you drink during a marathon?, Online publication
  5. Amby Burfoot, Dehydration and hyponatremia: How common? How dangerous?, Online publication
  6. T.D. Noakes & D.B. Speedy, Case proven: exercise associated hyponatraemia is due to overdrinking. So why did it take 20 years before the original evidence was accepted?, Scientific journal
  7. Health24 Staff, Tim Noakes on overhydration in athletes, Online publication
  8. M.D. Hoffman & K.J. Stuempfle, Sodium Supplementation and Exercise-Associated Hyponatremia during Prolonged Exercise, Scientific journal