Hyponatremia: causes and symptoms of low sodium levels

Research has found that hyponatremia and low sodium levels are more common among triathletes than first thought. Here we explain how it's caused and how to avoid it by keeping your sodium levels at recommended levels


At 2015 Ironman Frankfurt, temperatures reached 37°C, making a tough race even more brutal. But iron athletes possess a fortitude that subdues the cerebral cry to stop; instead, they stride onward.


Tragically, that sweltering day in Germany ended the life of 30-year-old Iain O’May, who collapsed soon after the finish line. His condition deteriorated and he died from swelling of the brain caused by insufficient salt intake – in other words, hyponatremia.

Such cases are rare in triathlon but, as a recent study shows, hyponatremia isn’t.

How does hyponatremia affect the body?

Researchers in Germany tested the post-race blood of nearly 1,100 competitors in the Ironman European Championships between 2005 and 2013.

They discovered that more than 10% had developed hyponatremia, which often derives from, as in the case of O’May, drinking too great a volume of fluid that lacks sufficient essential minerals like sodium.

This upsets the body’s water balance, leading to cellular swelling, and is often accompanied by nausea, headaches and, tragically, death.

Out of the 115 athletes that developed hyponatremia, 17 cases were deemed serious and three critical. These findings led the authors to conclude that hyponatremia is a serious health concern among triathletes.

Extreme heat and long periods of exercise are a dangerous combination but one that’s unavoidable, particularly at Ironman, and also, to a lesser degree, Olympic and 70.3. So is there a solution?

What causes hyponatremia?

Hyponatremia in triathlon is often caused by over drinking, so learning to listen to your body and ensuring that you don’t over consume drinks low in sodium is the number one priority.

Also note that while consuming sodium before, during and after events can maintain performance, it shouldn’t be used to compensate for going wild with fluid intake.

Do I need a sweat test?

I’m reticent to be overly prescriptive with the ideal sodium intake because athletes’ sweat rates – and, in turn, how much sodium they lose – can vary widely.

Around 10% of the athletes we’ve tested have very high sodium losses.

Also, the mean sodium concentration per litre of sweat is 1,000mg, which is about two to three times higher than the ‘hydration industry’ would have you believe when you look at the composition of standard sports drinks.

That’s why a sweat test is ideal because you can individualise your sodium plan.


  • It’s wise to take in a little extra sodium (or salt) with food and drinks in the last 24-48hrs pre-race. Just be aware that this won’t prevent hyponatremia if you still over-drink at the race.
  • Your sodium plan should be a combination of knowing your requirements beforehand plus reacting to thirst. Having a base idea is essential, so you’re aware of adjustments you make during the stress of an event.
  • Your maximal fluid absorption rate is 800-1,000ml an hour. Consuming anymore than this isn’t required.