Hyponatremia hit the headlines with the publication of data on the prevalence of the condition in Ironman finishers. Scarily, over 10% of athletes tested had hyponatremia!

But what is this potentially race-ruining condition and how can it be avoided?

What is hyponatremia?

Hyponatremia is a medical term describing low (hypo) blood sodium levels (natremia – Na is the chemical symbol for sodium).

There are a few different causes of the condition, but the one of interest to athletes is when dilution of sodium levels in the blood is driven by excessive drinking. This can be exacerbated by the loss of sodium in sweat during prolonged exercise. This variant of the condition is known as ‘Exercise Associated Hyponatremia’, or EAH.

The symptoms of hyponatremia

Maintaining blood sodium levels within a healthy range (135-145mmols per litre) is critical to homeostasis and optimal bodily function. When blood sodium levels drop below this ideal range initial symptoms can include:

  • Nausea
  • Lethargy
  • Muscle cramps
  • Weakness/fatigue
  • Headaches
  • Restlessness

Because of how finely balanced your blood sodium levels need to be for good health, even mild hyponatremia during exercise is bad news. It can seriously undermine your performance well before it starts to make you properly ill.

In severe cases of hyponatremia, the symptoms can potentially escalate into seizures and coma. The end result can even be death, if things go uncorrected for long enough. This is because, in order to try to preserve sodium concentrations in the blood, the body shifts excess fluid it can’t otherwise excrete from the blood plasma into it’s own cells, causing them to swell up.

This swelling is devastating when it occurs in brain cells and is what leads to the condition becoming fatal. About 14 deaths, including one at Ironman Frankfurt in 2015, have been directly attributed to hyponatremia during sporting events since 1981. However, as the results from the recent Ironman study show, the occurrence of non-fatal EAH is likely to be a lot higher.

What causes hyponatremia?

As already alluded to, for athletes the primary cause of hyponatremia is the over-consumption of fluids (especially drinks low in sodium).

A popular theory has emerged that the prevalence of over-drinking (and therefore hyponatremia) has increased in recent decades because the pitfalls of dehydration have been so effectively publicised since the 1980s that most athletes believe that they need to drink well before they feel thirsty, and that a ‘more is better’ approach applies to hydration.

In my conversations with athletes I do see many of them drinking very large amounts routinely in an attempt to make sure their pee is ‘clear’ all of the time, as they see this as a primary indicator of being 'well hydrated'. And that applies right up to the pro level.

So, I would say that there's still widespread lack of appreciation of the fact that too much fluid intake can be as detrimental for health and performance as too little. But this is slowly starting to change with more publicity about hyponatremia in the non-scientific press as sports like triathlon have become more popular.

Many experts also feel that the very large sodium losses in sweat seen in some individuals - as a result of high sweat rates over long periods of time and/or the very high sweat sodium concentrations - can contribute to increased susceptibility to developing hyponatremia. And I agree with that.

There are quite a lot of interesting case studies that back this up, both in healthy people with high sweat/sodium losses and those with Cystic Fibrosis, who’s genetic disorder causes them to lose lots of sodium in their sweat.

However, it’s also fair to say that this area is the subject of ongoing debate.

How can you avoid hyponatremia?

In theory, avoiding hyponatremia is pretty straightforward, you just have to avoid drinking more than you sweat and pee out, so that dilution of your blood does not occur.

For quite a while some experts (notably Prof Tim Noakes, with many others following his lead) have been advocating a ‘drink water to thirst’ approach for this very reason. When healthy people drink water purely to the dictates of thirst during exercise, it has been demonstrated time and again that they don’t tend to take in more than they sweat out and, as a result, become gradually dehydrated, making hyponatremia all but impossible.

This strategy is often backed up with evidence to support the fact that that mild dehydration does not necessarily negatively impact performance. That’s almost certainly true to a degree and if you're keen to dive deeper into that topic you can read my thoughts here

But this approach leaves athletes to train for and compete in any event, of any length or intensity, and in all conditions, with nothing more than this relatively vague statement to guide their hydration strategy.

It also ignores the valuable contribution of supplementing fluid intake with additional sodium to help aid fluid retention, maintain blood sodium levels and replace some of that lost in sweat. At a certain point the usefulness of ‘drink to thirst’ guidelines effectively ends. And that point is probably when you’re exercising for more than 2 hours, assuming you began well hydrated (for advice on how to do that, this blog is worth a read).

Should I just drink to thirst to avoid hyponatremia?

If avoiding hyponatremia is your one and only objective, then drinking to thirst is probably ok advice to follow. But, in my view, it starts to get more complicated when you consider that most athletes want to perform at their best, not just ‘survive’ a race.

Yes, dehydration has historically been over emphasised, but it can, beyond a certain point, still cause massive performance issues primarily because it manifests itself in reduced blood volume and increased blood viscosity (thickness), both of which impair cardiovascular function and heat dissipation. Anyone who has become dehydrated and tried to exercise effectively will know this only too well!

The negative effects of dehydration are especially relevant in events that are very long and hot (such as Ironman races and ultra marathons) and for athletes who're training hard and sweating a lot on back to back days. That’s because, in these scenarios, the volume of sweat and sodium losses can be quite dramatic.

Simply drinking water ‘to thirst’ on these occasions is not always adequate to maintain blood plasma volumes to a degree that’s compatible with peak performance. It’s a lot more productive to think of dehydration as existing on the opposite end of the scale to hyponatremia, and to aim to strike a balance between these two extremes.

The role of sodium in avoiding hyponatremia

It’s long been known that taking sodium in with drinks increases fluid retention in the blood stream. Also, sweat contains a relatively large (and variable) amount of sodium in it so, when sweat output is high, the net loss of sodium can be substantial too.

Sodium is a finite resource in the body and, as a result, supplementation can help to maintain both blood volume and blood sodium levels much better than drinking water alone, especially at times when sweat losses are high.

Sodium also helps reduce the rate of dilution in the bloodstream when compared to just taking in water alone. This was shown very neatly in a 2015 study where triathletes were given either extra sodium supplements or a placebo pill to take alongside regular sports drinks during a middle distance race in hot weather.

The ‘extra sodium’ group replaced around 71% of their sodium losses during the event, with the ‘placebo’ group only replacing about 20%. The results showed better maintenance of blood volume, higher blood sodium levels post-race and faster finishing times in the sodium group than in those taking the placebo.

Personalising your sodium intake

Where I would go one step further with this argument is in the idea of personalising your sodium supplementation to take into account your personal fluid and sodium losses.

The amount of sodium you lose in your sweat can vary massively from athlete to athlete and sweat rates can also vary dramatically. We’ve tested athletes who lose an estimated 40 grams of sodium in the course of a single ten hour Ironman, compared with others who lose just 3 grams during the same period.

The idea that a single strategy for fluids and sodium supplementation could work equally well for both of these athletes makes no sense at all. You can start to understand your individual electrolyte needs by taking our free online Fuel & Hydration Planner or you can find out exactly how much sodium you lose in your sweat by taking our Sweat Test.

Whilst sodium supplementation should not be seen as a way to compensate for over-drinking to avoid hyponatremia, it can be extremely useful in helping to maintain hydration levels at times when your sweat losses are high. It helps by increasing the retention of fluid in your bloodstream and maintaining your blood sodium levels.

So, personalising your sodium and fluid intake not only reduces the risk of hyponatremia, but maximise your performance when sweat losses are high. Personalisation is best achieved through a combination of data collection (taking a Sweat Test) and some good ol' fashioned trial and error in training and events.

Further Reading