Hyponatremia hit the headlines again recently with the publication of data on the prevalence of the condition in Ironman finishers. Scarily, over 10% of athletes tested had hyponatremia!
What exactly 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.
Why is hyponatremia bad?
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:
Because of how finely balanced your blood sodium levels need to be for good health, even mild EAH during exercise is bad news. It can seriously undermine your performance well before it starts to make you properly ill.
In severe cases of EAH, 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 EAH during sporting events since 1981. However, as the results from the recent Ironman study show, the occurrence of non-fatal EAH is likely to a lot higher.
What causes hyponatremia?
As already alluded to, for athletes the primary cause of EAH is 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 is still a widespread lack of appreciation that too much fluid intake can be as detrimental for health and performance as too little, although this is slowly starting to change with more publicity about EAH in the non-scientific press.
Many experts also feel that very large sodium losses in sweat, as a result of high sweat rates over long periods of time and/or the very high sweat sodium concentrations seen in some individuals, can contribute to increased susceptibility to developing EAH. 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 hyponatremia be avoided?
In theory, avoiding EAH is pretty straightforward, you just have to avoid drinking more that you sweat and pee out, so that dilution of 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’s 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 EAH 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. This approach leaves athletes to train or compete for 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.
So should I just drink to thirst to avoid hyponatremia?
If avoiding EAH 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 the fact 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 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 events and ultra marathons) and for athletes who are 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 aim to strike a balance between these 2 extremes.
The role of sodium supplementation for 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.
It helps reduce the rate of dilution that goes on in the blood stream 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 half Ironman 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 those taking the placebo.
Where I would go one step further with this argument is in the idea of individualising your supplementation to take into account your personal fluid and sodium losses. Sweat sodium levels can vary almost 10 fold from athlete to athlete and sweat rates can also vary dramatically. We’ve tested athletes who lose an estimated 40,000mg (40grams!) of sodium in the course of a single 10 hour Ironman event, compared with others who lose just 3,000mg (3grams) 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 find out how much sodium you lose in your sweat by getting your sweat tested (Full disclosure: my company offers sweat testing and a multi-strength range of electrolytes).
A quick recap
Hyponatremia is a potentially dangerous condition in which low sodium levels occur in the blood. Even at low levels it impairs performance; and in severe cases can be fatal. In athletes this is almost always the result of drinking too much fluid, with high sweat sodium losses possibly being a contributory factor for some individuals.
The best way to avoid hyponatremia is to avoid excessive drinking. This primarily means learning to listen to your body and not forcing in more fluid than you feel like you need.
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 maintaining hydration levels at times when sweat losses are high. It helps by increasing the retention of fluid in the blood stream and maintaining blood sodium levels.
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 (getting your sweat testing) and organised trial and error in training and competition.
Andy Blow has a few top 10 Ironman and 70.3 finishes and an Xterra World Age Group title to his name. He founded Precision Hydration to help athletes solve their hydration issues. He has a degree in Sport and Exercise Science and was once the Team Sports Scientist for Benetton and Renault F1 teams.