What is hypernatremia? (and how you can avoid it!)

Most endurance athletes have heard of a rare but potentially race-ruining (and potentially dangerous) condition called hyponatremia, which is characterised by diluted blood sodium levels often caused by overdrinking during exercise.  

Far fewer have heard of the condition at the other end of the spectrum - hypernatremia (or high blood sodium), despite the fact mild cases of hypernatremia are more likely to be an issue for athletes in long, hot races.

And, whilst cases severe cases are genuinely rare, it’s worth understanding what hypernatremia is and how to avoid it…

What is hypernatremia? 

Sodium is essential for your body to function, and it plays a crucial role in helping you stay hydrated when you’re sweating for several hours during exercise. 

There have been several documented cases of athletes suffering from exercise-associated hyponatremia (low blood sodium). This is often caused by a dilution of sodium levels in the blood, which is driven by excessive drinking (and exacerbated by the loss of sodium through sweating during exercise). 

Hypernatremia is the opposite of hyponatremia. 

In theory, it can be caused by excessive sodium intake, as eating a huge amount of salt in one sitting could temporarily raise blood sodium. But, in reality, your kidneys are very good at flushing out excess sodium within hours, so overdosing on sodium isn’t a concern for most people in their daily lives, assuming you have plenty of excess fluid. You almost certainly will have plenty of fluid available at rest, but this becomes more complex during exercise, when you’re losing fluid through sweating. 

For endurance athletes, rather than overdoing your sodium intake, the more likely cause of hypernatremia is under-drinking during prolonged periods of sweating. 

Your sweat contains far less sodium than your blood does. So when you sweat heavily and don’t drink enough to replace that fluid, the sodium remaining in your blood becomes more concentrated - pushing it above the normal range.

Your body uses two systems to monitor hydration and trigger thirst. The first are osmoreceptors in your brain, which detect when blood becomes more concentrated and prompt you to drink. 

Blood sodium sits at around 140 mmol/L, which works out at roughly 3,200 mg of sodium per litre. Sweat isn't close. From conducting thousands of Sweat Tests, we’ve seen more than a ten-fold difference in sweat sodium concentration between athletes, with the saltiest sweater losing more than 2,000mg/L to the lowest who losing 200mg/L. 

It's that essentially every one of those results sits below the blood line. Half of them are under 962mg/L (which is less than a third as salty as the blood that sweat comes out of).

So every litre of sweat you lose takes proportionally more water out of you than sodium. Lose enough of it without putting fluid back, and what's left behind gets saltier. That's the whole mechanism, and it's why hypernatremia during exercise is almost always a water problem wearing a sodium problem's clothing. 

As fluid losses continue and blood volume starts to drop, a second system kicks in. Pressure sensors called baroreceptors in your major arteries and in the chambers of your heart detect the fall and signal your kidneys to hang onto water, while also ramping up the drive to drink.

Dehydration, in the strict physiological sense, means being short of fluid relative to the sodium in your blood. Push that far enough, and you cross into hypernatremia, which is defined as a blood sodium concentration above 145 mmol/L.

Blood sodium drifts up a little during any long, hot session - your plasma volume contracts as you sweat, and some of that is entirely normal, expected, and reverses as soon as you rehydrate. 

A single reading a fraction over 145 at an IRONMAN® finish line isn't the same as clinical hypernatremia in a hospital bed. And it's part of why studies reporting eye-catching rates of it in ultra runners need reading carefully. What matters is how far above the line you go, and how long you stay there.

What are the symptoms of hypernatremia?

The most obvious warning sign is intense or excessive thirst, which is your body’s way of telling you to drink more water to dilute the sodium concentration in your blood. You might notice this more in a long, hot race when you’ve been sweating without drinking enough.

Symptoms depend on how high blood sodium climbs and how quickly it gets there. Mild elevation, just over the 145mmol/L line, often produces nothing more than thirst. As levels climb further, you might experience:

• Headache
• Nausea
• Lethargy or weakness
• Irritability
• Confusion

At the severe end, hypernatremia can cause seizures and coma. But that's a long way beyond where most athletes end up, and cases that serious almost always have something else going on too - illness, an underlying medical problem, or a situation where someone has had no access to fluid for a very long time.

What this looks like in practice

Take a 75kg athlete going nine hours at the IRONMAN® in the extreme heat of Hawaii, where athletes can experience massive fluid loss through sweating:

• They have a starting blood sodium of 140mmol/L, right in the middle of the normal range
• Average sweat rate of ~1.2 litres an hour, so a shade under 11 litres across the day
• Their sweat sodium concentration is 900mg of sodium per litre
• They're drinking an electrolyte drink that delivers 1,000mg of sodium per litre
• But they only manage to replace 70% of what they're losing

Here's where they end up. Over the nine hours, they sweat out about 9,700mg of sodium and take in about 7,600mg. So they cross the line having consumed less sodium than they lost - a net deficit of a couple of thousand milligrams.

And their blood sodium has gone up to 145.1mmol/L. They’re technically hypernatremic.

So, they lost more sodium than they took in, and still finished with saltier blood than they started with.

That's not a paradox, it's just what happens when you treat a concentration as if it were a running total. They finished around 3.2 litres down on fluid, which is 4.3% of their body mass. Losing that much water was more than enough to concentrate the sodium that was left behind, whatever the sodium ledger said.

Would they have known at the time? Almost certainly, though not because of the sodium. At 4.3% down in Kona's heat and humidity, they'd have been feeling pretty rough through the back half of the marathon and running a long way below what they were capable of. The hypernatremia isn't really the thing that ruined their day - it's a marker of the thing that ruined their day, which was simply not drinking enough.

The encouraging part is that both problems have the same fix, and it isn't a dramatic one. Nudge that replacement figure up to 85% - still well short of matching their losses litre for litre - and the same athlete finishes at 141.4mmol/L, comfortably normal, and only 2.2% down on body mass. And that’s despite drinking more sodium overall. 

How can I avoid hypernatremia?

The short version is drink enough during long, hot races, and add some sodium to your bottles.

Thirst is a useful signal, and you shouldn't ignore it, but it's a lagging one - by the time you're properly thirsty, you're already playing catch-up. And in a race, thirst gets drowned out by everything else going on. That's why it's worth having a flexible plan that puts a floor under your intake rather than relying purely on how you feel deep into a race. 

What's in the bottle matters as much as how much of it you get through. Sodium is what keeps fluid in your bloodstream rather than letting it pass straight through you, and it's the difference between the two lines in the graph above - same volume of drink, very different place to finish. An electrolyte drink, electrolyte capsules taken alongside plain water, or salty food will all help. Somewhere around 1,000mg of sodium per litre is a sensible middle-of-the-road starting point for most athletes.

Two things worth measuring

For athletes who want to go further and personalise their approach, two types of testing can be very useful:

• Sweat rate testing : Weigh yourself before and after a hard session, allow for what you drank, and you've got a rough figure for how much fluid you lose per hour. That tells you what you're aiming to replace. You're not trying to finish level - most athletes handle finishing 2-4% down on body mass perfectly well, provided they started properly hydrated and paced sensibly.
• Sweat Test  - tells you where you sit on that ~200 to ~2,300mg/L spread when it comes to sweat sodium concentration. It's largely genetic.

Key takeaways

Hypernatremia and hyponatremia look like opposite problems, and in one sense they are: one is too little water for the sodium you're carrying, the other is too much. But they share a root cause, which is a mismatch between what you're losing and what you're putting back.

Which is why I'm wary of advice that boils down to drink more, or drink less. Neither is wrong exactly, and neither is much use on its own. Two athletes lining up side by side can lose sodium at rates that differ by a factor of eleven. Hand them the same hydration plan, and at least one of them is going to have a rough day.

The key is getting the ratio of sodium to water right: drinking plain water without sodium during a long event can cause hyponatremia (low blood sodium), but neglecting fluids while sweating heavily can push sodium too high. 

Both extremes are avoidable with a sensible, personalised approach to hydration.

• Hypernatremia means blood sodium above 145mmol/L. In endurance sports, it's almost always caused by losing too much water, not by taking in too much salt
• Mild cases are more common in long, hot races than most athletes realise. Cases severe enough to be dangerous are genuinely rare
• Sweat is always more dilute than blood. Ours ranges from 205 to 3,070mg/L against blood at roughly 3,200mg/L, so every litre you lose leaves what's behind slightly saltier
• At the same fluid intake, drinking with sodium and drinking plain water finish in very different places. Getting the ratio right matters more than the volume alone
• Aim to replace a good proportion of your losses rather than all of them. Finishing 2-4% down on body mass is normal and manageable for most athletes

Further reading

• What is hyponatremia? (And how to avoid it!)
• Why sodium is crucial for athletes
• How much dehydration can you tolerate?