Hyponatremia: why drinking more water isn't the answer

What is hyponatremia? It's when the sodium in your blood drops too low, below 135 mmol per litre, during or in the day after a long session or race. In endurance sport the main cause isn't sweating out too much salt. It's drinking more fluid than your body can get rid of.

That surprises a lot of people, because for years the message was the opposite. Drink before you're thirsty. Never let yourself get dehydrated. Grab a cup at every aid station. For most athletes that advice is harmless. For a slower runner in a five-hour marathon, or an age-grouper on a long Ironman day, it's exactly how people end up in the medical tent.

How common is it?

More common than you'd think, and it's not just an ultra-runner problem.

• Boston Marathon, 2002. Researchers took blood from 488 runners at the finish. 13% had hyponatremia, and 0.6% had critically low sodium.

• New Zealand Ironman. In a study of 330 finishers with full data, 18% were hyponatremic. Eleven had severe hyponatremia, and seven of those had symptoms.

Most of these cases were mild, and many people never knew they had it. But the severe end is genuinely dangerous, and it's almost completely avoidable.

Why it happens

Your blood sodium is a concentration: sodium divided by water. You can push it down two ways. Lose a lot of sodium, or add a lot of water. In endurance sport, the second one does most of the damage.

During long exercise your body releases a hormone, vasopressin, that tells your kidneys to hold on to water. So if you keep drinking well beyond what you sweat, your kidneys can't clear the extra fast enough. The water stays in, and your sodium gets diluted. A 2017 review of the evidence names drinking beyond thirst, together with that hormone response, as the most common cause.

The Boston data shows this clearly. The runners who gained weight during the race, meaning they drank more than they lost, were about four times more likely to have hyponatremia. The Ironman study found the same thing: low sodium went with weight gain, which points to too much fluid, not too little salt.

Here's the bit that really matters. In Boston, it didn't matter whether runners drank water or a sports drink. What mattered was how much. Sports drinks contain far less sodium than your blood, so drinking too much of them still dilutes you. An electrolyte drink isn't a free pass to drink without limit.

Who's most at risk

The pattern across studies is consistent, and it's probably not who you'd guess.

So it's often not the elite at the front. It's the steady age-grouper who's out there for five, eight or twelve hours, stopping at every station and drinking because they've been told to. Same story in a long gravel or sportive day, a long open-water swim with regular feeds, or a slow ultra.

What about women? In the Ironman study, women had lower blood sodium than men. In Boston, once body weight and finish time were taken into account, sex on its own wasn't a separate risk factor. In practice, smaller athletes who are out for a long time and drink a lot are the group to watch.

What it feels like

This is the tricky part. Mild hyponatremia often gives you no symptoms at all, or vague ones you'd put down to a long day anyway. The danger is at the severe end, below about 120 mmol per litre, where it affects the brain because water moves into the brain and makes it swell.

That's why it matters so much that it can look like ordinary exhaustion, and why handing someone who's struggling at the finish more water can make it worse. If someone has been drinking a lot and is getting confused, they need medical help, not another cup.

How to make sure it never happens to you

The good news is that prevention is simple, and it costs nothing.

Drink to thirst. This is the core recommendation of the 2015 international consensus statement on hyponatremia in exercise. Thirst is a good guide for most people, and the panel concluded that the old "drink ahead of thirst" advice had pushed athletes into overdrinking.

Weigh yourself before and after long sessions. If you're heavier after a long run or ride than before it, you drank too much. Losing a little weight is normal and expected. Gaining it is the warning sign.

Know your sweat rate. A simple sweat rate test tells you roughly how much you lose in an hour. If your planned drinking is well above that, you're overdrinking.

Don't drink at every aid station by habit. Take what you need. In a marathon with stations every few kilometres, you don't need a cup at each one.

Then think about sodium. For long, hot sessions and heavy or salty sweaters, sodium has a place. But it's the second step, not the first. Our sodium guide covers how much.

There's real-world evidence this works. At the New Zealand Ironman, the race started teaching athletes about not overdrinking, cut back the number of fluid stations and weighed athletes before the start. Those changes went with a drop in the number of athletes with symptomatic hyponatremia.

The honest take

I think the "never get dehydrated" message did more harm than good for recreational endurance athletes. Most of us aren't going to come to harm from losing a bit of fluid over a few hours. The data says the people who get into real trouble are the slower athletes, out there for hours, who did exactly what they were told and drank too much.

So drink when you're thirsty. Check your weight after long days. And if you finish heavier than you started, drink less next time.

FAQ

Is hyponatremia the same as dehydration? No, it's close to the opposite. Dehydration is too little water in your body. Exercise hyponatremia is usually too much water for the amount of sodium in your blood.

Will salt tablets prevent it? Not if you're still overdrinking. Salt can help on long, hot days, but no amount of salt makes it safe to drink far more than you sweat.

Do sports drinks protect me? In the Boston study, the type of drink made no difference. The amount did. Sports drinks have much less sodium than your blood, so drinking too much of them can still dilute it.

Can it happen in a shorter race? It's mostly a problem in events lasting several hours. In shorter races you simply don't have time to drink that much, unless you're drinking huge amounts before the start as well.

Sources

• Almond CSD, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005.

• Speedy DB, et al. Hyponatremia in ultradistance triathletes. Medicine and Science in Sports and Exercise, 1999.

• Speedy DB, et al. Diagnosis and prevention of hyponatremia at an ultradistance triathlon. Clinical Journal of Sport Medicine, 2000.

• Hew-Butler T, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine, 2015.

• Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-associated hyponatremia: 2017 update. Frontiers in Medicine, 2017.

• Knechtle B, et al. Exercise-associated hyponatremia in endurance and ultra-endurance performance. Frontiers in Medicine, 2019.

In MOVE you can log a sweat test, so your fluid target per hour comes from what you actually lose rather than a guess. That's the simplest way to make sure you're replacing sweat, not drowning in it.