Hyponatremia in Endurance Sport: The Overhydration Risk Nobody Talks About

Most of us grew up with the same message: drink early, drink often, never let yourself get thirsty. It sounds sensible, and for decades it was printed on race packs and shouted from feed stations. The trouble is that it is wrong for a lot of athletes, and following it too faithfully can lead to something far more dangerous than mild dehydration. Exercise-associated hyponatremia (spelled hyponatraemia in most UK medical texts) is a drop in blood sodium caused mainly by drinking more than you lose. It has put marathon runners, Ironman finishers and even hill walkers in intensive care.

The good news is that hyponatremia in endurance sport is almost entirely preventable. You don't need a lab, a sweat test or a spreadsheet. You need to understand what causes it, who is most at risk, and how to keep fluid, sodium and carbohydrate separate in your head so you're not using one to fix the other.

What Hyponatremia Actually Is

Your blood sodium normally sits between about 135 and 145 mmol/L. Exercise-associated hyponatremia (EAH) is defined as a level below 135 mmol/L during exercise or within 24 hours afterwards. A mild drop often causes no symptoms at all, which is part of the problem: plenty of athletes cross the line slightly hyponatraemic and never know.

As sodium falls further, water moves into cells to balance the concentration. Most tissues cope with that, but the brain is enclosed in the skull and has very little room to swell. That is why the symptoms progress the way they do:

• Early: bloating, nausea, puffy hands (rings and watch straps feeling tight), headache and a general sense of feeling "off".
• Developing: vomiting, confusion, unusual tiredness, slurred speech or poor coordination.
• Severe: seizures, collapse and loss of consciousness. Levels below roughly 125 mmol/L are where the serious cases tend to sit.

The nasty twist is that several of those early signs look exactly like dehydration or heat illness. A runner who feels sick and dizzy at mile 22 is usually told to drink more, and if hyponatremia is the cause, that advice makes things worse.

What Causes Hyponatremia in Endurance Sport? The Overhydration Problem

The single biggest cause of hyponatremia in endurance sport is drinking more fluid than you sweat out. Sodium loss through sweat plays a part, but on its own it rarely drags blood sodium down far enough. The main driver is dilution: too much water in the body for the sodium that's there.

There is a second factor working against you. During long exercise, nausea, pain and stress can trigger release of antidiuretic hormone (arginine vasopressin). That hormone tells your kidneys to hold on to water rather than pass it, so the usual safety valve of simply weeing out the excess stops working. You keep drinking, your kidneys keep holding, and sodium keeps diluting.

The numbers from real races are sobering. A study of 488 Boston Marathon finishers published in the New England Journal of Medicine found that 13% had hyponatremia and 0.6% had critically low levels (120 mmol/L or below). The strongest predictor was weight gain during the race, which is direct evidence of drinking more than was lost. Finishing time over four hours was another major risk factor, simply because slower athletes have more time and more feed stations to overdrink at.

Common risk factors include:

• Long event duration, particularly marathons over four hours, Ironman and ultras.
• Drinking to a schedule rather than to thirst, especially "a cup at every station".
• Smaller body size, which partly explains why women appear more often in the data. The same volume of fluid dilutes a smaller blood volume more.
• Low sweat rates, typically in cool weather or at easier intensities.
• Anti-inflammatory painkillers such as ibuprofen, which can impair the kidneys' ability to clear water.
• Weight gain between the start and the finish line. If you're heavier after a long event, you drank too much.

How Much Should You Actually Drink?

Sweat rates vary enormously. Most athletes lose somewhere between 0.5 and 1.5 litres per hour, and heavy sweaters in the heat can go beyond 2 litres. A slower marathoner on a cool April morning might lose less than 500ml an hour. There is no single number that suits everyone, which is exactly why fixed drinking schedules cause trouble.

The current international consensus on EAH is simple: drink to thirst. Thirst is a well-tuned signal, and for most people in most conditions it keeps them within a safe range. Losing up to around 2% of body weight over a long event is normal and rarely harmful to performance or health. Gaining weight is never the goal.

If you want a more objective check, use the scales:

• Weigh yourself naked before a 60 to 90 minute training session at race intensity.
• Note everything you drink during the session.
• Weigh yourself again afterwards, towelled dry. Each kilo lost is roughly a litre of sweat not replaced.
• If you've gained weight, you're drinking too much. If you've lost more than 2 to 3%, you may need a little more.

Repeat this in a few different conditions, cool, warm and hot, and you'll have a realistic range for your own body. For most athletes, that will land somewhere around 400 to 800ml per hour, not the litre-plus that old advice suggested.

Sodium, Carbs and Fluid: Keep Them Separate

A lot of confusion comes from treating fluid, sodium and fuel as one thing. They do different jobs, and getting one right doesn't fix the others.

Sodium replaces what you lose in sweat, typically somewhere between 460mg and 1,840mg per litre depending on the individual. Taking electrolytes is sensible for long events and heavy sweaters, but here's the critical point: sodium supplements do not protect you if you're overdrinking. Research on Ironman athletes has shown that sodium intake has little effect on blood sodium compared with fluid intake. You can't salt your way out of drinking three litres more than you need.

Carbohydrate keeps you fuelled. For efforts over 90 minutes, 60 to 90g per hour is the target for most athletes, using a mix of glucose and fructose so you can absorb more. This is where many people accidentally overdrink. If your only carbohydrate source is a sports drink, hitting 90g an hour can mean swallowing well over a litre an hour, whether you're thirsty or not.

Fluid replaces sweat, and should be guided by thirst and your own sweat rate. Decoupling your carbs from your fluid means you can hit your fuelling targets while drinking only what your body asks for.

What We Use at FTP Endurance Fuel

This is one of the main reasons we fuel with pastilles rather than relying on drink mixes. A solid source of carbohydrate lets you hit 60 to 90g an hour without forcing extra fluid down. You drink to thirst, take an electrolyte if it's hot or you're a salty sweater, and get your energy from something you chew.

Our Mixed Fruit Energy Pastilles are what most of us carry on long runs and rides, with a small handful every 20 to 30 minutes covering the carbohydrate side of the equation. They're easy to portion, easy to chew at race effort, and they don't add any volume to your stomach beyond the sip of water you'd take anyway. If you want to test how solid fuel sits with you before committing to a bulk bag, the Taster Pack is a good place to start.

The Bottom Line

Hyponatremia is rare in shorter events but a real risk in anything long, slow or cool, and it's caused far more often by too much water than too little salt. Drink to thirst, weigh yourself in training to learn your sweat rate, skip the ibuprofen on race day, and never aim to finish heavier than you started. If you or someone you're racing with becomes confused, very bloated or starts vomiting after a long event, don't push more fluid on them. Get medical help, and tell the medics how much has been drunk. Then sort your fuelling so carbs come from food rather than volume: browse our bundles and build a plan where fluid, sodium and energy each do their own job.