Hyponatremia in Running: When Too Much Water Becomes Dangerous

Every running magazine writes about dehydration and almost none writes about hyponatremia — yet it is the more dangerous side of the same coin. If you drink more sodium-poor water during a long race than you sweat out, you dilute your blood sodium. The consequences range from nausea and headache to confusion, seizures and brain swelling. In marathon medical tents, exercise-associated hyponatremia has been the most common serious complication for years — more frequent than heat collapse and far more frequent than genuine dehydration. Here is how it develops, how to recognise it, who is most at risk, and how a realistic drinking and sodium strategy keeps you clear of it.

What happens inside: sodium governs your fluid balance

Sodium is the most abundant electrolyte in blood plasma. It determines how much water stays inside the vessels and how much moves into the cells — the rule is simple: water follows sodium. With sodium between 135 and 145 millimoles per litre, cell volume, nerve conduction and muscle excitability all work reliably. Below 135, doctors call it hyponatremia. Below 130 it becomes clinically relevant, below 125 it is life-threatening.

The key point: it is not the absolute amount of sodium in your body that matters, but the ratio of sodium to water. You can lose 1,500 milligrams of sodium during a marathon and still develop hyponatremia if you refill with three litres of plain water at the same time. Conversely, a heavy salty sweater can show normal blood values as long as fluid intake stays modest. That is why the classic instruction "drink as much as you can" is simply wrong for long efforts — it optimises one risk and creates the other.

The mechanism during a race: sweat is always lower in sodium than blood plasma. You therefore lose more water than salt — pure sweating actually raises your sodium level slightly at first. Add plenty of water without sodium and the balance tips. On top of that, exercise and nausea trigger the hormone ADH, which reduces water excretion through the kidneys. The water you drank stays in the body and the dilution worsens. That exact combination — hours of effort plus sodium-poor refilling plus reduced urine output — is what makes exercise-associated hyponatremia so deceptive.

Who is most at risk

The cases described in sports medicine follow a remarkably stable pattern. They are rarely the fast runners losing two litres of sweat per hour, but the group with long duration and a low sweat rate:

• Slower finishers beyond four hours: Intake per hour stays the same while duration doubles. Over five hours a moderate oversupply adds up to several litres of surplus.
• Small and lightweight runners: Less body mass means smaller distribution volumes — the same intake dilutes far more at 50 kilograms than at 85 kilograms.
• Starters in cool or wet weather: At 12 degrees you sweat a fraction of what you lose at 28. Taking two cups at every station regardless means systematically overdrinking.
• Fear of dehydration: Anyone who has had one bad hot race tends to drink preventively afterwards. In the studies this is the single strongest risk factor.
• Heavy salty sweaters who drink only water: White salt rings on shirt and cap and a salty taste on the skin indicate a high sweat sodium concentration. This group actively needs sodium, not just less water.
• Use of painkillers: NSAIDs such as ibuprofen or diclofenac affect kidney function under load and impair water excretion — an underrated amplifier, see painkillers in endurance sport.

Warning signs: how to recognise hyponatremia

The insidious part is how much it resembles ordinary race fatigue. Nausea, headache and exhaustion are familiar to every marathon runner — yet they are also the first symptoms of developing sodium dilution. There are, however, some distinguishing features that work reliably in practice:

• Swollen hands and fingers: Rings that suddenly no longer fit, or hands that feel puffy. This points to fluid retention, not to a fluid deficit.
• Weight gain during the race: If you weigh more after three hours than at the start, you have retained water. This is the clearest sign of all.
• Bloated stomach and sloshing: The fluid you drink is not passing through and sits in the stomach — a sign that dilution is already underway.
• Headache with pressure: Unlike the dull dehydration headache, it often sits across the forehead and worsens when you bend forward.
• Confusion, word-finding problems, loss of coordination: From here it is an emergency, not a bad patch. Wobbling, slurred speech and disorientation signal a developing brain swelling.

The most useful practical distinction: with dehydration, thirst is strong, the mouth is dry and urine output is minimal and dark. With hyponatremia you often have no thirst, no dry mouth, but nausea and swelling — and you have drunk a lot in the past hours. If you are unsure, do not force more litres in. A cup of salty broth at a station is the better choice than plain water when dilution is beginning.

What to do in a real emergency

With mild symptoms — discomfort, slight nausea, swollen hands but a clear head — the rule is: stop drinking, add sodium, ease off the pace and monitor the situation. A sports drink with at least 500 milligrams of sodium per litre, salty broth at an aid station or salted crackers all help. Plain water makes it worse.

With confusion, vomiting, loss of coordination or altered consciousness, the sporting part ends immediately: withdraw from the race, get medical help, do not keep running. The affected person must not drink any more — especially not because a helper assumes "dehydration". That exact reflex turns a moderate hyponatremia into an emergency. In hospital the diagnosis is made from the blood sodium concentration; treatment in symptomatic cases is controlled hypertonic saline, not water.

The right drinking strategy for long runs

No complicated formula follows from the research — just a simple stance: drink to thirst, but with sodium and in moderation. Four rules that have proven themselves in practice:

• Establish your baseline: Measure your sweat rate with the weighing test — before and after a one-hour run without drinking. Every kilogram of difference equals one litre. Details are in hydration for runners.
• Never replace more than you lose: Your sweat rate is the upper limit of your hourly intake, not more. If you sweat 0.6 litres per hour, you do not need 1.2 litres in cool weather.
• Plan sodium in: Beyond 90 minutes, 300 to 800 milligrams of sodium per hour should be added, up to 1,000 milligrams for salty sweaters. Sports drinks with 400 to 700 milligrams per litre, sodium gels or salt capsules are equivalent routes.
• Drinking against thirst is off-limits: If you have no thirst and your stomach is already sloshing, the next station is not an obligation. A sip to rinse is enough.

A worked example for a marathon at 15 degrees: a 75-kilogram runner with a sweat rate of 0.8 litres per hour loses roughly 3.2 litres over four hours. Sensible intake is 600 to 800 millilitres per hour — 2.4 to 3.2 litres across the race, spread over the stations, each with sodium. Under those conditions, taking two cups at every station and emptying your own bottle on top quickly lands you at five litres and into the risky range. The dense aid-station network at big marathons is part of the problem.

Before the start: the most common mistakes in the final 24 hours

The groundwork for hyponatremia is often laid the day before. The classic is "pre-hydration": litre after litre through the afternoon and evening in the hope of building a reservoir. The body does not store water — it excretes it or retains it in tissue. On race morning you then start with a lower sodium level than necessary and with bladder pressure that reminds you of every toilet in the first ten kilometres.

Better: normal fluid intake the day before, urine colour as a check (pale yellow is the target), and 400 to 500 millilitres with breakfast on the morning, plus a salty breakfast — cheese, bread with salt, no sugar-free smoothie. If you are a salty sweater you can add a pinch of salt to a glass of water in the morning. This is not a performance hack, just the simple correction of a rule that has been told wrongly for decades.

How Peakora factors your fuelling in

The Peakora fuelling calculator plans fluid and sodium from your target time and your sessions — not as a fixed litre figure but as an hourly schedule that scales with race duration. A 4:30 goal gets different numbers than a 3:00 goal, and the dress rehearsal in your long runs shows whether your stomach tolerates the strategy. That turns intake into a planned quantity instead of station-by-station improvisation — the most effective protection against both extremes. How it interacts with carbohydrates is covered in fuelling for the marathon, and the blood-sodium side connects directly to electrolytes in running.

Frequently asked questions about hyponatremia

What is hyponatremia in running?

Hyponatremia means blood sodium drops below 135 millimoles per litre. In sport it usually develops because you drink more sodium-poor water over several hours than you sweat out — the blood becomes diluted.

How do I recognise hyponatremia?

Early signs are nausea, headache, a bloated stomach and swollen hands. Later come confusion, loss of coordination and altered consciousness. Drinking more water at that point makes it worse.

How much sodium do I need per hour?

For long efforts, 300 to 800 milligrams of sodium per hour is a sound range, up to 1,000 milligrams for heavy salty sweaters. One litre of sports drink with 500 milligrams of sodium per litre already covers typical needs.

Should I drink less during a marathon?

Yes, if you have been drinking against thirst. For most runners 400 to 800 millilitres per hour is enough. In cool weather over four hours many need considerably less — extra water does not protect you, it dilutes.