Exercise Hyponatremia in UK Runners: Overhydration Warning for Summer 2026

Stethoscope and medical equipment – understanding the risks of overhydration in endurance sport

The May 2026 heatwave caught the UK off guard. On 26 May the mercury hit 35°C, shattering spring records, and the Met Office outlook for early June points to above-average temperatures continuing with no firm end in sight. For runners, the heat has arrived just as summer race season opens: Race to the Stones, Bath Half, Liverpool Rock and Roll, the Lake District 100 — all have issued hydration warnings to competitors in recent weeks.

But here is what those warnings often get wrong. The instinct to drink as much as possible during a hot race is not just unhelpful; it can be genuinely dangerous. This article explains why drinking more is not always safer. You will find the precise definition of exercise-associated hyponatremia (EAH), who faces the greatest risk, how to tell EAH apart from dehydration, and a practical hydration plan grounded in current evidence from UK Athletics and the Wilderness Medical Society.

What exercise-associated hyponatremia actually is

Exercise-associated hyponatremia is defined as a serum sodium concentration falling below 135 mmol/L during or up to 24 hours after physical activity. In plain language: you drink more fluid than your kidneys can excrete while running, and the sodium in your blood becomes diluted. It is the runner’s overhydration problem, not the under-hydration problem — the opposite of what most people worry about in a heatwave.

The condition first entered mainstream awareness after several high-profile marathon deaths in the early 2000s, but it remains widely misunderstood. The Wilderness Medical Society published its updated EAH guideline in 2021, and the 2023 International Exercise-Associated Hyponatremia Consensus Development Conference reinforced the same core message: the old advice to drink ahead of thirst is the principal cause.

EAH shows up most consistently in marathon runners, ultramarathon runners and Iron-distance triathletes — anyone who is exercising long enough to drink large volumes of fluid without sufficient sodium replacement. The kidneys in a healthy adult can excrete roughly 800 to 1,000 ml of water per hour at rest. During exercise, that capacity drops. If you are drinking at or above that rate for several hours, you are outpacing what your body can process. Your blood sodium falls, your cells swell with excess water, and if the drop is rapid or severe enough, your brain and lungs are the organs that suffer first.

Why heatwave running raises the risk

A hot day does not just make running harder. It reshapes the entire risk profile for overhydration.

High ambient temperature slows your pace. A slower pace means more time on the course. More time on the course means you pass more aid stations. And if you have been told to drink at every aid station regardless of whether you feel thirsty, you end up consuming far more fluid than a faster runner covering the same distance. This is the central paradox: the runners who are most cautious about hydration in the heat are often the ones most at risk of diluting their blood sodium to dangerous levels.

Inexperienced runners are particularly vulnerable. First-timers at a summer marathon or ultra tend to follow fixed drinking schedules they picked up from older training plans or well-meaning running club advice. That advice — drink at every station whether you feel like it or not — was standard for decades. It is now recognised as a key contributor to EAH.

Smaller-framed athletes face a narrower margin for error because they carry less total body water to begin with. Research consistently shows higher EAH rates among women compared to men in the same event distances, though the reasons are still debated.

Every major UK summer race in 2026 has issued heat-specific guidance. Race to the Stones, Bath Half, Liverpool Rock and Roll, and the Lake District 100 have all warned competitors to adjust their hydration strategy. The UKHSA Heat-Health Alert system has been active since early May. The message from UK Athletics medical guidance is straightforward: in hot conditions, drink to thirst and pay attention to your body, not to a rigid schedule.

Blood vial for pathology testing — serum sodium is measured to confirm a diagnosis of hyponatremia

How to tell EAH apart from dehydration

This is where most runners — and most well-meaning bystanders — get it wrong.

Dehydration and exercise-associated hyponatremia share some surface symptoms: fatigue, dizziness, nausea, headache. But the differences are critical, and confusing the two can be fatal.

With dehydration, your urine is dark, your mouth is dry, you feel genuinely thirsty, and you have lost weight since the start of your run. The fix is straightforward: drink fluid, preferably with electrolytes, and rest.

With EAH, the picture flips. Your urine is pale or clear, you may not feel particularly thirsty (or you may feel oddly bloated), and critically, you have gained weight since the start. That weight gain is water your body cannot get rid of. Your fingers may look puffy. Your rings feel tight. The headache will not settle no matter how much water you drink — because water is exactly the problem.

The single most important rule, endorsed by UK Athletics medical guidance and the British Association of Sport and Exercise Medicine (BASEM), is this: if a runner is unwell during or after a hot race and has gained weight on the scale since the start, treat it as EAH until proven otherwise. Do not give them more fluid. Do not encourage them to keep drinking. Get medical help.

If you are running a race that has weigh-in stations, use them. Weigh yourself before the start and again at checkpoints. Even a gain of half a kilogram on a hot day is a red flag that you are drinking more than your body can process.

A practical UK runner hydration plan for summer 2026

Getting hydration right in a heatwave does not require complicated calculations. It does require abandoning the old drink-as-much-as-you-can mindset.

Be sceptical of anyone — a running buddy, a marshal at an aid station, a training plan from 2005 — telling you to drink as much as you can. That advice is outdated and it is putting runners in hospital.

When to call 999 on a hot race

If you suspect exercise-associated hyponatremia in yourself or a fellow runner, the first step is simple: stop drinking water immediately. Stop running. Get to a cool, shaded area if possible, but do not force fluids.

If the person is alert, oriented and able to swallow, give them a salty snack — crisps, salted peanuts, pretzels — or a cup of salty broth if one is available. This can help raise sodium levels gently while you assess whether further help is needed. Do not let them keep sipping water.

If the person is confused, is vomiting repeatedly, has a severe headache, is seizing, is breathless with frothy sputum, or shows any altered consciousness, call 999 without hesitation. These are signs of severe EAH, possibly with cerebral or pulmonary oedema, and the treatment is hypertonic saline administered intravenously in a hospital. That is not something you can manage at the roadside or on a race course. NHS emergency teams are trained to recognise hyponatremia, but they need to know the context — tell them the person has been exercising in heat, has been drinking water throughout, and has either gained weight or shows signs of overhydration.

Getting someone to hospital fast in this scenario is the difference between a full recovery and a preventable tragedy.

Frequently Asked Questions

What is exercise-associated hyponatremia?

It is a condition where your blood sodium drops below 135 mmol/L during or after prolonged exercise, caused by drinking more fluid than your kidneys can process. The excess water dilutes your blood sodium, causing cells — including brain cells — to swell. It is most common in marathon and ultra runners who drink on a fixed schedule rather than to thirst.

Can drinking too much water kill you during a marathon?

Yes. Severe exercise-associated hyponatremia can lead to cerebral oedema, pulmonary oedema, seizures and respiratory failure. There have been documented deaths in marathon runners from overhydration. It is entirely preventable with the right hydration strategy.

How much should I drink during a long hot run in the UK?

There is no single number that fits everyone. Most adults need between 400 and 800 ml per hour during prolonged exercise in the heat, depending on body size, sweat rate and pace. The most reliable guide is your own thirst and your pre- and post-run weight. Aim for no more than 2 percent weight loss and no weight gain.

How do I tell if I have hyponatremia or just dehydration?

Dehydration means dark urine, thirst, dry mouth and weight loss during the run. Hyponatremia means pale urine, no particular thirst, weight gain during the run, puffy fingers and a headache that does not settle with fluids. If in doubt, weigh yourself. Weight gain during exercise is the clearest warning sign of EAH.

What should I do if a fellow runner collapses in the heat?

Call 999 if they are confused, seizing, vomiting repeatedly or losing consciousness. Move them to a cool area. Do not force water into them. If they are alert and can swallow, a salty snack may help. Tell paramedics they have been exercising in heat and drinking fluids throughout the event.

The May 2026 heatwave has put UK runners on unfamiliar ground. The instinct to drink more, and more often, feels like the safe choice. But the evidence from UK Athletics, BASEM and the Wilderness Medical Society is clear: drink to thirst, weigh yourself before and after long runs, use isotonic drinks for anything over 90 minutes in the heat, and never force fluids on an unwell runner who has gained weight.

Heatwave conditions combined with a slower pace and a fixed drinking schedule is the combination that puts people in hospital. If you are ever in doubt about whether someone is overhydrated or dehydrated, err on the side of calling 999. Sodium levels can be corrected quickly in A&E. The alternative is not worth the risk.

For more on staying safe in warm weather, read our guides to heat exhaustion vs heatstroke NHS first aid and the UK first heatwave 2026 amber alert.