Dehydration vs Hyponatraemia: The Over-Drinking Risk in Long Events

Dehydration and exercise-associated hyponatraemia are opposite problems with overlapping symptoms. Dehydration is losing more fluid than you take in; hyponatraemia is blood sodium diluted below 135 mmol/L, in long events almost always by drinking well beyond thirst for hours. Headache, nausea and confusion sit on both lists, so your weight, not your symptoms, tells them apart: lighter means dehydration, the same or heavier means over-drinking. Drink to thirst, and treat confusion, vomiting or a seizure late in an event as a 999 call, never as a reason to drink more.^{1, 2, 3} Part of electrolytes for athletes in the Wellgard electrolyte guide.

Evidence at a glance

• Studies: race cohorts of 488 marathon runners and 669 ultramarathon finishers; a 161 km race survey of 157 finishers with blood samples; a 9-runner treadmill trial; a 210-triathlete cramp cohort; the 2015 international consensus statement.
• Largest single study: 887 finishers of a 161 km ultramarathon over five years, 669 with blood sodium.⁴
• Strongest measured outcome: 13% of 488 Boston Marathon runners tested had blood sodium at or below 135 mmol/L; weight gain during the race carried an odds ratio of 4.2 and a finish over four hours 7.4.³
• Not tested: no trial has randomised runners to drink to thirst against a schedule in a real race; the figures are observational.

Sources checked on 17 October 2026.

What is exercise-associated hyponatraemia?

A blood sodium below 135 mmol/L during or up to 24 hours after prolonged exercise, in the definition the 2015 international consensus conference uses; the normal range is about 135 to 145.¹ The 2017 update of that work says the research since the 1980s has "conclusively identified" two causes as the commonest: drinking beyond thirst, and a hormone (arginine vasopressin) that makes the kidneys hold on to water when exercise, nausea or pain tells them to; a smaller group loses enough salt and fluid through sweat for that to play a part.² Mild cases feel like nothing much or like a bad patch; severe cases swell the brain, which is where confusion, vomiting, seizures and the race-related deaths come from.^{2, 3} The signs of low electrolytes outside sport are a separate page.

How common is it in marathons and ultramarathons?

Common enough to plan for. At the 2002 Boston Marathon, 488 runners gave blood at the finish: 13% had sodium at or below 135 mmol/L and 0.6% at or below 120, the critical level. Weight gain during the race (odds ratio 4.2), a finish over four hours (7.4) and extremes of body mass index were the independent risk factors; the composition of what they drank, female sex and anti-inflammatory painkillers were not.³ Across five years of a 161 km ultramarathon in northern California, 15.1% of 669 finishers tested were hyponatraemic, from 4.6% to 51% by year, more in hot years; 23.8% of those had gained weight, 40.6% had held it and 35.6% had lost more than 3%, so at that distance in heat the condition arrived with dehydration as well as over-drinking.⁴ In a later year of the same race, 6.6% of finishers tested were hyponatraemic, and 93.9% of runners had used sodium supplements.⁵

Dehydration vs hyponatraemia: how do they differ?

The one measurement that separates them is weight. The 2007 sports-medicine position stand's rule is to finish no more than about 2% lighter and never heavier; a runner who finishes heavier and feels unwell has over-drunk, whatever the thirst and headache suggest.⁶

Does a sodium drink or salt capsule prevent it?

The cohorts did not find that it does, and the consensus says over-drinking is the cause to control. At Boston, the composition of the fluid drunk was not associated with hyponatraemia.³ In the 161 km race, the rate of sodium intake from supplements was the same in hyponatraemic and normal finishers, 93.9% of the field used sodium supplements, and the authors concluded that avoiding over-drinking is the most important means of prevention.⁵ In 9 runners doing two 5-hour treadmill runs at 30 °C, capsules replacing all their measured sweat sodium raised blood sodium by 4 mmol/L against 1 with placebo but made no difference to how much they drank or their fluid balance.⁷ The 2015 consensus advises drinking to thirst, and a sports drink at 20 to 50 mmol/L of sodium is still far more dilute than blood at 135 to 145, so it dilutes blood sodium too, more slowly than water.¹ Sodium for a heavy sweater stays sensible, sized to the sweat; it is not a licence to drink past thirst, and no sachet prevents hyponatraemia.

Why do the symptoms get confused?

Because nausea, headache and confusion appear on both lists, and because the reflex answer to feeling unwell in a race is to drink. The NHS dehydration signs are thirst, a headache, dark yellow pee, peeing less, dizziness, tiredness and a dry mouth; its urgent signs include dizziness on standing that does not go away and confusion.⁸ The 2017 review lists the same confusion, vomiting and seizures for severe hyponatraemia.² So the question to ask a runner who is unwell late in an event is not "have you drunk enough?" but "how much have you drunk, and are you heavier than you started?" Someone who has been drinking at every station for five hours and is confused needs a medical tent and a blood sodium, not another bottle. Pouring water over the head cools; pouring it in may not.

What should you do on the day?

Weigh yourself before the start in minimal kit, so there is a number to compare.⁶ Drink to thirst, and for the hours that cost salt put sodium in the drink or the food, sized to your sweat (the weigh-yourself method).^{1, 9} If you feel bloated, your rings are tight or you have gained weight at a checkpoint, stop drinking until you pee. If you feel dizzy and have not drunk for hours in heat, drink, and treat heat-exhaustion signs (dizziness, headache, nausea, cramps, a fast pulse, pale clammy skin) that do not settle within 30 minutes of cooling and drinking as a 999 call, as the NHS does for heatstroke.¹⁰ Confusion, vomiting or a seizure after hours of heavy drinking is 999 whatever the scales say. Treatment of a measured low sodium is a medical team's decision, not something a drink or a sachet corrects.¹

Cramp is a third thing

Cramp is neither sign. In 210 Ironman triathletes, the 43 who cramped had the same blood sodium, potassium and magnesium and the same weight change as the 166 who did not; faster race times and a history of cramping were the only independent risk factors.¹¹ In the 161 km race, the 14% who cramped matched the rest on sodium supplement use and post-race blood sodium.¹² So cramp on its own says nothing about your sodium or your water, and sodium for a salty sweater remains sensible without any promise that it stops cramp.

What we don't know

• No randomised trial has compared drinking to thirst with a drinking schedule in a real race; the cohort figures describe what happened, not what prevents it.
• Why some runners release the water-holding hormone during exercise and others do not is not established.²
• No study gives a litre figure for how much is too much; it depends on hours, heat, sweat and the kidneys.

Frequently asked questions

How much water is too much in a marathon?

No litre figure exists; the Boston cohort found more than 3 litres during the race and drinking at every mile went with low sodium, and weight gain was the measurable sign.³ Drink to thirst and never finish heavier.⁶

Can you get hyponatraemia from drinking too much water in a day?

The documented cases come from hours of drinking beyond thirst during long exercise, when the kidneys hold water; a large intake spread across an ordinary day is a different thing. How much water a day covers the everyday question.²

Does hyponatraemia feel like dehydration?

Often, which is the danger: headache, nausea and confusion sit on both lists. Weight, bloating and tight rings point to over-drinking; thirst, dark pee and a lighter weight point to dehydration.^{2, 8}

Do electrolyte drinks prevent hyponatraemia?

Not in the evidence: at Boston the type of drink was not associated with it, and in the ultramarathon sodium intake from supplements was the same in those with and without it.^{3, 5} Sodium replaces sweat; it does not undo water you did not need.

Sources

1. Hew-Butler T, et al. Consensus statement, 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, 17 authors. Br J Sports Med 2015. PubMed 26227507.
2. Hew-Butler T, et al. Narrative review. Front Med 2017. PubMed 28316971.
3. Almond CS, et al. Cohort, 488 marathon runners with finish-line blood samples; publicly funded. N Engl J Med 2005. PubMed 15829535.
4. Hoffman MD, et al. Cohort, 887 ultramarathon finishers over 5 years, 669 with blood sodium; publicly funded. Med Sci Sports Exerc 2013. PubMed 23135369.
5. Hoffman MD, Stuempfle KJ. Observational, 161 km race, 157 finishers with survey and blood sample; publicly funded. Med Sci Sports Exerc 2015. PubMed 25551404.
6. Sawka MN, et al. American College of Sports Medicine position stand on exercise and fluid replacement. Med Sci Sports Exerc 2007. PubMed 17277604.
7. McCubbin AJ, da Costa RJS. Double-blind randomised crossover trial, 9 runners, two 5-hour runs; academic authors. Int J Sports Physiol Perform 2024. PubMed 37944507.
8. NHS. Dehydration. Reviewed 1 May 2026; checked 17 October 2026. nhs.uk.
9. Baker LB, et al. Observational, 506 athletes (404 male); sports-drink-maker research institute. J Sports Sci 2016. PubMed 26070030.
10. NHS. Heat exhaustion and heatstroke. Reviewed 28 May 2026; checked 17 October 2026. nhs.uk.
11. Schwellnus MP, et al. Prospective cohort, 210 Ironman triathletes, 43 with cramp; publicly funded. Br J Sports Med 2011. PubMed 21148567.
12. Hoffman MD, Stuempfle KJ. Observational, 280 ultramarathon starters surveyed, 157 with blood samples. Sports Med Open 2015. PubMed 26284165.

General information, not medical advice. Confusion, vomiting or a seizure during or after a long event is a 999 call; a measured low blood sodium is treated by a medical team.

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