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Nutrition6 min read

Exercise-associated hyponatremia: why drinking too much water is dangerous

Exercise-associated hyponatremia (EAH) means the sodium level in your blood falls too low during or after exercise. It is rare but can be life-threatening. The main cause is not losing too much salt: it is drinking more fluid than you sweat out over a long event.

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A marathon runner reaching for a cup of water at a drinks station.

The short answer

Exercise-associated hyponatremia is low blood sodium during or soon after long exercise, caused mainly by drinking more than you sweat. In the Boston Marathon, 13% of runners tested had it, and gaining weight during the race was the strongest risk factor (Almond et al., 2005). Drink to thirst, and never finish heavier than you started.

  • Sodium in drinks or tablets does not prevent EAH if you drink far too much (Hew-Butler et al., 2015).
  • Headache, nausea, vomiting, confusion or seizures after a long race need urgent medical help.

What is exercise-associated hyponatremia?

Hyponatremia in runners: who is at risk?

Water intoxication in a marathon: warning signs

  • Early signs: bloating, puffy hands, headache, nausea and feeling unwell despite drinking plenty.
  • Serious signs: vomiting, confusion, unusual behaviour, extreme tiredness, seizures or collapse.
  • Serious signs are a medical emergency: tell race medical staff or call 999. Do not drink more water.

Not everyone who gains weight during a race develops low sodium, because the body responds in different ways (Noakes et al., 2005).

How to avoid drinking too much water in a marathon

  • Drink to thirst, or use a planned amount based on your measured sweat rate.
  • Weigh yourself before and after long training runs; you should not finish heavier.
  • Do not drink at every station just because it is there, especially if you are slower or it is cool.

Measure your losses with the sweat rate calculator, then set an hourly drinking plan for race day.

People also ask

Do salt tablets prevent hyponatremia?

Not if you drink far too much. Sodium helps replace losses, but limiting overdrinking matters most.

Can you drink too much water during a marathon?

Yes. Drinking more than you sweat over several hours can dilute your blood sodium dangerously.

How much water should I drink during a marathon?

Drink to thirst or to a plan based on your measured sweat rate, and do not finish heavier than you started (Hew-Butler et al., 2015).

Is hyponatremia the same as dehydration?

No. It usually happens with too much fluid, not too little.

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Scientific References & Evidence

  1. Almond, C.S.D., Shin, A.Y., Fortescue, E.B., Mannix, R.C., Wypij, D., Binstadt, B.A., Duncan, C.N., Olson, K.P., Salerno, A.E., Newburger, J.W. and Greenes, D.S. (2005) 'Hyponatremia among runners in the Boston Marathon', New England Journal of Medicine, 352(15), pp. 1550-1556. doi: 10.1056/NEJMoa043901. Available at: https://doi.org/10.1056/NEJMoa043901
  2. Hew-Butler, T., Rosner, M.H., Fowkes-Godek, S., Dugas, J.P., Hoffman, M.D., Lewis, D.P., Maughan, R.J., Miller, K.C., Montain, S.J., Rehrer, N.J., Roberts, W.O., Rogers, I.R., Siegel, A.J., Stuempfle, K.J., Winger, J.M. and Verbalis, J.G. (2015) 'Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015', Clinical Journal of Sport Medicine, 25(4), pp. 303-320. doi: 10.1097/JSM.0000000000000221. Available at: https://doi.org/10.1097/JSM.0000000000000221
  3. Noakes, T.D., Sharwood, K., Speedy, D., Hew, T., Reid, S., Dugas, J., Almond, C., Wharam, P. and Weschler, L. (2005) 'Three independent biological mechanisms cause exercise-associated hyponatremia: evidence from 2,135 weighed competitive athletic performances', Proceedings of the National Academy of Sciences, 102(51), pp. 18550-18555. doi: 10.1073/pnas.0509096102. Available at: https://doi.org/10.1073/pnas.0509096102

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Educational information for adults aged 18 and over. It does not diagnose, treat or manage any medical condition. If you have a health condition, or symptoms that worry you, speak to your doctor or a health professional.