What is exercise-associated hyponatremia?
Dangerously diluted blood sodium, usually below 135 mmol/L, caused by drinking more fluid than you are losing. It is rarer than dehydration but considerably more dangerous.
Normal blood sodium sits around 135–145 mmol/L. Exercise-associated hyponatremia occurs when excess fluid intake dilutes it below that range. Because sodium concentration governs the movement of water between compartments, dilution draws water into cells — including brain cells, which is why severe cases present neurologically and can be fatal.
The primary cause is overdrinking, not sodium loss. This is counterintuitive and it matters: the runners most at risk are not the fastest or the sweatiest, they are the ones out on course longest with the most opportunity to drink at every station. Slower finishers, smaller runners and those taking NSAIDs are over-represented in case series.
Early symptoms are unhelpfully vague — nausea, headache, bloating, confusion — and overlap almost exactly with dehydration, which is the dangerous part: the instinctive response of drinking more makes it worse. Weight gain during a race is the distinguishing sign, since a dehydrated runner cannot have gained weight.
Prevention is straightforward: drink to thirst rather than to a schedule, include sodium in your fluids on longer efforts, and be sceptical of the advice to drink ahead of thirst that circulated widely in earlier decades. If you finish a marathon heavier than you started, you drank too much.
Key numbers
| Threshold | <135 mmol/L | blood sodium |
|---|---|---|
| Primary cause | overdrinking | not sodium loss |
| Warning sign | weight gain during the race |
Work it out for yourself
HR pacing simulator — Track modelled sodium and fluid balance across a full marathon.
Frequently asked questions
How do I tell hyponatremia from dehydration?
Body mass is the practical discriminator: a dehydrated runner has lost weight, a hyponatremic one has often gained it. Since the early symptoms overlap almost completely, this matters — and any confusion, disorientation or seizure during or after a race is a medical emergency, not something to self-manage.
Do salt tablets prevent hyponatremia?
They help, but they do not license overdrinking — the dominant driver is fluid excess rather than sodium deficit. Moderating intake is the primary prevention; sodium is a useful adjunct, particularly for efforts over four hours.
Sources
- Hew-Butler et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clinical Journal of Sport Medicine.
- Almond et al. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine.
Related explainers
Glossary: Hyponatremia