Hyponatremia in Triathlon: Recognising and Preventing Salt Loss on Long Course
Most triathletes worry about dehydration on long, hot race days — and rightly so. But the opposite problem, drinking too much plain water without replacing the sodium lost in sweat, is a genuine and under-discussed risk on long-course races, and it can be more dangerous than mild dehydration itself.
What Hyponatremia Is
Hyponatremia is abnormally low blood sodium. In endurance sport, it typically develops when an athlete drinks more fluid than they lose through sweat and urine over several hours, while also losing sodium through sweat without adequately replacing it. The result is that blood sodium concentration drops, which can affect nerve and muscle function — including, in severe cases, the brain and heart.
Why Long Course Racing Raises the Risk
The longer you’re racing, the more aid stations you pass, and the easier it is to over-drink “just in case” without tracking how much sodium you’re actually taking in alongside it. Slower finishers are statistically at higher risk than faster ones, partly because more time on course means more opportunities to drink, and partly because a longer race gives more time for an imbalance to develop. Hot conditions increase sweat rate and sodium loss, but paradoxically also increase the temptation to over-drink plain water at every aid station.
Recognising the Warning Signs
- Nausea, bloating or a “sloshing” stomach feeling despite feeling like you need to drink more.
- Puffy or swollen hands, feet or face during a long race.
- Confusion, disorientation or unusually poor coordination — easy to mistake for simple fatigue.
- In severe cases: seizures or loss of consciousness, which requires immediate medical attention.
These symptoms can look similar to heat exhaustion or simple over-fatigue, which is exactly why it’s easy to miss. If you or someone near you shows confusion or disorientation on course, tell medical staff at the next aid station rather than assuming it will pass.
Prevention
- Drink to thirst rather than a fixed volume schedule — the old advice to drink as much as possible “to stay ahead of dehydration” is now understood to increase hyponatremia risk.
- Take in sodium alongside fluids, not just water — electrolyte drinks, salt tablets or sodium-containing gels rather than relying on plain water at every station. See our guide on salt tablets vs electrolyte drinks for how to choose between them.
- Know your individual sweat rate and sodium loss — this varies hugely between athletes, so generic advice is a starting point, not a prescription.
- Practise your race-day fuelling plan in training, on long rides and runs, so it’s second nature rather than improvised mid-race.
Hyponatremia is preventable with sensible, practised fuelling — the goal isn’t to drink as much as possible, but to match fluid intake to what you’re actually losing, with enough sodium alongside it to keep your blood chemistry in balance.
