Key takeaways
- Hyponatremia is dangerously low blood sodium — in endurance, usually from over-drinking water without replacing salt.
- Excess water dilutes the body's sodium; symptoms include nausea, headache, bloating, and confusion.
- Severe cases can cause seizures and life-threatening brain swelling.
- It's essentially the opposite of dehydration — prevent it with sensible (not excessive) drinking plus electrolytes.
hypo- (low) + natrium (sodium) + -emia (blood condition).
This is general educational information, not medical advice. Severe hyponatremia is life-threatening — seek emergency medical help; learn more through wilderness first-aid training.
What hyponatremia is
Hyponatremia is abnormally low sodium concentration in the blood. In endurance sports it’s usually caused by drinking excessive amounts of water (and other low-sodium fluids) without replacing the sodium lost in sweat — diluting the body’s sodium to dangerous levels. Sodium is essential for nerve and muscle function, so low levels cause problems.
Symptoms
Early: nausea, headache, bloating or puffiness, confusion or feeling ‘off,’ and fatigue — confusingly similar to dehydration, though the cause is opposite. A telltale sign: the person has been drinking a lot but still feels unwell (often with clear urine or swelling). Severe: vomiting, severe confusion, seizures, and life-threatening brain swelling.
Deep into a hot ultra, a runner who’s been drinking plain water at every aid station feels nauseated, headachy, and bloated. Recognizing possible hyponatremia rather than dehydration, they stop guzzling water and take in salty food and an electrolyte mix — and avoid making it worse with still more water.
Prevention
Don’t over-drink, and replace electrolytes: drink to your thirst and needs rather than forcing down large amounts of plain water, and on long, hot, sweaty efforts take in sodium (sports drinks, tablets, or salty food) with your fluids. The goal is balance — enough to avoid dehydration, with adequate sodium. Crucially, if someone’s ill after drinking a lot, more plain water can make it worse, so distinguish it from dehydration and seek help when in doubt. It’s a different problem from the energy-depletion bonk.
The bottom line
Hyponatremia is dangerously low blood sodium — in endurance, usually from over-drinking water without replacing the salt lost in sweat, diluting the body's sodium. Symptoms (nausea, headache, bloating, confusion) mimic dehydration but the cause is opposite, and severe cases cause seizures and brain swelling. Prevent it by drinking to thirst (not excessively) and taking electrolytes. If someone's ill after drinking a lot, more plain water can worsen it.
Frequently asked questions
What is hyponatremia?
Hyponatremia is an abnormally low concentration of sodium in the blood. In endurance sports it's most often 'exercise-associated hyponatremia,' caused by drinking too much fluid (especially plain water) during prolonged exercise without replacing the sodium lost in sweat, which dilutes the sodium in your blood to dangerously low levels. Sodium is essential for nerve and muscle function and fluid balance, so low levels cause problems.
What are the symptoms of hyponatremia?
Early symptoms can include nausea, headache, bloating or puffiness, confusion or feeling 'off,' and fatigue — which can be confusingly similar to dehydration, even though the cause is the opposite. A telltale sign is that the person has been drinking a lot but still feels unwell (and may have clear urine and be urinating frequently or have swelling). As it worsens, hyponatremia can cause vomiting, severe confusion, seizures, and life-threatening swelling of the brain. It's a serious condition that can be fatal if severe.
How do you prevent hyponatremia?
Mainly by not over-drinking and by replacing electrolytes. Drink according to your thirst and needs rather than forcing down large amounts of plain water 'to be safe,' and on long, hot, or sweaty efforts, take in sodium/electrolytes (through sports drinks, electrolyte tablets, or salty food) along with your fluids. The goal is balance — enough fluid to avoid dehydration, with adequate sodium so you don't dilute your blood. Crucially, if someone shows symptoms after drinking a lot, giving them more plain water can make hyponatremia worse, so it's important to distinguish it from dehydration; when in doubt about a seriously ill person, seek medical help.
Sources
- Exercise-associated hyponatremia — American Council on Exercise
- Sodium & hydration — Centers for Disease Control and Prevention