| What it is | Dangerously low blood sodium |
| Common cause | Overdrinking plain water; salt loss |
| Signs | Nausea, headache, confusion, swelling |
| Key | Looks like dehydration; treatment is opposite |
Hyponatremia is a dangerously low concentration of sodium in the blood, which in endurance athletes often results from drinking large amounts of plain water without replacing salt lost in sweat. Symptoms — nausea, headache, confusion, and swelling — overlap with dehydration but the treatment is opposite, making it a serious and easily misread condition on long efforts.
This is general educational information, not medical advice. In an emergency, seek professional medical help.
The opposite of dehydration
It mimics dehydration but is caused by overdrinking — replace electrolytes, don’t just add water. Both can accompany heat illness.
Frequently asked questions
What is hyponatremia?
Hyponatremia means abnormally low sodium in the blood. In hikers and endurance athletes it most often comes from drinking far more plain water than needed while losing sodium through sweat, which dilutes the body's sodium. Cells, including in the brain, swell, which can become life-threatening.
How is hyponatremia different from dehydration?
They can look alike — nausea, headache, fatigue, confusion — but are opposites: dehydration is too little fluid, hyponatremia is too much water relative to sodium. Crucially, treating suspected dehydration by drinking still more plain water can worsen hyponatremia, which is why recognizing the difference matters.
How do you prevent hyponatremia?
Don't overdrink — match fluids to need rather than forcing large volumes — and replace electrolytes on long, hot, or multi-hour efforts through sports drinks, electrolyte tabs, or salty food. Drinking to thirst while including sodium is the practical strategy to avoid both dehydration and hyponatremia.
Sources
- Exercise-associated hyponatremia — Wilderness Medical Society
- Hydration safety — CDC