Exercise-associated hyponatremia is low blood sodium below 135 mmol/L during or up to 24 hours after prolonged exertion. Stop running and stop drinking if you develop nausea, vomiting, headache, bloating, swelling, weight gain, confusion or lethargy, and get emergency help for altered mental status, seizures, breathing distress or collapse. It affects marathon, triathlon and ultradistance runners in hot and cold conditions, according to the Wilderness Medical Society in its 2019 Wilderness Medical Society update. Drinking beyond thirst is the main driver, not simply losing sodium in sweat.
Medical information disclaimer: This article is for general educational purposes only and does not provide medical advice, diagnosis, or treatment. Always consult a physician or other qualified health professional about symptoms, medications, tests, or treatment decisions.
Table of Contents
- Why does sodium drop during long runs?
- Who faces higher risk?
- Which early signs mean stop?
- Which signs need emergency care?
- How should you drink to lower risk?
Why does sodium drop during long runs?
Drinking water or sports drinks beyond thirst dilutes blood sodium. Weight gain during a race signals excess fluid retention rather than good hydration.
Runners often believe more fluid always prevents collapse. The 3rd International Exercise-Associated Hyponatremia Consensus reports in the 2015 consensus statement in the British Journal of Sports Medicine that overdrinking and weight gain are major risk factors. Flat pace, long time on course and ready fluid access raise intake further.
Who faces higher risk?
Risk rises with exercise over four hours, slow pace, low body mass and event inexperience. High fluid availability and NSAID use in some studies also increase risk.
Check weight change when possible. Any gain means you drank more than you lost.
- Running longer than four hours at an easy pace
- Drinking on schedule rather than to thirst
- Gaining weight while running
- Small body size, limited race experience and frequent aid stations
- NSAID use during the event
Which early signs mean stop?
Early warnings mean stop running and stop drinking, according to the Gatorade Sports Science Institute in its hyponatremia review for athletes. Do not try to push through them with more fluid.
Sit down, avoid plain water and seek medical assessment. Worsening headache, vomiting or mental change needs urgent care.
- Nausea or vomiting
- Headache
- Bloating or puffy fingers and ankles
- Weight gain since the start
- Confusion, lethargy or feeling distinctly off
Which signs need emergency care?
Severe illness involves brain and lung swelling. Gettysburg College health faculty describe altered mental status, seizures, respiratory distress, coma and death as severe outcomes.
Call emergency services immediately for confusion, disorientation, seizure, shortness of breath, extreme drowsiness or inability to stay awake. Do not leave the runner alone. Suspected cases must not receive plain water.
How should you drink to lower risk?
Drink to thirst rather than on a fixed schedule. Avoid gaining weight while running, as reported by the Wilderness Medical Society in this March 2020 report on thirst-based drinking.
Thirst or headache alone cannot separate hyponatremia from dehydration because early symptoms overlap, including confusion, lethargy, nausea and cramps. Use weight change and mental status as stronger cues. Clinicians treat hyponatremic encephalopathy with a 100 mL bolus of 3% hypertonic saline.