Exercise associated hyponatremia happens when blood sodium falls below 135 mmol/L during or within 24 hours after prolonged exercise. It can become life-threatening, and drinking extra water—or a sports drink—does not necessarily prevent it. For most endurance participants, current sports-medicine guidance favors drinking to thirst rather than forcing fluids to meet a preset hourly target.
The basics of exercise associated hyponatremia
Sodium helps regulate the balance of water in and around your body’s cells. During a long event, taking in more fluid than your body can get rid of can dilute the sodium in your blood. Exercise can also prompt the release of hormones that make the kidneys retain water, even when your body has taken in plenty.
Overdrinking is a major risk, but it is not the only factor: sodium loss through sweat and individual responses can contribute. The balance varies with the person, event and conditions, which is one reason a single drinking formula cannot fit every runner or race. Gaining weight during an event can be a sign of taking in more fluid than you are losing, though it is not a diagnosis.
A 2025 review by Lawrence E. Armstrong, Brendon P. McDermott, Suzanne L. Young and Douglas J. Casa examined 56 field studies reporting 220 cases across seven outdoor endurance activities lasting 5 to 29.5 hours. The reported mild symptoms often included nausea, weakness, dizziness and headache; more serious cases included confusion, vomiting, seizures and loss of consciousness. These cases help describe possible symptoms, but they do not provide a universal risk estimate for every event or athlete.
Why it can look like dehydration
Early exercise associated hyponatremia symptoms—such as headache, nausea, weakness and dizziness—are not specific. They can resemble dehydration, heat illness or other problems during exercise. Feeling unwell alone cannot tell you which condition is causing it. A blood test is needed to confirm low sodium, and symptoms do not reliably indicate the sodium level or severity.
Watch especially for worsening confusion or unusual behavior, repeated vomiting, agitation, collapse, a seizure or loss of consciousness during or after an endurance event. These can signal a medical emergency. Alert event medical staff or call your local emergency number immediately. Do not assume the person needs water: extra fluid can worsen hyponatremia, and treatment depends on the cause and severity. Diagnosis and treatment belong to medical professionals.
Drink to thirst, not to a rigid schedule
Older advice sometimes encouraged athletes to drink as much as possible to avoid dehydration. Sports-medicine reviews now emphasize avoiding overdrinking. Tamara Hew-Butler’s 2019 review identifies drinking according to thirst as an individualized prevention strategy; a 2025 review likewise recommends replacing “drink ahead of thirst” advice with drinking when thirsty.
That does not mean thirst gives you a precise measurement or that every event will feel the same. It means you generally do not need to force down fluid just because a schedule, aid station or bottle says it is time. There is no one amount of water that suits every marathon runner: pace, duration, weather and personal fluid losses differ. Plan for access to fluids, but avoid drinking beyond thirst to hit a fixed quota.
What electrolyte drinks can—and can’t—do
Sports drinks contain sodium, which can replace some sodium lost in sweat. But they still add fluid, and many are less concentrated than blood. Drinking too much of them can still contribute to dilution; their sodium content does not guarantee protection from exercise associated hyponatremia. The amount you drink matters as well as what is in the bottle.
Electrolyte drinks may suit some event nutrition plans, but they are not a reason to override thirst or follow a rigid drinking target. If you feel seriously unwell during or after an event, seek medical help rather than trying to correct the problem yourself with water, sports drinks or salt. Confusion, seizures, collapse or loss of consciousness need emergency care.
