Hyponatremia and exercise can become a dangerous combination when you drink more fluid than your body can use. Exercise-associated hyponatremia (EAH) is a low blood sodium concentration—below 135 mmol/L—that develops during or within 24 hours of physical activity. It is most often linked to taking in excess water or sports drinks while the body is temporarily less able to get rid of water.
Hyponatremia and exercise: How excess fluid lowers sodium
Blood sodium helps regulate the balance of water in and around your cells. If you drink faster or in greater amounts than you lose fluid, extra water can dilute the sodium in your blood. During exertion, hormones including antidiuretic hormone can also prompt the kidneys to retain water. The combination can make overdrinking especially risky.
EAH is associated with endurance events, but it is not limited to ultramarathons. It can occur during shorter activities too, and symptoms may appear after you finish. Longer exertion, overdrinking, and very hot or cold conditions can contribute to risk. Sodium loss in sweat may play a role in some cases, but taking salt or an electrolyte drink does not make it safe to drink beyond your needs.
The Third International Exercise-Associated Hyponatremia Consensus Development Conference and a 2017 update by sports-medicine researchers Tamara Hew-Butler and colleagues identify overdrinking, combined with the body’s reduced ability to excrete water during exercise, as a central cause. Their guidance supports drinking according to thirst rather than following advice to drink as much as possible.
Why EAH can look like dehydration or heat illness
Early exercise associated hyponatremia symptoms can be vague: nausea, headache, dizziness, weakness or unusual tiredness. Bloating or swelling may also occur. These complaints can resemble dehydration or heat illness, so symptoms alone cannot reliably tell you which problem is happening. A blood sodium test is needed to confirm EAH.
Be especially alert to a change in someone’s behavior or level of awareness during or after an event. Confusion, agitation, repeated vomiting, collapse, a seizure or loss of consciousness can indicate severe illness. Trouble breathing is also an emergency warning sign. Severe EAH can cause swelling in the brain or lungs and can be fatal.
Do not assume that a person who has been exercising in heat simply needs more water. Heat illness and EAH can overlap, and their treatments differ. More fluid—including fluid given for presumed dehydration—can worsen EAH when excess water is already lowering blood sodium.
What to do if you suspect hyponatremia during exercise
For confusion, a seizure, collapse, difficulty breathing or other serious symptoms, call your local emergency number and get urgent medical care. Tell event medical staff how long the person has been exercising and what they have been drinking, if you know. Do not encourage them to drink more while EAH is a possibility. If they are confused or vomiting, don’t try to make them drink; follow emergency responders’ instructions.
Medical teams can check blood sodium and distinguish EAH from other exertional problems. Treatment depends on severity. Clinicians may restrict fluids in less severe cases; serious neurological symptoms can require carefully administered hypertonic saline. These treatments need medical assessment, not trial and error at the roadside.
How drinking to thirst may reduce risk
During training and events, let thirst guide your drinking instead of forcing fluids on a schedule or trying to finish a set amount. Don’t treat every aid station as a reason to drink. Thirst-led drinking does not guarantee prevention, but it can help you avoid the overconsumption linked to EAH. There is no single fluid target that fits every athlete, event, pace and set of conditions.
Water and sports drinks both add fluid; an electrolyte label does not prevent dilution if you drink too much. For a closer look at overhydration and what sports drinks can—and can’t—do, read our guide to drinking too much water during exercise.
If you or someone else becomes unwell during or after an event, don’t try to diagnose the cause from symptoms alone. Seek medical help, especially if symptoms are worsening or involve confusion, collapse, seizures or breathing problems.
