Drinking too much water during a marathon can lower blood sodium: Doctor shares signs to watch for

Overhydration during a marathon can mimic dehydration, but it can point to a much more serious condition which affects your sodium levels.

Marathons are long-distance runs, and staying hydrated is high on a runner's checklist. Reaching for water at every checkpoint may seem like the sensible thing to do, but drinking more than your body needs can also pose grave health risks.

Before you sign up for a marathon this season, make sure you are aware of the risks of overhydrating and how to stay hydrated safely.

Let's take a quick look at a health risk runners should know about: a condition called exercise-associated hyponatraemia (EAH).

Dr Judhajit Sengupta, consultant, critical care medicine, Narayana RN Tagore Hospital, Mukundapur, Kolkata, explained how excessive fluid intake during prolonged exercise can lead to exercise-associated hyponatraemia (EAH).

What is exercise-associated hyponatraemia?

Hydration during a run requires balance, as consuming too much fluid can affect the concentration of sodium in your blood.

The doctor described the condition: “The condition called exercise-associated hyponatremia is a syndrome that is characterised by a decrease in the amount of sodium in the blood. Even special oral rehydration solution (ORS) and sports drinks are not an exception, because they contain sodium in a lower concentration than in the blood."

This is why the general advisory cautions against excessive water consumption during physical activity, as it may lead to a reduced level of sodium in the blood.

The body retains excess water during exercise, an effect attributed to the hormone, the expert identified to be arginine vasopressin, which causes the kidneys to retain water. And when fluid intake exceeds the body's losses and its ability to excrete water, then the retained fluid can lower sodium concentration in the blood.

He cited a finding of a study where this condition was observed: “A landmark 2005 study published in The New England Journal of Medicine found that 13% of runners who ran in the Boston Marathon had exercise-associated hyponatremia, with 0.6% experiencing critical hyponatremia.”

What symptoms should runners watch for?

Dizziness is one of the symptoms you need to monitor. (Picture credit: Shutterstock)

Some symptoms may overlap with dehydration and heat-related illness. Here are the symptoms according to the doctor:

• Nausea and vomiting
• Headache
• Dizziness or lightheadedness
• Fatigue

Confusion, seizures, or loss of consciousness can indicate a severe medical emergency. Any runner who becomes confused or collapses needs urgent medical help.

Sometimes, these symptoms may resemble those of dehydration, but according to Dr. Sengupta, a history of fluid intake and prompt blood sodium testing can help clinicians identify EAH. The distinction is essential because people may assume a runner is weak due to dehydration and give them more fluid, unaware that this could worsen the situation.

How can runners avoid drinking too much?

The expert warned against forcing yourself to drink large quantities of water. There is no single volume that is dangerous for everyone, but you should drink in response to thirst rather than stock up on water at every checkpoint in response to thirst rather than stock up on water at every checkpoint to stay hydrated. Even sports drinks do not protect against the risks of excessive fluid intake.

Note to readers: This article is for informational purposes only and not a substitute for professional medical advice. Always seek the advice of your doctor with any questions about a medical condition.