What It Is
Hyponatremia is the medical term for low sodium concentration in the blood. Exercise-associated hyponatremia (EAH) refers to cases that develop during or shortly after prolonged endurance exercise. It happens when total fluid intake outpaces the body's ability to excrete water, diluting the sodium in the bloodstream.
How Common Is It
Research from large endurance events has found EAH in a meaningful fraction of finishers, particularly in slower marathoners and ultramarathon participants. Most cases are mild and self-resolve. Severe EAH is much less common but is the dangerous version that occasionally makes the news. This isn't a content piece making medical recommendations - for individual concerns, see a sports physician.
The takeaway is that the old default of "drink as much as you can during a race" is no longer reflective of how endurance researchers think about hydration.
Who's at Higher Risk
Slower finishers (more time on course, more opportunities to over-drink), people who follow a strict "never miss an aid station" schedule regardless of thirst, and people who only carry water and avoid sodium-containing drinks on long courses. Smaller-bodied athletes also tend to feature in published case series.
Current Guidance
Modern endurance hydration guidance leans toward drinking to thirst as the default, with sodium-containing drinks (rather than plain water) used when fluid is consumed during long events. This is what sports drinks have always been designed for - the sodium is the difference between rehydrating and over-diluting.
Higher-sodium products (LMNT, the higher-sodium Liquid I.V. formulation) get attention in this conversation, but standard sports drinks (Gatorade, Powerade, Endura, SiS) also contain enough sodium to put you above plain-water risk territory.
The Honest Frame
Don't drink as much as you possibly can. Don't drink only plain water on long endurance events. Eat the gels, the snacks, and use sodium-containing drinks. The body's signals - thirst, urine, how you feel - are the most reliable feedback during the event itself.
If You're Training for a Long Event
Read the race's published fluid stations and product list, train with those products, and consider a chat with a sports dietitian if you're a heavy sweater or you've felt unwell during long events before. Sports Drinks for Runners covers race-day planning in more detail.
Sources
Authoritative references this article draws on. External links open in a new tab.
Frequently Asked
How common is hyponatremia in marathons?
Published research has found EAH in a meaningful fraction of finishers, particularly slower marathoners and ultra-runners. Most cases are mild and self-resolve.
Who is at risk of hyponatremia?
Slower endurance finishers, athletes following strict 'drink at every aid station' schedules, those drinking only plain water on long courses, and smaller-bodied athletes.
How can I avoid hyponatremia?
Drink to thirst, not to schedule. Use sodium-containing drinks during long events. Don't pre-load excessive water in the morning. Eat the gels and snacks.
