Hyponatremia: The Hidden Danger of Over-Hydrating in Endurance Sports
Guest post by Maren Williams — Endurance athlete (triathlete & marathoner) In partnership with Proven Science Nutrition · Science First. Results Always.
About the author — My name is Maren Williams, an endurance athlete who's spent the last several years chasing finish lines — 6 marathons and 3 triathlons, including an Olympic distance triathlon at 2026 USAT Nationals. Somewhere between the 5 a.m. swims and the long Sunday runs, I learned that what you put in your body matters as much as the training itself — sometimes more. I write about smart, sustainable endurance training and fueling, and I'm proud to partner with Proven Science Nutrition because they obsess over the science the same way I do. Come say hi and follow the journey: Instagram @marenwilliamstri and on YouTube @marenwilliamstri.
Let me tell you about the scariest thing I've seen at a finish line — and it wasn't someone who didn't drink enough.
A few years into racing, I watched a fellow runner collapse after a hot marathon. Everyone assumed dehydration. Medics assumed otherwise — and they were right. She'd been drinking water at every single aid station, sometimes two cups, the whole way. She didn't have too little fluid. She had too much. Her blood sodium had dropped to a dangerous level, and she was experiencing exercise-associated hyponatremia (EAH).
That day changed how I think about hydration. We're all taught to fear dehydration — but in endurance sports, drinking too much water can be just as dangerous, and almost nobody talks about it. So let's talk about it.
What Is Hyponatremia?
Hyponatremia simply means your blood sodium level is too low. Sodium is the main electrolyte in the fluid outside your cells, and your body keeps it in a tight, healthy range. When athletes develop it during or after prolonged exercise, we call it exercise-associated hyponatremia (EAH) — defined as a blood sodium concentration below 135 mmol/L.[1]
Here's the counterintuitive part: EAH is usually caused by drinking too much fluid, not by losing too much sodium. When you take in more water than your body can process, you dilute the sodium that's already there — like adding too much water to a glass of saltwater. The concentration drops, and your cells start to swell with the excess fluid, including cells in your brain. That's what makes severe cases so dangerous.[1]
It's More Common Than You'd Think
This isn't a rare, freak occurrence. In a landmark study of the Boston Marathon, researchers found that 13% of runners finished with hyponatremia, and a small but serious fraction were in the critical range.[2] Thirteen percent. That's more than one in ten runners in a major marathon.
And the single biggest risk factor they identified? Weight gain during the race — the telltale sign of drinking more than you're sweating out.[2] Runners who gained weight over the course of the marathon (because they were over-drinking) were far more likely to end up hyponatremic.
Who's Most at Risk
EAH can happen to anyone in a long event, but some of us are more vulnerable. You're at higher risk if you:[1][2]
Drink on a rigid schedule ("a cup at every aid station") rather than to thirst
Are doing a long event — marathons, ultras, Ironman and long-course triathlons
Are a slower finisher (more time, more opportunities to over-drink)
Are a smaller athlete (less body water to buffer the excess)
Over-drink in cooler weather, when you feel like you should hydrate but aren't sweating much
Have had a hyponatremia scare before
I fit several of those boxes myself, which is exactly why I had to learn this the hard way.
Know the Warning Signs
The tricky thing about EAH is that its early symptoms — nausea, headache, feeling puffy or "off" — can look a lot like dehydration or just general race fatigue. That's dangerous, because the instinct is to drink more water, which makes hyponatremia worse.
Watch for:
Nausea, headache, and bloating or puffiness (rings, watch, or socks feeling tight)
Confusion, disorientation, or unusual behavior
Swelling in the hands and feet
Feeling worse the more you drink
Severe signs are a medical emergency — seizures, extreme confusion, loss of consciousness. If you or a training partner show these, get medical help immediately and do not keep drinking plain water.[1]
A note on this, athlete to athlete: I'm a runner and triathlete, not a doctor. This article is about awareness and prevention — it is not medical advice. If you ever suspect hyponatremia in yourself or someone else, treat it as the emergency it can be and get professional medical care right away.
How to Prevent It (Without Swinging Too Far the Other Way)
The goal isn't to drink less out of fear — it's to hydrate smart. Here's the approach that's kept me safe across dozens of long races:
1. Drink to thirst, not to a schedule. This is the single most important shift. Your thirst is a remarkably good guide. The rigid "drink as much as you can, as often as you can" advice is exactly what gets people into trouble.[1]
2. Replace sodium, don't just replace water. When you do drink during long efforts, make it count — include sodium so you're replacing what you sweat out, not just diluting what's left. Sodium also helps your body actually absorb and hold onto the fluid you take in, rather than flushing it through.[3][4]
3. Don't over-drink in the cool. On cooler days you sweat less, so you need less. Feeling like you "should" hydrate isn't the same as needing to.
4. Know your sweat rate. The more you understand your own fluid and sodium losses, the better you can match your intake to them — not too little, not too much. (My teammates and I dug into exactly how to figure this out in How Much Sodium Do Endurance Athletes Actually Lose? — worth a read.)
5. Practice it in training. Never test a new hydration strategy on race day. Dial it in on your long training days so it's automatic.
The through-line of all of this: it's not about water OR sodium — it's about balance. Replace what you lose, drink to thirst, and keep your sodium up so the fluid you drink actually works for you.
The Electrolyte I Train and Race With
This is why I'm picky about what goes in my bottles. A lot of hydration products are mostly sugar and flavor with a token pinch of sodium — which is useless for an endurance athlete trying to actually replace losses. I use Proven Science Electrolytes because it's built the way the science says it should be:
750 mg of Himalayan pink salt for meaningful sodium — the electrolyte that actually matters for holding onto fluid
A full mineral profile (potassium, magnesium, calcium) plus 750 mg of coconut water powder for naturally occurring potassium
No added sugar, and every dose disclosed on the label — no hidden "blends"
Pairing real sodium with drinking to thirst is exactly the balance that keeps hyponatremia at bay on my longest days. It's hydration with intention — which is the whole point.
The Bottom Line
Dehydration gets all the attention, but over-hydration — and the hyponatremia it can cause — is the hidden danger that catches too many endurance athletes off guard. It affects more than one in ten marathoners, its early signs are easy to mistake, and the usual instinct (drink more water) makes it worse.[1][2] The fix isn't fear — it's balance: drink to thirst, replace your sodium, know your sweat rate, and practice your plan. Do that, and you'll cross the finish line strong and safe.
See you out there. 🏃♀️
— Maren
Guest post by Maren Williams, endurance athlete, in partnership with Proven Science Nutrition. Want a complete, science-first approach to fueling your training? Grab the free Nutritional & Supplement Guide, or explore personalized coaching at Fit by Mi.
*This article is for educational purposes only and is not medical advice. Exercise-associated hyponatremia can be life-threatening. Individual fluid and sodium needs vary widely. Consult a qualified healthcare professional for guidance, and seek emergency medical care immediately if you suspect hyponatremia — particularly with confusion, seizures, or loss of consciousness.
Frequently Asked Questions
Can you really drink too much water during a race? Yes. Drinking more fluid than your body can process dilutes your blood sodium and can cause exercise-associated hyponatremia (EAH) — a potentially dangerous condition. In one major marathon study, 13% of runners finished hyponatremic, and the biggest risk factor was over-drinking (shown by weight gain during the race).
What's the difference between dehydration and hyponatremia? Dehydration is too little body fluid; hyponatremia is too little sodium in your blood, usually from too much fluid. Confusingly, their early symptoms (nausea, headache, feeling off) can look similar — but the fixes are opposite. Drinking more water helps dehydration and worsens hyponatremia, which is what makes EAH so dangerous.
How do I know if I'm drinking too much during endurance exercise? Warning signs include bloating or puffiness (tight rings, watch, or socks), nausea, headache, swelling in the hands and feet, and feeling worse the more you drink. A practical rule: drink to thirst rather than on a rigid schedule, and if you're gaining weight during a long event, you're almost certainly over-drinking.
Does taking electrolytes prevent hyponatremia? Replacing sodium helps, but the most important factor is not over-drinking in the first place. The best protection is a combination: drink to thirst, avoid excessive plain water, and include meaningful sodium so you're replacing losses rather than diluting your blood. Electrolytes are part of the plan, not a license to over-drink.
Who is most at risk of exercise-associated hyponatremia? People doing long events (marathons, ultras, long-course triathlons), slower finishers, smaller athletes, those who drink on a fixed schedule rather than to thirst, and anyone over-drinking in cool conditions. If you've had an EAH scare before, you're at higher risk and should be especially careful.
What should I do if I think someone has hyponatremia? Treat it as a potential emergency. Stop giving them plain water, and get medical help immediately — especially if there's confusion, severe headache, vomiting, seizures, or loss of consciousness. Severe hyponatremia requires urgent professional treatment, not more fluids.
References
Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine. 2015;25(4):303–320. doi:10.1097/JSM.0000000000000221
Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine. 2005;352(15):1550–1556. doi:10.1056/NEJMoa043901
Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand: Exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007;39(2):377–390. doi:10.1249/mss.0b013e31802ca597
Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. American Journal of Clinical Nutrition. 2016;103(3):717–723. doi:10.3945/ajcn.115.114769
Baker LB. Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature. 2019;6(3):211–259. doi:10.1080/23328940.2019.1632145
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