Hyponatremia in Endurance Athletes: The Danger of Drinking Too Much Water

Some endurance athletes are scared of dehydration, so they drink at every aid station like the finish line has a hydration audit.

Water. Sports drink. More water. Maybe a salt cap, just to be safe.

That can backfire.

What if the problem is not too little water, but too much?

Exercise-associated hyponatremia is low blood sodium during or after exercise, usually defined as blood sodium below 135 mmol/L. In endurance races, the big practical problem is often not "you forgot salt." It is that you drank more fluid than your body could clear, which diluted the sodium in your blood.

I do not want to be dramatic about it, but this is one of the few hydration mistakes that can become genuinely dangerous. Mild cases can look like normal race misery. Severe cases can involve confusion, seizures, coma, and death. If an athlete is confused, collapsing, seizing, or acting seriously off, that is medical-tent-now territory, not "try another salt tab and see what happens."

This post is about risk reduction, not diagnosis. You cannot diagnose hyponatremia from vibes at mile 20. But you can avoid the dumb setup that makes it more likely.

What hyponatremia actually is

Sodium helps regulate fluid balance, nerve signaling, and muscle function. Your body keeps blood sodium in a tight range because your cells, especially brain cells, do not like big water shifts.

Hyponatremia means blood sodium is too low. General medical references put the cutoff below 135 mEq/L or 135 mmol/L, and symptoms can range from mild nausea or headache to serious neurological problems when the drop is severe or fast (MedlinePlus, Cleveland Clinic).

In endurance sports, the version people worry about is exercise-associated hyponatremia, or EAH. The 2015 international consensus statement defines EAH as low sodium during or up to 24 hours after prolonged physical activity (Hew-Butler et al.).

Normal-person version: your blood gets too diluted while you are racing or shortly after.

That matters because symptoms can overlap with dehydration, heat illness, bonking, and plain old race-day suffering. Headache, nausea, weakness, and feeling terrible are not exactly rare at mile 22 of a marathon. That is what makes this tricky. If you assume every bad feeling means "drink more," you can make the wrong problem worse.

Why endurance athletes get into trouble

Long races create the perfect little science fair of bad hydration decisions.

You are sweating and aid stations are everywhere. Coaches, race emails, old-school advice, and product labels all tell you hydration is important. If you are a newer marathoner or a slower ultra runner, you might be on course for 5, 8, 12, or 24 hours, which gives you a lot of chances to overcorrect.

The Boston Marathon study is the one that still gets cited for good reason. Researchers tested runners after the 2002 race and found hyponatremia in 13% of sampled finishers. The strongest predictor was weight gain during the race, which is a pretty clear overdrinking signal. The study also found that hyponatremia happened in runners who drank sports drink too, not just plain water (Almond et al.).

Average is a trap here too.

A 90 kg salty sweater running a hot Ironman and a 52 kg marathoner jogging through cool-weather aid stations do not have the same fluid ceiling. Body size, pace, sweat rate, race duration, temperature, and how aggressively you drink all matter. Smaller athletes and slower athletes can be at higher risk because a fixed number of cups per aid station is a bigger dose relative to body size and sweat loss.

Women have shown up as higher risk in some EAH reports, but I would be careful with lazy rules there. Body size, race duration, and drinking behavior probably explain a lot of it. The useful takeaway is simple: smaller athletes should be especially skeptical of big, generic fluid targets.

Salt tabs are not a free pass

This is the part people get backwards.

Sodium matters. The sodium-per-hour guide exists because sweat sodium losses can be huge, and some athletes absolutely need more sodium than the course provides.

But salt does not make overdrinking safe.

Sports drink is still mostly water. Salt caps still dissolve into the fluid already in your gut. If you keep drinking more than you are losing, you can still dilute blood sodium. The WMS clinical practice guideline update and EAH consensus papers both put excessive fluid intake at the center of prevention, with drinking to thirst as a major guardrail (Wilderness Medical Society guideline, Hew-Butler et al.).

So if your plan is "I drink at every station and take a SaltStick every hour," that might work on a hot day if you are losing a lot of sweat. Or it might be too much fluid with a little sodium sprinkled on top. Same behavior, different athlete, different risk.

That is the whole problem with blanket hydration rules.

Symptoms you should not ignore

Mild hyponatremia can be sneaky. Some athletes have no obvious symptoms. Others feel awful in ways that sound like half the finish chute: nausea, headache, vomiting, fatigue, dizziness, bloating, swollen hands, swollen feet, or just a weirdly heavy, puffy feeling.

The red flags are mental-status changes.

Confusion. Severe headache. Unusual sleepiness. Poor coordination. Collapse. Seizure. Acting drunk or not making sense. Those are not normal "tough day out there" symptoms. They need medical evaluation fast.

And this is where the dehydration confusion gets dangerous. If someone with hyponatremia is mistaken for a dehydrated athlete and encouraged to keep drinking, that can make the dilution worse. Medical teams can check sodium. Your training partner at mile 23 cannot.

If the symptoms are serious, stop trying to solve it from a race belt.

A safer hydration plan

The goal is not to underdrink. Dehydration is real, heat illness is real, and pretending you can macho your way through a hot race with three sips of water is its own brand of denial and error.

The safer move is to build a plan around your actual fluid losses instead of aid-station vibes.

Start with a sweat rate test. Weigh before and after a one-hour run or ride, track what you drank, and estimate your hourly sweat loss. Do it in conditions that look like race day if you can. A cool spring sweat test is useful, but it will not tell the whole story for a humid July marathon.

Then use a range, not a commandment. Many athletes do better replacing a portion of sweat losses rather than chasing 100% replacement while moving. If your sweat rate is 1.0 L/hr, a target around 500-800 ml/hr may be enough depending on heat, race length, and tolerance. If your sweat rate is 1.8 L/hr, that plan probably needs to be bigger.

Look for weight gain in training simulations. If you finish a long session heavier than you started, and you did not eat enough food to explain it, you probably overdrank. That is not a badge of discipline. That is useful feedback.

Also pay attention to your hands. Swollen fingers are not a perfect diagnostic tool, but if your rings feel tight, your stomach is sloshy, and you have been drinking constantly, maybe stop trying to win the aid station punch card.

Race-day rules that actually help

Here is what I would tell most endurance athletes.

Drink early enough that you are not playing catch-up, but do not drink mechanically just because a cup is available. Match fluids to thirst, sweat rate, weather, and the plan you practiced. If your stomach is sloshing, your fingers are swollen, and you are peeing clear every 20 minutes, that is not "hydration discipline." That is a warning light.

For marathons, water with gels makes sense because gels are concentrated. But water with every gel plus full cups at every station plus sports drink on top can add up fast, especially for smaller or slower runners. The marathon gel timing guide is useful here because it separates carb timing from fluid panic.

For Ironman and 70.3, the bike gives you more control. Use bottle marks, watch alerts, and a realistic hourly target. On the run, reassess. The athlete who drank well on the bike does not need to keep forcing bottle math into a hot half marathon just because the spreadsheet said so. The 70.3 nutrition guide gets into that bike-run split.

For ultras, be extra careful with "small sips forever." Small sips are smart until they become endless background drinking for 12 hours. Aid-station soup, sports drink, water, cola, broth, and bottles all count. Your gut does not care that they came from different tables.

Want a hydration plan that starts with your race, body size, weather, and pace instead of random cup-counting? Build a personalized carb, fluid, and sodium plan in about 60 seconds with the free EnduranceOS planner.

What to do if things feel off

During a race, mild discomfort is hard to interpret. That is annoying, but honest.

If you are simply thirsty, your mouth is dry, and you have not been drinking much, fluid may help. If you are sloshy, bloated, puffy, nauseous, and have been drinking constantly, more fluid is probably not the first move. Slow down, stop drinking for a bit, and get medical help if symptoms are escalating.

Confusion changes everything.

If an athlete is confused, collapsing, vomiting repeatedly, seizing, unable to walk straight, or not acting like themselves, get medical support. Do not keep giving them water because they "look dehydrated." Do not assume a salt cap fixes it. Let the people with blood sodium testing handle that one.

This is also why you should be cautious with NSAIDs like ibuprofen during long races. EAH consensus papers list NSAID use among risk factors, likely because kidney water handling can get messier during prolonged exercise. If you need painkillers to get through the race, that is a different conversation with a clinician, not a race-morning improvisation.

The boring middle is the goal

The internet loves extremes. Drink constantly. Never drink. Take a gram of sodium an hour. Throw your salt tabs in the trash. Buddy, no.

The useful answer lives in the boring middle: know your sweat rate, respect thirst, count all fluids, include sodium where it makes sense, and do not treat aid stations like a dare.

Hyponatremia is scary because the fix is not heroic. It is restraint. It is planning. It is knowing when "more" is no longer helping.

And honestly, that is good news. You do not need a medical degree to reduce your risk. You need a hydration plan that fits your body and your race, plus enough humility to adjust when the day starts sending signals.

The EnduranceOS planner builds carb, fluid, and sodium targets around your race distance, body size, pace, and conditions. It takes about 60 seconds, and it is free.