We hear everywhere that we need to drink more. During long efforts, this reflex can backfire. Hyponatremia affects serious, well-prepared runners who have simply swallowed too much plain water during their race. Here's how it happens, how to recognize it, and above all, how to avoid it.
What exactly is hyponatremia?
A matter of concentration, not quantity
Sodium is the mineral that controls how water is distributed between the inside and outside of your cells. In a healthy person, its blood concentration is around 135 to 145 mmol per liter. Hyponatremia occurs when this figure falls below 135 mmol per liter.
The important point: in the context of sport, it is almost never because you lack sodium in absolute terms. It is because you have added so much water that the sodium present becomes diluted. In other words, the problem comes from the denominator, not the numerator.
Why the body can no longer keep up
At rest, your kidneys have no problem getting rid of excess fluid. During prolonged exercise, things change. Renal blood flow drops because blood is redirected to the muscles and skin. At the same time, the body secretes more antidiuretic hormone, which encourages the body to retain water rather than eliminate it.
The result: you can literally drink faster than your ability to eliminate fluid. The excess water then moves into the cells and makes them swell. In the muscles, this is not dramatic. In the brain, enclosed in a rigid skull, it becomes serious.
Symptoms that should alert you
Mild signs
They look so much like ordinary fatigue that they are often ignored: nausea, headaches, bloating, a sloshing sensation of fullness in the stomach, fingers swelling until your ring feels tight, cramps, and an unexplained drop in pace. Taken individually, none of these signs proves anything. When they occur together during a long outing where you have been drinking a lot, they deserve your full attention.
Serious signs
Repeated vomiting, confusion, disorientation, incoherent speech, visual disturbances, difficulty walking straight, or labored breathing. At this stage, stop exercising immediately, stop drinking plain water, and call emergency services. Brain swelling cannot wait, and the condition can worsen quickly.
The danger of confusing it with dehydration
That is the real danger. Nausea, headaches, and fatigue also appear when you lack water. The natural response is then to drink even more, which significantly worsens the situation. One useful clue: dehydration comes with a dry mouth, pronounced thirst, dark and infrequent urine, and weight loss. Hyponatremia is more likely to cause a feeling of being overly full, clear and abundant urine, and a stable or higher weight compared with the start.
This is exactly why water alone is not enough during a long effort. We explain the mechanism in our electrolyte drink guide.
Who is really at risk?
The most exposed profiles
Contrary to what you might think, they are not elite athletes. Cases are concentrated among runners who stay on the course for a long time, often more than four hours, and stop at every aid station as a precaution. Smaller women are overrepresented in studies, mainly because the same volume of water represents a much larger share of their body volume.
People who sweat very salty are also more susceptible. If you regularly find white marks on your T-shirt or cap after an outing, you are losing a lot of sodium and need to take that into account.
Situations to monitor
Efforts lasting more than four hours, hot weather that encourages anxiety-driven drinking, trail races with many closely spaced aid stations, long triathlons, and more broadly any first race at a new distance where you follow a memorized hydration plan without listening to your body. Taking anti-inflammatory drugs before or during exercise also increases the risk, because these substances reduce kidney function.
Rather than following a memorized plan, it is better to know your actual fluid losses. The weigh-in test takes one session, and we explain it in our article on sweat rate, with volume guidelines in our article on how much to drink depending on your distance.
How to avoid hyponatremia
1. Drink to thirst rather than by the clock
This is the number-one recommendation in international consensus statements on the subject. Thirst remains remarkably reliable during exercise. Forcing yourself to empty a bottle every twenty minutes because the plan says so, regardless of what your body is asking for, is exactly the behavior that leads to overhydration.
2. Take in sodium, not just fluid
Beyond an hour and a half of exercise, plain water is no longer enough. Usual guidelines place the need at around 500 to 700 mg of sodium per liter consumed, increasing it if you sweat heavily or if it is hot. A properly formulated sports drink, electrolyte sticks, or a hydration drink with sodium, potassium, and magnesium do this job much better than a bottle of plain water.
3. Test your strategy in training
Race day is not the time to try a new drink or a new drinking schedule. Reproduce your plan during your long outings, in comparable temperatures, and note how you feel. Your stomach will tell you very quickly if you are drinking too much.
4. Weigh yourself before and after a long outing
This is the simplest way to understand your profile. Weigh yourself without clothes before setting out and immediately afterward, taking into account what you drank. A loss of 1 to 2% of body weight is normal and harmless. A loss of more than 3% indicates that you are not drinking enough. The same or higher weight at the finish clearly means that you are drinking too much.
5. Act quickly if in doubt
If mild signs appear, stop consuming plain water, take a concentrated source of salt such as salty broth, crackers, or a pinch of salt, slow down significantly, and let your body eliminate the excess. If confusion sets in, do not try to handle it alone: go to the medical aid station.
Our powdered electrolytes provide 252 mg of sodium per stick, precisely the amount that matters in this situation. It is compared with that of other brands on drinkscore.info.
FAQ: your questions about hyponatremia
How much water should you drink per hour of running?
Guidelines generally range from 400 to 800 ml per hour, but this range is only a starting point. Your sweat rate, the temperature, the intensity, and your body size completely change the equation. The weigh-in test remains much more reliable than a general figure applied to everyone.
How long do you have to run to be at risk?
Documented cases almost all involve efforts lasting more than four hours. Below two hours, the risk remains very low for a healthy person. So this is not something to worry about during a forty-minute outing.
Are salt tablets enough to protect you?
They help, but they are not a guarantee. Several studies show that the determining factor remains the total volume of fluid consumed. You can take salt tablets and still develop hyponatremia if you drink far too much alongside them. Sodium helps correct the problem; it does not cancel it out.
Does coconut water protect against the problem?
Coconut water is rich in potassium but relatively low in sodium, while sodium is what matters here. During a very long effort, it is therefore not enough on its own and must be supplemented with a proper sodium intake.
Can you develop hyponatremia outside of sport?
Yes, particularly during extreme heat, in older people, with certain medical treatments, or in cases of kidney and heart disease. The mechanism differs slightly, but the principle remains the same: too much water relative to the available sodium.
What to remember
Staying properly hydrated is not simply about drinking a lot. During long efforts, the right strategy comes down to three habits: listen to your thirst instead of following a rigid plan, take in sodium once the effort exceeds an hour and a half, and understand your own profile through weighing. Three simple habits are enough to reduce a rare but genuinely serious risk.
