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Hyponatremia: when there's too much water

In a marathon or an ultra, drinking too much can sometimes be more dangerous than drinking too little. Let's unpack what hyponatremia is, why endurance sport is a risk zone, and how to drink wisely.

EG
Ekaterina Gromova

For years we've been taught that dehydration is the number one enemy on a course, so drink, drink, and drink some more. But endurance has a flip side that gets talked about far less often. Sometimes the body suffers not from a lack of water, but from too much of it. This condition is called hyponatremia, and in a marathon or an ultra it can turn out to be more dangerous than mild under-drinking.

What hyponatremia is

Sodium is an electrolyte that helps regulate the amount of water inside cells and around them. A healthy blood sodium level is 135 to 145 mmol/L. Hyponatremia sets in when it drops to values of < 135 mmol/L.

Here's the mechanism: one or several factors dilute the sodium in the body, the water level in the body rises, and cells begin to swell. It's precisely this swelling that causes symptoms — from mild to life-threatening. The most dangerous of all is swelling of brain tissue.

Why endurance is a risk zone

The logic is simple and treacherous. Through sweat we lose not only water but also sodium. If you pour in water beyond what's needed during a long effort, the kidneys can't keep up with removing the excess fluid — and the sodium in the blood gets diluted even further. That's exactly why the Mayo Clinic explicitly lists marathons, ultramarathons, and triathlons as risk factors: these are long, high-intensity efforts where it's easy to overdrink.

It plays out in two scenarios. With chronic hyponatremia, sodium declines gradually, over 48 hours or longer, and symptoms are usually milder. With acute hyponatremia, it drops quickly, and that threatens rapid brain swelling, coma, and death. The marathon situation is exactly the acute variant: several hours of active overdrinking in a row.

One more nuance: according to the Mayo Clinic, those at the greatest risk of brain damage are premenopausal people. This is thought to be related to the influence of sex hormones on the body's ability to balance sodium.

Warning signs on the course and the biggest myth

Symptoms of hyponatremia, per the Mayo Clinic:

  • nausea and vomiting;
  • headache;
  • confusion;
  • loss of energy, drowsiness, fatigue;
  • restlessness and irritability;
  • muscle weakness, spasms, or cramps;
  • seizures;
  • coma.

The tricky part is that many of these signs — nausea, headache, confusion — look exactly like the symptoms of dehydration. And that's where the biggest endurance myth kicks in: "I feel bad, so I must need to drink more." If the cause is overdrinking, the extra water will only make things worse.

Pay special attention to muscle cramps: they're routinely blamed on a lack of water and washed down with an even larger volume of fluid. Yet cramps are on the list of symptoms of hyponatremia itself.

How to apply this in practice

  • Drink to thirst. The Mayo Clinic advises athletes to replace roughly as much fluid as they lose through sweat. Thirst is a good guide to volume.
  • Check your urine color. If you don't feel like drinking and your urine is pale yellow, you probably have enough water.
  • On long events, not just water. In marathons and triathlons it makes sense to replace part of the water with sports drinks containing electrolytes (discuss this with your doctor).
  • Don't drink "to stock up." Excess water overloads the kidneys. What works here is moderation, not maximum.
  • Know the red flags. Nausea and vomiting, confusion, a seizure, or loss of consciousness are reasons to seek emergency help immediately.

The bottom line

  • Hyponatremia is a drop in blood sodium to < 135 mmol/L; cells swell, and the most dangerous consequence is brain swelling.
  • Sodium is lost through sweat, and overdrinking on a long course dilutes it even further — which is why in a marathon or an ultra drinking too much can be more dangerous than drinking too little.
  • The acute form develops over hours and threatens coma; an endurance race is a textbook scenario.
  • Symptoms masquerade as dehydration, so the instinct to "drink more" can do harm.
  • The benchmarks for normal are thirst and pale-yellow urine; on long events, add electrolytes.

Source: Mayo Clinic — Hyponatremia. https://www.mayoclinic.org/diseases-conditions/hyponatremia/symptoms-causes/syc-20373711

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