Hyponatremia: Too much water, too little sodium

Drinking more is healthier. This is one of the most persistent health myths — and it can be dangerous in sports. Hyponatremia, a clinically low sodium level in the blood, is not a fringe phenomenon. It has been documented at endurance sports events, has led to hospitalizations, and in rare cases, has been fatal. The cause is almost always the same: too much water, too little sodium.

What is Hyponatremia?

Hyponatremia describes a blood sodium level below 135 mmol/l. The normal range is between 135 and 145 mmol/l. Sodium is the most abundant electrolyte in the extracellular space – i.e., in the blood and tissue fluid. It regulates the water balance between body compartments and is directly involved in the function of nerves and muscles.

When sodium levels drop, the body reacts by shifting water: water moves into the cells to equalize the concentration difference. That sounds harmless – but it's not, especially when it affects the brain. Brain neurons swell. In a closed skull, this has consequences.

How does hyponatremia occur during exercise?

There are two main mechanisms relevant in sports:

Dilutional Hyponatremia — the most common form in sports. The athlete drinks more water than they lose through sweat. Blood volume increases, sodium concentration decreases — not because there is too little sodium, but because it is diluted by the additional water.

Depletional Hyponatremia — rarer, but possible with extreme sweat losses without adequate sodium intake. The body loses so much sodium in sweat over hours that even normal fluid intake is not enough to maintain the level.

In practice, the dilutional form is more common — and paradoxically, it's the one caused by well-intentioned excessive drinking.

Who is at risk?

Hyponatremia doesn't just affect ultra-runners. Certain factors significantly increase the risk:

• Long endurance competitions: Marathon, triathlon, ultramarathon – the longer the exertion, the more time for over-drinking
• Slow runners: Those who are on the go for a long time drink more – and sweat less per unit of time than fast runners. The risk of a positive water balance increases.
• High salt loss in sweat: Some people, due to genetics, lose significantly more sodium per liter of sweat than others – so-called "salty sweaters"
• Combat athletes after weigh-in: Severe dehydration followed by uncontrolled water drinking is a classic scenario
• Heat and high humidity: Increased sweat rates exacerbate sodium losses

Symptoms: From mild to life-threatening

Hyponatremia develops gradually. Symptoms become more severe as sodium levels drop:

Mild Hyponatremia (130–135 mmol/l):

• Nausea, slight discomfort
• Headaches
• Mild fatigue that doesn't improve with drinking
• Bloating, heaviness in the legs

Moderate Hyponatremia (125–130 mmol/l):

• Vomiting
• Severe headaches
• Confusion, disorientation
• Muscle cramps

Severe Hyponatremia (below 125 mmol/l):

• Seizures
• Loss of consciousness
• Pulmonary edema
• Life-threatening condition – immediate emergency medical attention required

Important note: Severe hyponatremia is a medical emergency. If you experience signs of confusion, seizures, or loss of consciousness after physical exertion, immediately call the emergency services (112).

The problem with confusion

Hyponatremia is often confused with dehydration in sports — by paramedics, by caregivers, sometimes even by medical personnel without specific experience. Both can cause exhaustion, nausea, and confusion. The crucial difference: with dehydration, water helps. With hyponatremia, water makes the situation worse.

Providing an exhausted athlete who has already drunk a lot with more liters of water risks worsening their condition. Treating hyponatremia requires sodium — not more fluids.

How to avoid hyponatremia

Drink according to thirst, not according to a plan — with one important exception. Current sports science recommends that most athletes drink according to thirst rather than a fixed hydration schedule. This prevents excessive fluid intake. Exception: During very long periods of exertion or in extreme heat, where the sensation of thirst can be deceptive, a moderate plan is useful — but not one that forces over-drinking.

Replace sodium, not just water. What you lose in sweat is not just pure water. Electrolytes — primarily sodium — must be actively replaced. From 60–90 minutes of exertion, electrolytes should be included in every hydration plan.

Know your sweat rate. Knowing how much you sweat per hour allows you to adjust your fluid intake accordingly. Simple method: Weigh yourself before and after training. Approximately one liter of fluid loss corresponds to one kilogram of weight loss.

After weigh-ins in combat sports: Electrolytes first. Anyone who drinks after a weigh-in should always drink with sodium. This is not an option; it's a physiological necessity.

Urine color as a simple check. Pale yellow urine indicates good hydration. Clear, colorless urine — especially in combination with excessive drinking — can be a sign of over-hydration.

The myth "more water is always better"

This myth costs people their health — and in rare cases, their lives. It arose from a legitimate concern about dehydration, which was heavily communicated in the 1980s and 1990s. The pendulum swung too far.

The current sports science recommendation is more nuanced: Drink enough to maintain performance – but no more. And whatever is consumed must correspond to the electrolyte balance. Water is not a neutral drink if consumed in quantities that lower the sodium concentration in the blood.

Conclusion

Hyponatremia is the downside of the well-intentioned hydration myth. Too much water without electrolytes is not a sign of health consciousness – it is a physiological problem with real consequences. The solution is not to drink less, but to drink correctly: with enough sodium, adjusted to exertion, duration, and individual sweat rate.

Water is not neutral. Sodium is your friend. That is the current state of science.



Sources:
¹ Hew-Butler T et al.: Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 2015.
² Noakes TD: Waterlogged: The Serious Problem of Overhydration in Endurance Sports. Human Kinetics, 2012.
³ Rosner MH, Kirven J: Exercise-associated hyponatremia. Clinical Journal of the American Society of Nephrology, 2007.
⁴ Maughan RJ, Shirreffs SM: Dehydration and rehydration in competitive sport. Scandinavian Journal of Medicine & Science in Sports, 2010.
⁵ EU-Verordnung (EG) Nr. 1924/2006 über nährwert- und gesundheitsbezogene Angaben über Lebensmittel (HCVO).