Excessive hydration in a heatwave: how water intoxication can become dangerous

During a heatwave, the most common advice is to drink plenty of water. Few people know, however, that excessive hydration can also be dangerous. Consuming a large quantity of water in a very short space of time can cause hyponatraemia, also known as "water intoxication" – a serious electrolyte imbalance in the body that can be life-threatening.

What is hyponatraemia

When we sweat, the body loses not only water but also minerals, particularly sodium – an essential electrolyte for regulating the body's fluid balance and maintaining normal blood pressure.

When a person drinks several litres of plain water in a short period without replacing the lost minerals, the sodium in the blood becomes excessively diluted. An osmotic effect then occurs: water leaves the bloodstream and enters the cells, causing them to swell. Brain cells are the most severely affected, as the skull cannot accommodate this expansion, resulting in dangerously high intracranial pressure.

Warning signs

The initial symptoms of hyponatraemia resemble those of heat exhaustion or heatstroke, which makes the condition difficult to recognise. Signs that may appear include:

• Severe headaches and dizziness
• Nausea, vomiting, and lethargy
• Muscle cramps and general weakness
• Confusion, disorientation, and difficulty speaking

If the person continues to drink water despite these symptoms, the pressure on the brain can lead, in severe cases, to seizures, coma, and in extreme situations, death.

How much water is too much?

The kidneys of a healthy adult can process, on average, a maximum of 0.8–1 litre of water per hour through urine. If this rate is consistently exceeded, the renal system becomes overloaded and sodium reserves are rapidly diluted.

According to international medical guidelines, the daily fluid requirement during a heatwave ranges between 2.5 and 4 litres for an adult, depending on body weight, physical activity, and how much the individual sweats. What matters most, however, is the rate of consumption, not just the total amount: 3 litres drunk throughout the day is healthy, but 2 litres consumed all at once can be dangerous.

Guidelines for proper hydration

1. Drink water steadily, in small amounts: a glass (200–250 ml) every 30–45 minutes, rather than waiting until you feel intensely thirsty and then drinking an entire bottle at once.
2. Include electrolytes in your diet: if you sweat heavily or work outdoors, plain water is not enough. Clear vegetable broths, unsweetened fruit compotes, still or sparkling natural mineral water, or isotonic sports drinks can all help.
3. Eat fresh fruit and vegetables: watermelon, cucumbers, tomatoes, and peaches provide both water and the minerals needed to maintain cellular balance.
4. Monitor the colour of your urine: a pale straw-yellow colour indicates good hydration. Dark urine signals dehydration. If it is completely clear, you are drinking too much water and should reduce your intake.