The phrase sounds backwards, but there is a real physiological reason it can be true in certain situations. Plain water is essential for life, and for most day to day drinking it helps hydration, not dehydration. But hydration is not only about how much water enters the body. It is also about whether the body can keep the right fluid balance inside and outside cells, and whether sodium levels stay in a healthy range. When water intake becomes high relative to sodium intake or sodium losses, the result can be dilution of blood sodium, a condition called hyponatremia. That can make a person feel weak, nauseated, confused, crampy, and in severe cases it can become dangerous.

Sodium matters because it is one of the body’s main electrolytes. It helps regulate fluid balance, nerve signaling, and muscle function. If you drink large amounts of plain water after sweating heavily, after endurance exercise, during heat exposure, or while losing fluids through diarrhea and vomiting, you may replace water without adequately replacing the sodium and other electrolytes that were lost. In that setting, more plain water can sometimes worsen the imbalance rather than fix it. That is why sports medicine guidance warns not only about dehydration from too little fluid, but also about overdrinking and exercise associated hyponatremia.

This does not mean plain water is bad. It means context matters. A person eating normal meals and drinking according to thirst will usually get enough sodium from food, and plain water works perfectly well most of the time. The problem tends to show up when sodium has been depleted, fluid losses are high, or water intake becomes excessive. Mayo Clinic notes that drinking too much water can overwhelm the kidneys’ ability to excrete it, lowering sodium levels in the blood. Cleveland Clinic similarly describes water intoxication and hyponatremia as conditions caused by too much water relative to the body’s electrolyte balance.

This is also why oral rehydration solutions exist. Medical rehydration treatment for diarrhea is not just water. The World Health Organization recommends oral rehydration solution made with clean water, sugar, and salt, because the body absorbs and retains fluid more effectively when water is paired with the right amount of electrolytes and glucose. The CDC also notes that rehydration solutions used to prevent or treat dehydration should contain sodium, not just plain water alone in cases of significant fluid loss.

So, can water without salt actually be dehydrating? In a strict everyday sense, not usually. But in practical physiology, yes, it can contribute to a state where hydration gets worse rather than better if it further dilutes sodium after heavy sweating, endurance exercise, illness, or extreme overconsumption of fluids. The better way to say it is this: water alone does not always equal effective rehydration. Sometimes the body needs both water and sodium.

A smart takeaway is not to salt every glass of water. It is to match hydration to the situation. Normal daily life usually calls for water and regular food. Heavy sweating, prolonged exercise, heat exposure, or gastrointestinal illness may call for fluids that also replace electrolytes. And if someone develops headache, vomiting, confusion, swelling, muscle cramps, or unusual fatigue after drinking a lot of water, that can be a sign of hyponatremia and needs medical attention.