Water is among the least toxic substances, yet too much too fast can kill
Rats tolerate more than 90 grams of water per kilogram of body weight, making it one of the least poisonous compounds known. Even so, a person has died after drinking six litres in three hours. Flooding the body dilutes its sodium, and brain cells swell inside a skull that cannot expand.
The mechanism is osmosis. When excess water leaves the fluid around cells too dilute compared with the fluid inside them, water moves into the cells to even things out, and they swell. In the brain that raises pressure, producing the first signs: headache, confusion, irritability, drowsiness and changes in personality or behaviour. Muscle cramps, nausea, vomiting and dulled senses can follow. If swelling continues it may choke off blood flow or press on the brainstem, leading to seizures, coma or death.
Accidental overdrinking is exceptionally rare in ordinary life. Most deaths in healthy people have come from water-drinking contests or from long exercise with heavy fluid intake. Among runners completing the 2002 Boston Marathon, a study found 13 percent crossed the line with low blood sodium, or hyponatraemia. The strongest predictor was gaining weight during the race, and sports drinks offered no protection compared with plain water. Some guidelines that encouraged aggressive fluid replacement have been blamed.
Military training is another setting. One US Army study counted 17 trainees hospitalised in a year, and another recorded three soldiers' deaths, prompting a recommended ceiling of 1 to 1.5 litres per hour of heavy sweating. MDMA raises the risk in two ways: users overheat, sweat and drink heavily, while the drug boosts antidiuretic hormone, so the kidneys hold on to water.
Infants, especially those under nine months, are vulnerable because a modest amount of water is large relative to their small bodies and sodium stores. People with psychogenic polydipsia feel compelled to drink huge volumes, and carers can mistake the resulting symptoms for psychiatric ones. Patients fed by drip or tube need their fluids and electrolytes carefully matched, and some medicines, including the anticonvulsant oxcarbazepine, can lower sodium in certain people.
Source: Water intoxication
