Marathon runners can get sick from drinking too much plain water
About one in ten people competing in endurance events end up with hyponatremia, blood sodium diluted below 135 mmol/L, often after sweating hard and gulping water without electrolytes. It is the commonest electrolyte disorder, found in roughly a fifth of hospital admissions, and in severe cases it swells the brain.
Sodium is the main positive ion outside the body's cells, and it cannot slip through their membranes without dedicated channels or pumps. A feedback loop keeps its concentration within tight limits. Sensors in the hypothalamus detect rising sodium and trigger release of antidiuretic hormone, which tells the kidneys to reclaim water and also stirs thirst. When sodium starts to fall, that hormone normally switches off so the kidneys can dump excess water.
Hyponatremia appears when that loop is swamped or broken. Swamping happens with extreme water drinking, a diet of little more than beer, or too much salt-free intravenous fluid. Breakage includes syndromes where antidiuretic hormone stays switched on, plus heart, liver and kidney failure. Vomiting, diarrhoea and diuretics can cause it by draining volume, and ecstasy and many psychiatric drugs have been implicated. Some low readings are false, produced by very high blood fats or proteins that distort lab measurement.
Mild cases may cause only headaches, nausea, sluggish thinking or unsteadiness, although even levels of 131 to 135 mmol/L have been tied to more falls and poorer attention. Below about 115 mmol/L water floods brain cells and they swell. Pressure inside the skull climbs and can push brain tissue out of place, which is usually fatal without urgent treatment. Speed matters: a drop spread over days or weeks is often tolerated because neurons adapt. Long-term, low sodium has been linked to double the risk of osteoporosis.
Doctors sort cases by whether the patient is low, normal or high in fluid volume, since each points to a different cause and treatment. Volume loss is usually replaced with saline; fluid overload is managed with fluid restriction and a low-salt diet. Correcting sodium too fast carries its own dangers, so gradual correction is the norm when levels have been low for over two days. In US hospitals the condition is associated with higher death rates and costs an estimated $2.6 billion a year.
Source: Hyponatremia
