Hydration guidance has moved away from encouraging maximum fluid intake. The change followed recognition that excessive drinking during prolonged exercise can produce a serious problem of its own.
Sodium concentration is tightly regulated
The concentration of sodium in blood is held within a narrow range, because it determines how water distributes between the inside and outside of cells.
When that concentration falls, water moves into cells to equalise the difference. Cells swell, and in the brain this is constrained by the skull.
The resulting symptoms include headache, nausea, confusion and, in severe cases, far more serious consequences. This is why the condition is treated as a medical emergency. Onset can be during the event or in the hours afterwards.
The kidneys have a maximum excretion rate
Excess water is normally removed by the kidneys, which can produce a substantial volume of dilute urine per hour when required. That capacity is considerable and easily sufficient at rest.
That rate has a ceiling, and drinking beyond it means water accumulates. Sustained intake above the ceiling for several hours is what produces the dilution. A single large drink rarely causes a problem on its own.
Exercise reduces the ceiling further, because hormonal responses to exertion promote water retention at precisely the time an athlete may be drinking heavily. The two effects therefore work against each other.
Sodium is also being lost in sweat
Dilution is driven by both sides of the balance. Water is being added while sodium is being lost through sweat, and the two effects compound.
An athlete losing saltier sweat while drinking plain water is exposed to both simultaneously, which is one reason sports drinks contain sodium at all. Neither effect alone would usually be sufficient.
The sodium content of typical drinks is well below sweat concentration, so they slow the dilution rather than preventing it during very long events. Drinking to a plan matters more than which drink is chosen.
Symptoms overlap with dehydration
Confusion, nausea and weakness occur in both conditions, which makes them difficult to distinguish without measurement. The instinctive response to those symptoms is to drink more.
That response is harmful if the cause is dilution rather than deficit, which is why medical teams at endurance events test blood sodium before treating. Field measurement has become standard at large events for that reason.
Who is most exposed
The pattern appears most often in slower participants in long events, who have both the time to drink large volumes and lower sweat rates than faster competitors.
Guidance has consequently shifted toward drinking according to individual losses rather than as much as possible. Any suspected case during an event requires immediate medical attention. Weight gained across an event is a warning sign rather than a neutral observation.
