Although this is under the ‘Running’ snippet category it will apply to many different activities. Long distance events are typically associated with exercise-associated hyponatraemia (EAH) – so that’s marathons, ultraruns, cycling, triathlons, etc.
Hyponatraemia is when the levels of sodium drop to dangerous levels. Most typically it is caused by drinking in excess of thirst. Symptoms can range from the relatively trivial, like dizziness and nausea, up to the very serious where there can be altered mental status, seizures, coma and, ulitmately, death.
This paper is a summary of the best advice from 17 international experts who met in California and thrashed out the best evidence. They cover lots of areas but the most important prevention strategy is to tackle drinking behaviour. There has been a perception that somehow our thirst mechanisms are dangerously unreliable and we should drink aggressively to counter this. The evidence suggests that drinking palatable fluids when thirsty is the safest strategy. You are unlikely to become significantly dehydrated and this will prevent EAH.