Background There have been several recorded fatalities due to Exercise-associated hyponatraemia (EAH) attributed to excess fluid intake by marathon runners. A significant proportion of runners finishing marathons have been found to be hyponatraemic, mainly due to inappropriate fluid intake.

Objective To explore the hydration strategies of marathon runners, their sources of information and knowledge about fluid intake on marathon day and their understanding of hyponatraemia.

Design Anonymised questionnaire.

Setting The London Marathon Registration.

Participants A random sample of 217 runners registering for the marathon.

Results 93% of the runners had read or been told about drinking fluids on marathon day. The majority of competitors had a plan regarding fluid intake prior to, during and after the marathon. 83.9% said that they knew enough about what and how much to drink on marathon day. However, 34% planned to drink a volume large enough to put themselves at higher risk of EAH. Only 21% knew the volumes of water and sports drink bottles from which they planned to drink during the marathon. 20.7% were planning to take a drink from all 24 water stations. Only 25% planned to drink according to their thirst as recommended by the international EAH conference consensus statement.

68% of runners had heard of hyponatraemia or low sodium levels, but only 35% had a basic understanding of its cause and effects.

Conclusions Runners of the 2010 London Marathon lack knowledge about appropriate fluid intake on race day. A significant proportion of participants have drinking strategies that put them at risk of EAH. Runners need to be more effectively educated about safe fluid intake prior to future London Marathons.