Background Exercise-associated hyponatraemia (EAH) is a dilutional hyponatraemia that is caused primarily by the intake of hypotonic fluid beyond the dictates of thirst and exacerbated by the syndrome of inappropriate antidiuretic hormone secretion as well as an inability to mobilise osmotically inactive sodium stores. Runners who drink more than to their thirst do so for a reason, and understanding and curtailing this behaviour will probably decrease the incidence of this highly preventable condition.

Objective To determine the beliefs about fluid replacement held by runners and whether these beliefs are reflected in hydration behaviours.

Methods An online survey was filled out by 197 runners solicited by personal solicitation, e-mail and flyers distributed at three local races in autumn 2009.

Results Most runners (58%) drink only when thirsty. Runners drinking to a set schedule are significantly older, more experienced and faster than those drinking when thirsty. Gastrointestinal distress is the most frequently cited (71.5%) reason to avoid overhydration. Runners have a poor understanding of the physiological consequences of hydration behaviours that frequently reflect messages of advertising.

Conclusions Runners at highest risk of EAH exhibit behaviour that is shaped by their beliefs about the benefits and risks of hydration. These beliefs are frequently based on misconceptions about basic exercise physiology.