Background Exercise-Associated Hyponatremia (EAH) is a potentially fatal condition during endurance exercise (Petzold et al, 2007). Excessive fluid intake over and above normal fluid losses is the most common risk factor in the development of EAH. A “drink to thirst” fluid regime is considered best practice (Hew-Butler et al, 2015).

Objective To investigate fluid intake and changes in serum sodium concentrations ([Na]⁺) in runners during a marathon

Design Prospective observational study.

Setting 2016 London Marathon.

Participants 28 amateur athletes were recruited at the marathon registration.

Independent Variables Online and printed medical advice was available to all participants. A drink to thirst fluid regime was recommended to all athletes by the event organisers.

Main Outcome Measures Serum sodium samples were collected and body mass measured at the marathon start and finish lines. Questionnaires regarding estimated fluid intake data were collected at the finish.

Results The average volume of fluid intake during the marathon was 1.4 litres (range 0–5.5 litres). Change in serum sodium concentration was inversely correlated with change in body mass (r=−0.383, p=0.044) and with estimated fluid intake (r=−0.406, p=0.032). A significant correlation between fluid intake and weight change was not found. The average pre-race and post-race [Na]⁺ were 141.7 mmol/L and 141.5 mmol/L, respectively (p=0.8673). One runner (1/28, 3.6%) was found to have asymptomatic hyponatraemia ([Na]⁺⁺=130 mmol/L). This runner had consumed an estimated 5l of fluid over the course of the marathon. A trend in correlation was seen between fluid intake and race time (r=0.349, p=0.069).

Conclusions Runners consumed potentially dangerous volumes of fluid during a marathon despite the written guidance recommending against such practices. The consequence of this behaviour is overhydration and an increased risk of developing potentially fatal EAH. The optimal medium for runner education on safe drinking practices is yet to be determined.