Objectives: To investigate the drinking behavior of the participants in a multi-day mountain bike (MTB) cross-country competition, to monitor its effect on the body's fluid compartments and body mass, and to evaluate the prevalence of exercise-associated dysnatremia.
Design: Descriptive field study.
Setting: The Jeantex Bike Transalp Competition 2008 (8 stages; 665.40 km; 21 691 m height).
Participants: Twenty-five male, amateur MTB cyclists.
Independent Variables: Reported fluid intake during the race, air temperature.
Main Outcome Measures: Changes in body mass and body composition from pre to post race and throughout the competition week, serum sodium concentration at finish line of stages 5 and 6.
Results: Mean (±SD) hourly fluid intake during the race correlated with air temperature (r = 0.868, P < .05) and ranged between 494 ± 191 mL/h and 754 ± 254 mL/h. In absence of exercise-induced hyponatremia (EAH) cases, we report 5 and 4 cases of asymptomatic post-race hypernatremia, on days 5 and 6, respectively. When related to race time and body mass, the liquid intake during the race (in mL·kg−1·h−1) correlated with post-race serum sodium concentration (stage 5: r = −0.463, P < .05, n = 24; stage 6: r = −0.589, P < .01, n = 23); no correlation was found between the change in body mass from pre to post race and serum sodium concentration at finish line.
Conclusions: Ad libitum fluid consumption during competition was spontaneously adjusted to the unsettled weather conditions in the course of the 2008 “Bike Transalp.” The inverse linear relationship between hourly fluid intake and post-race serum sodium concentrations suggests underdrinking to be one contributing factor to the high reported incidence of hypernatremia in the absence of EAH. Experimental studies are requested to confirm this hypothesis and to further examine the pathogenesis of exercise-associated dysnatremia. In this setting, body mass monitoring was not an accurate instrument to control body fluid homeostasis.