Fluid balance during and after an ironman triathlon

Objective: To record weight changes, fluid intake and changes in serum sodi um concentration in ultradistance triathletes. Design: Descriptive research. Setting: Ironman triathlon (3.8 km swim, 180 km cycle, 42.2 km run). Air te mperature at 1200 h was 21 degreesC, (relative humidity 91%). Water tempera ture was 20.7 degreesC. Participants: 18 triathletes. Interventions: None. Main Outcome Measures: Subjects were weighed and had blood drawn for serum sodium concentration [Na], hemoglobin, and hematocrit, pre-race, post-race, and at 0800 h on the morning following the race ("recovery"); subjects wer e also weighed at transitions. Fluid intake during the race was estimated b y athlete recall. Results: Median weight change during the race = -2.5 kg (p < 0.0006). Subje cts lost weight during recovery (median = -1.0 kg) (p < 0.03). Median hourl y fluid intake = 716 ml/h (range 421-970). Fluid intakes were higher on the bike than on the run (median 889 versus 632 ml/h, p = 0.03). Median calcul ated fluid losses cycling were 808 ml/h and running were 1,021 ml/h. No sig nificant difference existed between pre-race and post-race [Na] (median 140 versus 138 mmol/L) or between post-race and recovery [Na] (median 138 vers us 137 mmol/L). Plasma volume increased during the race, median + 10.8% (p = 0.0005). There was an inverse relationship between change in [Na] pre-rac e to post-race and relative weight change (r = -0.68, p = 0.0029). Five sub jects developed hyponatremia([Na] 128-133 mmol/L). Conclusions: Athletes lose 2.5 kg of weight during an ultradistance triathl on, most likely from sources other than fluid loss. Fluid intakes during th is event are more modest than that recommended for shorter duration exercis e. Plasma volume increases during the ultradistance triathlon. Subjects who developed hyponatremia had evidence of fluid overload despite modest fluid intakes.