Research Fluid balance in the Ironman triathlon Speedy DB, Noakes TD, Kimber NE, Rogers IR, Thompson JMD, Boswell DR, Ross JJ, Campbell RGD, Gallagher PG, Kuttner JA.

Clinical Journal of Sports Medicine 2001;11:44-50

Objective:

To record weight changes, fluid intake and changes in serum sodium concentration in ultradistance triathletes.

Design:

Descriptive research.

Setting:

Ironman
triathlon (3.8 km swim, 180 km cycle, 42.2 km run). Air temperature at
1200 h was 21 degrees C, (relative humidity 91%). Water temperature was

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 were also weighed at
transitions. Fluid intake during the race was estimated by athlete
recall.

Results:

Median
weight change during the race = -2.5 kg (p < 0.0006). Subjects lost
weight during recovery (median = -1.0 kg) (p < 0.03). Median hourly
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
calculated fluid losses cycling were 808 ml/h and running were 1,021
ml/h. No significant difference existed between pre-race and post-race[Na] (median 140 versus 138 mmol/L) or between post-race and recovery[Na] (median 138 versus 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-race to post-race and relative weight change
(r = -0.68, p = 0.0029). Five subjects developed hyponatremia ([Na] 128-133 mmol/L).

Conclusions:

Athletes
lose 2.5 kg of weight during an ultradistance triathlon. most likely
from sources other than fluid loss. Fluid intakes during this event are
more modest than that recommended for shorter duration exercise. Plasma
volume increases during the ultradistance triathlon. Subjects who
developed hyponatremia had evidence of fluid overload despite modest
fluid intakes.