Maintenance of plasma volume and serum sodium concentration despite body weight loss in ironman triathletes.
OBJECTIVE
To examine the relationship between body weight, plasma volume, and serum sodium concentration ([Na]) during prolonged endurance exercise.


METHODS
Observational field study.


METHODS
2000 South African Ironman Triathlon.


METHODS
181 male triathletes competing in an Ironman triathlon.


METHODS
Body weight, plasma volume, and serum ([Na]) change from pre- to postrace.


RESULTS
Significant body weight loss occurred (-4.9 +/- 1.7%; P < 0.0001), while both plasma volume (1.0 +/- 11.2%; P = 0.4: NS) and serum [Na] (0.6 +/- 2.4%; P < 0.001) increased from pre- to postrace. Blood volume (-0.6 +/- 6.6%) and red cell volume (-2.6 +/- 5.5%; P < 0.001) decreased in conjunction with the body weight loss. There was a strong correlation between blood and plasma volume change, both as a percentage, and absolute change in fluid volume (r = 0.9; P < 0.001). Body weight change was positively correlated with plasma volume change (r = -0.4; P < 0.001), but inversely correlated with serum [Na] change (r = -0.4; P < 0.001). Plasma volume change was not significantly correlated with serum [Na] change (r = 0.0; NS). Serum [Na] change was inversely correlated with both percentage of red cell volume change (r = -0.2; P < 0.05) and percentage body weight change (r = -0.4; P < 0.001).


CONCLUSIONS
Plasma volume and serum [Na] were maintained in male Ironman triathletes, despite significant (5%) body weight loss during the course of the race. Body weight was not an accurate "absolute" surrogate of fluid balance homeostasis during prolonged endurance exercise. Clinicians should be warned against viewing these three regulatory parameters as interchangeable during an Ironman triathlon.
Hew-Butler
Tamara
T
University of Capetown/Medical Research Council Research Unit for Exercise Science and Sports Medicine, Department of Human Biology, University of Cape Town, Cape Town, South Africa. thew@sports.uct.ac.za
Collins
Malcolm
M
Bosch
Andrew
A
Sharwood
Karen
K
Wilson
Gary
G
Armstrong
Miranda
M
Jennings
Courtney
C
Swart
Jeroen
J
Noakes
Timothy
T
D000328:Adult; D001810:Blood Volume / Q000502:physiology*; D015444:Exercise / Q000502:physiology; D006801:Humans; D008297:Male; D010807:Physical Endurance / Q000502:physiology*; D012964:Sodium / Q000097:blood*; D015431:Weight Loss / Q000502:physiology*
D012964:Sodium