title: Maintenance of plasma volume and serum sodium concentration despite body weight loss in ironman triathletes.
abstract: 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.
authors.affiliation: 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
authors.forename: Tamara
authors.initials: T
authors.lastname: Hew-Butler
authors.forename: Malcolm
authors.initials: M
authors.lastname: Collins
authors.forename: Andrew
authors.initials: A
authors.lastname: Bosch
authors.forename: Karen
authors.initials: K
authors.lastname: Sharwood
authors.forename: Gary
authors.initials: G
authors.lastname: Wilson
authors.forename: Miranda
authors.initials: M
authors.lastname: Armstrong
authors.forename: Courtney
authors.initials: C
authors.lastname: Jennings
authors.forename: Jeroen
authors.initials: J
authors.lastname: Swart
authors.forename: Timothy
authors.initials: T
authors.lastname: Noakes
mesh_terms: 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*
chemical_list: D012964:Sodium