title: Interindividual variability in sweat electrolyte concentration in marathoners.
abstract: BACKGROUND
  Sodium (Na(+)) intake during exercise aims to replace the Na(+) lost by sweat to avoid electrolyte imbalances, especially in endurance disciplines. However, Na(+) needs can be very different among individuals because of the great inter-individual variability in sweat electrolyte concentration. The aim of this investigation was to determine sweat electrolyte concentration in a large group of marathoners.
  
  
  METHODS
  A total of 157 experienced runners (141 men and 16 women) completed a marathon race (24.4 ± 3.6 °C and 27.7 ± 4.8 % of humidity). During the race, sweat samples were collected by using sweat patches placed on the runners' forearms. Sweat electrolyte concentration was measured by using photoelectric flame photometry.
  
  
  RESULTS
  As a group, sweat Na(+) concentration was 42.9 ± 18.7 mmol·L(-1) (minimal-maximal value = 7.0-95.5 mmol·L(-1)), sweat Cl(-) concentration was 32.2 ± 15.6 mmol·L(-1) (7.3-90.6 mmol·L(-1)) and sweat K(+) concentration was 6.0 ± 0.9 mmol·L(-1) (3.1-8.0 mmol·L(-1)). Women presented lower sweat Na(+) (33.9 ± 12.1 vs 44.0 ± 19.1 mmol·L(-1); P = 0.04) and sweat Cl(-) concentrations (22.9 ± 10.5 vs 33.2 ± 15.8 mmol·L(-1); P = 0.01) than men. A 20 % of individuals presented a sweat Na(+) concentration higher than 60 mmol·L(-1) while this threshold was not surpassed by any female marathoner. Sweat electrolyte concentration did not correlate to sweat rate, age, body characteristics, experience or training. Although there was a significant correlation between sweat Na(+) concentration and running pace (r = 0.18; P = 0.03), this association was weak to interpret that sweat Na(+) concentration increased with running pace.
  
  
  CONCLUSIONS
  The inter-individual variability in sweat electrolyte concentration was not explained by any individual characteristics except for individual running pace and sex. An important portion (20 %) of marathoners might need special sodium intake recommendations due to their high sweat salt losses.
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Lara
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: César
authors.initials: C
authors.lastname: Gallo-Salazar
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Puente
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: Francisco
authors.initials: F
authors.lastname: Areces
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: Juan José
authors.initials: JJ
authors.lastname: Salinero
authors.affiliation: Camilo José Cela University Exercise Physiology Laboratory, C/ Castillo de Alarcon, 49, Villafranca del Castillo, 28692 Spain.
authors.forename: Juan
authors.identifier: 0000-0002-5785-984X
authors.initials: J
authors.lastname: Del Coso
mesh_terms: D000328:Adult; D002712:Chlorides / Q000032:analysis*; D004326:Drinking; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009994:Osmolar Concentration; D010807:Physical Endurance / Q000502:physiology; D012420:Running / Q000502:physiology*; D012737:Sex Factors; D012964:Sodium / Q000032:analysis*; D013542:Sweat / Q000737:chemistry*; D013546:Sweating / Q000502:physiology; D014882:Water-Electrolyte Balance / Q000502:physiology
chemical_list: D002712:Chlorides; D012964:Sodium
keywords: Athletes; Exercise; Fluid balance; Hyponatremia; Osmolality; Sex