Fluid intake during an ultramarathon running race: relationship to plasma volume and serum sodium and potassium

J Sports Med Phys Fitness. 1991 Sep;31(3):417-9.


To assess the relationship between self-selected fluid intake patterns and changes in plasma volume and serum electrolytes during prolonged exercise, five men completed ultramarathon runs ranging from 50 to 100 km. There was a significant relationship between fluid intake and plasma volume changes but no changes occurred in either serum sodium or potassium. Subjects who ingested the most fluid during the race had a modest hemodilution without any changes in serum or potassium. This response may have been influenced by the consumption of beverages containing osmotically active solutes such as sodium and glucose.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Exercise
  • Fluid Therapy*
  • Heart Rate
  • Humans
  • Male
  • Middle Aged
  • Plasma Volume*
  • Potassium / blood*
  • Running*
  • Sodium / blood*


  • Sodium
  • Potassium