Water and electrolyte shifts with partial fluid replacement during exercise

Eur J Appl Physiol Occup Physiol. 1999 Sep;80(4):318-23. doi: 10.1007/s004210050598.


In this study, we examined whether athletes, who typically replace only approximately 50% of their fluid losses during moderate-duration endurance exercise, should attempt to replace their Na+ losses to maintain extracellular fluid volume. Six male cyclists performed three 90-min rides at 65% of peak O2 uptake in a 32 degrees C environment and ingested either no fluid (NF), 1.21 of water (W), or saline (S) containing 100 mmol of NaCl x l(-1) to replace their electrolyte losses. Both W and S conditions decreased final heart rates by approximately 10 betas min(-1) (P<0.005) and reduced falls in plasma volume (PV) by approximately 4% (P<0.05). Maintenance of PV after 10 min in the W trial prevented further rises in plasma concentrations of Na+ [Na+], Cl- and protein but in the S and NF trials, plasma [Na+] continued to increase by approximately 4 mEq x l(-1). Differences in plasma [Na+] had little effect on the approximately 2.4 l fluid, approximately 120 mEq Na+ and approximately 50 mEq K+ losses in sweat and urine in the three trials. The main effects of W and S were on body fluid shifts. During the NF trial, PV and interstitial fluid (ISF) and intracellular fluid (ICF) volumes decreased by approximately 0.1, 1.2 and 1.0 l, respectively. In the W trial, the approximately 1.2 l fluid and approximately 120 mEq Na+ losses contracted the ISF volume, and in the S trial, ISF volume was maintained by the movement of water from the ICF. Since the W and S trials were equally effective in maintaining PV, Na+ ingestion may not be of much advantage to athletes who typically replace only approximately 50% of their fluid losses during competitive endurance exercise.

Publication types

  • Clinical Trial
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Algorithms
  • Bicycling / physiology
  • Electrolytes / blood
  • Electrolytes / urine
  • Exercise / physiology*
  • Exercise Test
  • Fluid Shifts / physiology*
  • Fluid Therapy*
  • Humans
  • Male
  • Oxygen Consumption / physiology
  • Sodium / physiology
  • Sweat / chemistry
  • Sweat / metabolism
  • Water-Electrolyte Balance / physiology*


  • Electrolytes
  • Sodium