title: Water and electrolyte shifts with partial fluid replacement during exercise.
abstract: 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.
authors.affiliation: Medical Research Council and University of Cape Town Bioenergetics of Exercise Research Unit, Sports Science Institute of South Africa, Newlands.
authors.forename: B
authors.initials: B
authors.lastname: Sanders
authors.forename: T D
authors.initials: TD
authors.lastname: Noakes
authors.forename: S C
authors.initials: SC
authors.lastname: Dennis
mesh_terms: D000328:Adult; D000465:Algorithms; D001642:Bicycling / Q000502:physiology; D004573:Electrolytes / Q000097:blood / Q000652:urine; D015444:Exercise / Q000502:physiology*; D005080:Exercise Test; D018495:Fluid Shifts / Q000502:physiology*; D005440:Fluid Therapy; D006801:Humans; D008297:Male; D010101:Oxygen Consumption / Q000502:physiology; D012964:Sodium / Q000502:physiology; D013542:Sweat / Q000737:chemistry / Q000378:metabolism; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D004573:Electrolytes; D012964:Sodium