Water and electrolyte shifts with partial fluid replacement during exercise

In this study, we examined whether athletes, who typically replace only approximate to 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 O-2 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 Na-Cl . l(-1) to replace their electrolyte losses. Both W and S conditions decreased final heart rates by approximate to 10 betas . min(-1) (P < 0.005) and reduced falls in plasma volume (PV) by approximate to 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 approximate to 4 mEq . l(-1). Differences in plasma [Na+] had little effect on the approximate to 2.4 l fluid, approximate to 120 mEq Na+ and approximate to 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 approximate to 0.1, 1.2 and 1.0 l, respectively. In the W trial, the approximate to 1.2 l fluid and approximate to 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 approximate to 50% of their fluid losses during competitive endurance exercise.