Water and electrolyte shifts with partial fluid replacement during exercise

Abstract

In this study, we examined whether athletes, who typically replace only ≈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°C environment and ingested either no fluid (NF), 1.2 l of water (W), or saline (S) containing 100 mmol of NaCl · l⁻¹ to replace their electrolyte losses. Both W and S conditions decreased final heart rates by ≈10 betas · min⁻¹ (P < 0.005) and reduced falls in plasma volume (PV) by ≈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 ≈4 mEq · l⁻¹. Differences in plasma [Na⁺] had little effect on the ≈2.4 l fluid, ≈120 mEq Na⁺ and ≈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 ≈0.1, 1.2 and 1.0 l, respectively. In the W trial, the ≈1.2 l fluid and ≈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 ≈50% of their fluid losses during competitive endurance exercise.