Water and electrolyte shifts with partial fluid replacement during exercise

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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.

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