Effects of Carbonated and Electrolyte-Added Water on Body Fluid Regulation and Ingestive Behavior During Ad Libitum Rehydration Following Mild Hypohydration

Purpose: Maintaining fluid homeostasis is crucial, and replenishing electrolytes—particularly sodium (Na⁺)—is critical for effective rehydration after thermal hypohydration. Despite the growing consumption of carbonated water (CW), its impact on post-hypohydration body fluid balance, either alone or combined with electrolytes, remains inadequately investigated. Therefore, this study was designed to examine the separate and interactive effects of carbonation and electrolyte provision in rehydration fluids—utilizing a 2 × 2 factorial design (carbonation × electrolytes)—on ingestive behavior and systemic fluid balance following mild thermal stress-induced hypohydration. Methods: Subjects (eight women and seven men, age range: 19–25 years) were dehydrated by performing three bouts of stepping exercise for 20 min, separated by 10 min of rest, at 25 °C. Following the dehydration protocol, subjects ingested pure water (W), CW (gas volume, 3.43 ± 0.20 (mean ± standard deviation)), electrolyte-added water (EW), or electrolyte-added CW (ECW) (gas volume, 3.04 ± 0.29 (mean ± standard deviation)) ad libitum. EW and ECW contained 19 mEq/L Na⁺. We assessed ingestive behavior and body fluid balance during the 180 min rehydration period. Results: The dehydration protocol induced hypohydration by ~10 g/kg body weight (~1% of body weight). Cumulative fluid intake was greater in the W trial than in the CW, EW, and ECW trials. Cumulative urine output was greater in the W and CW trials than in the EW trial. The fluid retention ratio was greater in the EW and ECW trials than in the CW trial. Consequently, the final fluid recovery was lower in the CW trial compared to the W, EW, and ECW trials; however, the combination of carbonation and electrolytes (ECW) did not significantly surpass the non-carbonated trials (W and EW) due to the reduced intake volume caused by carbonation. Conclusions: The data suggest that ad libitum ingestion of carbonated water is less effective than plain water for recovering from mild hypohydration (~1% body weight loss), as it reduces spontaneous fluid intake. However, electrolyte supplementation mitigates this reduced recovery by attenuating diuresis and consequently improving fluid retention ratio.