Sodium replacement and plasma sodium drop during exercise in the heat when fluid intake matches fluid loss

Context: Sodium replacement during prolonged exercise in the heat may be critically important to maintaining fluid and electrolyte balance and muscle contractility. Objective: To examine the effectiveness of sodium-containing sports drinks in preventing hyponatremia and muscle cramping during prolonged exercise in the heat. Design: Randomized crossover study. Patients or Other Participants: Thirteen active men. Intervention(s): Participants completed 4 trials of an exercise protocol in the heat (306°C) consisting of 3 hours of exercise (alternating 30 minutes of walking and cycling at a heart rate of 130 and 140 beats per minute, respectively); a set of standing calf raises (8 sets of 30 repetitions); and 45 minutes of steep, brisk walking (5.5 km·h ^-1 on a 12% grade). During exercise, participants consumed fluids to match body mass loss. A different drink was consumed for each trial: carbohydrate-electrolyte drink containing 36.2 mmol/L sodium (HNa), carbohydrate-electrolyte drink containing 19.9 mmol/L sodium (LNa), mineral water (W), and colored and flavored distilled water (PL). Main Outcome Measure(s): Serum sodium, plasma osmolality, plasma volume changes, and muscle cramping frequency. Results: During both HNa and LNa trials, serum sodium remained relatively constant (serum sodium concentration at the end of the protocol was 137.3 mmol/L and 136.7 mmol/L, respectively). However, a clear decrease was observed in W (134.5 ± 0.8 mmol/L) and PL (134.4 ± 0.8 mmol/L) trials compared with HNa and LNa trials (P < .05). The same trends were observed for plasma osmolality (P < .05). Albeit not significant, plasma volume was preserved during the HNa and LNa trials, but a reduction of 2.5% was observed in the W and PL trials. None of the volunteers experienced cramping. Conclusions: The data suggest that sodium intake during prolonged exercise in the heat plays a significant role in preventing sodium losses that may lead to hyponatremia when fluid intake matches sweat losses.