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Background  It remains unclear if ad libitum water drinking, as a hydration strategy, prevents exercise-associated hyponatremia (EAH) during prolonged exercise. The aim of this study was to determine the incidence of EAH within the broader context of fluid regulation among soldiers performing a 40-km route-march ingesting water ad libitum.    Methods  Twenty-eight healthy male soldiers participated in this observational trial. Pre- and post-exercise body mass, blood and urine samples were collected. Blood samples were assessed for serum sodium ([Na + ]), glucose, creatinine, urea nitrogen (BUN), plasma osmolality, creatine kinase (CK), and plasma arginine vasopressin (AVP) concentrations. Plasma volume (PV) was calculated using hematocrit and hemoglobin. Urine samples were analyzed for osmolality and [Na + ]. Water intake was assessed by weighing bottles before, during and after the march. The mean relative humidity was 55.7% (21.9–94.3%) and the mean dry bulb temperature was 27.1 °C (19.5 °C - 37.0 °C) during the exercise.    Results  Twenty-five soldiers (72 ± 10 kg) (Mean ± SD) completed the march in 09:11 ± 00:43 (hr:min). Participants consumed 736 ± 259 ml/h of water and lost 2.8 ± 0.9 kg (4.0% ± 1.4%,  P  < 0.05) of body mass. Significant (pre-march vs. post-march;  P  < 0.05) decreases in serum [Na + ] (141 mmol/L vs. 136 mmol/L), plasma osmolality (303 mOsmol/kg H 2 O vs. 298 mOsmol/kg H 2 O), and serum creatinine (111 μmol/L vs. 101 μmol/L) and urine [Na + ] (168 mmol/L vs. 142 mmol/L), as well as significant increases in plasma AVP (2 pg/ml vs. 11 pg/ml), plasma CK (1423 U/L vs. 3894 U/L) and urine osmolality (1035 mOsmol/kg H 2 O vs. 1097 mOsmol/kg H 2 O) were found. The soldier (72 kg) with the lowest post-exercise sodium level completed the march in 08:38. He drank 800 ml/h, lost 2% body mass, and demonstrated (pre-post) increases in plasma osmolality (294–314 mOsmol/kg H 2 O), BUN (20–30 mg/dl), AVP (2–16 pg/ml) and PV (41%). His urine osmolality decreased from 1114 mOsmol/kg H 2 O to 1110 mOsmol/kg H 2 O. No participants finished the route-march with a serum [Na + ] indicating hypernatremia (range, 134–143 mmol/L).    Conclusions  Ad libitum drinking resulted in 4% body mass loss with a 2 mmol/L serum [Na + ] reduction in conjunction with high urine osmolality (> 1000 mOsmol/kg H 2 O) and plasma AVP. No single hydration strategy likely prevents EAH, but hypernatremia (cellular dehydration) was not seen despite > 2% body mass losses and high urine osmolality.

Ad libitum water consumption prevents exercise-associated hyponatremia and protects against dehydration in soldiers performing a 40-km route-march