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To investigate change in serum sodium concentration and its potential causes during a cold weather ultradistance race.Descriptive research.A 100-mile (161-km) race over a snow-packed course in the Alaskan wilderness. Athletes competed in 1 of 3 divisions: foot, bike, or ski.Twenty athletes (11 runners, 6 cyclists, 3 skiers) volunteered for the study.None.Subjects were weighed and had blood drawn for hematocrit, serum sodium, serum aldosterone, and plasma arginine vasopressin concentrations pre- and postrace. Fluid and sodium intake were determined by race dietary analysis.Serum sodium concentration decreased significantly prerace (140.8 ± 1.2 mmol/L) to postrace (138.4 ± 2.2 mmol/L), although no athletes were classified as hyponatremic. Mean weight loss was significant during the race (−1.2 kg), although 1 athlete maintained his weight, and 3 athletes gained small amounts of weight (0.2 kg, 0.2 kg, and 0.5 kg, respectively). Hematocrit decreased significantly prerace (42.2 ± 3.5) to postrace (40.3 ± 4.1). Plasma arginine vasopressin and serum aldosterone increased significantly during the race (2.6 ± 0.7 to 6.0 ± 4.6 pg/mL and 5.1 ± 2.6 to 40.8 ± 25.1 ng/dL, respectively). Fluid consumption was 300 ± 100 mL/h, and sodium intake was 310 ± 187 mg/h.Decreased serum sodium concentration after a cold weather ultradistance race was due to fluid overload caused by excessive fluid consumption. Current recommendations that ultradistance athletes consume 500 to 1000 mL/h may be too high for athletes competing in the extreme cold.