Objective: To investigate change in serum sodium concentration and its potential causes during a cold weather ultradistance race.
Design: Descriptive research.
Setting: 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.
Participants: Twenty athletes (11 runners, 6 cyclists, 3 skiers) volunteered for the study.
Main outcome measures: 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.
Results: 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.
Conclusions: 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.