A prospective study of exercise-associated hyponatremia in two ultradistance triathletes

Objective: To study fluid and sodium balance in two ultra-distance triathle tes. Design: Prospective case study. Setting: An ultradistance triathlon (3.8 km swim, 180 km cycle, 42.2 km run ), and during overnight recovery. Ambient air temperature at 12:00 p.m. rac e day was 21 degrees C, with a relative humidity of 91%. Water temperature was 20.7 degrees C. Subjects: Two female ultradistance triathletes, ages 30 and 39 years, who w ere participating in a larger study investigating weight and electrolyte ch anges in the Ironman triathlon. Interventions: None. Main Outcome Measures: Subjects were weighed and had blood drawn for serum sodium concentration, hemoglobin, hematocrit, arginine vasopressin, and ald osterone concentration prior to and after the race, and at 8:OO a.m. the fo llowing morning. Sodium and fluid intake and urinary output were measured d uring recovery. Results: Both subjects; developed mild hyponatremia (Na 131 and 130 mmol/L) during the race, with a weight gain (0.5 and 1.5 kg). Neither subject had large sodium losses (24 mmol and 20 mmol). Fluid consumption was 733 ml/h a nd 764 ml/h. Plasma volume increased during the race (25 and 16%). Arginine vasopressin (AVP) levels were not elevated in either subject (1.2 and 1.9 pmol/L). Both subjects demonstrated a water excess during the race (1.5 and 2.5 L), and lost weight during recovery (2.0 and 4.5 LE) Conclusions: Hyponatremia resulted from fluid retention in the extracellula r space, without evidence of large sodium losses or inappropriate AVP secre tion.