ObjectiveTo study fluid and sodium balance in two ultradistance triathletes.DesignProspective case study.SettingAn 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. race day was 21°C, with a relative humidity of 91%. Water temperature was 20.7°C.SubjectsTwo female ultradistance triathletes, ages 30 and 39 years, who were participating in a larger study investigating weight and electrolyte changes in the Ironman triathlon.InterventionsNone.Main Outcome MeasuresSubjects were weighed and had blood drawn for serum sodium concentration, hemoglobin, hematocrit, arginine vasopressin, and aldosterone concentration prior to and after the race, and at 8:00 a.m. the following morning. Sodium and fluid intake and urinary output were measured during recovery.ResultsBoth 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 and 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 kg).ConclusionsHyponatremia resulted from fluid retention in the extracellular space, without evidence of large sodium losses or inappropriate AVP secretion.