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Objective: To study fluid and sodium balance in two ultradistance triathletes.

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. race 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 were participating in a larger study investigating weight and electrolyte changes 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 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.

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 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).

Conclusions: Hyponatremia resulted from fluid retention in the extracellular space, without evidence of large sodium losses or inappropriate AVP secretion.

(C) 2000 Lippincott Williams & Wilkins, Inc.