A study of serum sodium level among Hong Kong runners.

Objective: Hyponatremia and the associated life-threatening complications have emerged as an important issue among marathon runners. This study was conducted to estimate the serum sodium level among local marathon runners and to identify the associated risk factors of dysnatremia.

Design: Prospective observational cohort study.

Setting: Hong Kong Marathon 2008.

Participants: Subjects were approached at their convenience to participate in the study. Only full-marathon runners were recruited. They were to have had an unremarkable medical and drug history.

Interventions: Demographic data, training information, previous marathon experience, anticipated drinking strategy, details of fluid consumption throughout the race, weight change, finishing time, and physical complaint.

Main Outcome Measures: Post-race serum sodium level.

Results: Of the 6488 entries to the race, 370 runners (5.7%) were recruited. Among them, 272 (73.5%) completed the race and attended for blood sampling and data collection. One runner (0.4%) had hyponatremia (133 mmol/L) and 35 runners (12.9%) had hypernatremia (>145 mmol/L), whereas 236 runners (86.7%) had normal serum sodium (135-145 mmol/L) after the race. No symptomatic dysnatremia was found. A mean weight reduction of 0.70 kg was found after the race. An average of 1.9 L of fluid was consumed during the race and 2.5 L if the fluid consumed immediately before and after the race was also included. Hypernatremia was seen in runners who were better trained before the race, those who performed better, and those who drank less water after the race.

Conclusions: This is the largest prospective observational cohort study of dysnatremia conducted on athletes completing a standard marathon in Asia. No case of symptomatic dysnatremia was found.