Mitchell, N., Winger, J., Dugas, J., Luke, A., & Dugas, L. (2012). Predetermined drinking plans impact blood sodium concentration and body weight during a half marathon. Presentation 2286 at the 59th Annual Meeting of the American College of Sports Medicine, San Francisco, California; May 29-June 2, 2012.

This study investigated the relationships between predetermined drinking plans, body mass changes, and changes in blood sodium levels during a half-marathon race. Runners (N = 67) completed both pre- and post-race body mass and blood sample measurements. Blood samples were used to determine serum sodium concentrations.

No runners finished the race hyponatremic. The majority of runners (52%) indicated that they planned to drink according to a predetermined schedule. Ss (37%) planned to drink only when thirsty, while 11% of Ss planned to drink as much as possible. The only-when thirsty group was significantly younger, predominantly female (58%), and had the least running experience. Runners in the three groups differed in their serum sodium changes during the race (only-when-thirsty; 2.7±2.7 mmol/L, planned-schedule; 1.4±2.0 mmol/L, as-much-as-possible; 0.8±4.5 mmol/L) but did not exhibit statistical difference in their race times. However the running time for the only-when-thirsty group represented almost 11 minutes faster than the combined mean race-time of the other two groups. Alternatively runners losing >2% of starting body mass on average ran 10 minutes faster compared to runners with a more modest body mass loss (<2%) and experienced greater differences in their serum sodium concentrations.

Implication. A pre-race hydration plan has a measurable effect on post-race parameters of fluid and electrolyte balance. Drinking to thirst or losing >2% of starting body mass may protect runners from exercise-associated hyponatremia through favorable serum sodium changes and may have favorable outcomes on running performance. These findings bring into question current drinking recommendations and advice given to recreational runners.

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