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United States Department of Agriculture

Agricultural Research Service

Research Project: Mineral and Vitamin Interventions for At-risk Populations

Location: Obesity and Metabolism Research Unit

Title: Changes in Body Mass, Hydration and Electrolytes Following a 161-km Endurance Race

Authors
item Lebus, Daniel -
item Casazza, Gretchen -
item Hoffman, Martin -
item Ganong, Alison -
item Van Loan, Marta

Submitted to: Trade Journal Publication
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2010
Publication Date: May 1, 2010
Citation: Lebus, D.K., Casazza, G.A., Hoffman, M.D., Ganong, A.C., Van Loan, M.D. 2010. Changes in Body Mass, Hydration and Electrolytes Following a 161-km Endurance Race. Clinical Journal of Sport Medicine. Vol 20:193-199.

Interpretive Summary: Exercise is an important component in weight maintenance and overall health. The Dietary Guidelines for Americans recommends a minimum of 30 minutes a day most days of the week and longer periods of exercise for weight loss and successful maintenance of weight loss. However, some individuals participate in excessive amounts of exercise that can result in ill-effects like dehydration and imbalance in electrolytes. Maintaining proper water and electrolyte balance is essential for health. We studied changes in body weight, body water and electrolytes before and after a 100 mile ultra-endurance marathon. We found that the change in body weight and electrolytes declined 2-3% from the beginning of the race to the end. However, body water, a measure of hydration status, was unchanged at the end of the race. We also found that change in body weight was not associated with change in body water or electrolytes that are important because most long endurance races monitor change in body weight as a surrogate measure for change in hydration and electrolyte status. Based on post-race electrolytes, specifically sodium, 50% of all race finishers had medically low blood levels of sodium that in the extreme can cause death. We recommend that marathon and ultra-marathon events monitor changes in body water and electrolytes during any endurance event.

Technical Abstract: Purpose: To examine electrolyte concentrations and changes in body mass and total body water (TBW) during a 161-km ultra-marathon, and relate these to finish time and incidence of hyponatremia. Methods: Subjects were recruited from the 161-km 2008 Rio Del Lago Endurance Race. Body mass, TBW, and serum electrolyte concentrations were measured prior to the race and immediately post-race. Results: Body mass, extracellular fluid, and serum sodium concentration ([Na+]) decreased 2-3% (p < 0.001) from pre- to post-race, but TBW was unchanged. A significant relationship was observed between finish time and percent change in body mass (r = 0.36; p= 0.01), and between finish time and change in TBW (r = 0.51; p = 0.0003) and change in ECF (r = 0.78, p = 0.0001). Both TBW and ECF were associated with changes in serum [Na+]. No associations were found between post-race serum [Na+] and change in body mass (r = 0.03, p = 0.83), or finish time (r = 0.03, p = 0.72). Based on post-race serum [Na+] of 134.9 mmol/L or less, ~50% of finishers were classified as having either biochemical (low blood value) or clinical (symptoms)hyponatremia. Conclusions: Post-race serum [Na+] was not a good predictor of race performance, but faster runners tended to lose more body water and body mass than slower runners. Long duration racing events use change in body mass as an indicator of changes in total body water and risk for hyponatremia, but post-race serum [Na+] was not associated with change in body mass. Because change in body mass was not a good indicator of hydration status; effort should be made to monitor changes in TBW as a predictor of serum sodium during ultra-endurance events.

Last Modified: 8/29/2014
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