ORIGINAL ARTICLE

Exercise associated hyponatraemia: quantitative analysis to understand the aetiology

S J Montain¹, S N Cheuvront¹, M N Sawka¹

¹ US Army Research Institute of Environmental Medicine, Natick, MA, USA

Correspondence to:
Correspondence to:
Dr Montain
US Army Research Institute of Environmental Medicine, Building 42, Kansas St, Natick, MA 01760-5007, USA; scott.montain{at}us.army.mil

Background: The development of symptomatic hyponatraemia consequent on participation in marathon and ultraendurance races has led to questions about its aetiology and prevention.

Objectives: To evaluate: (a) the assertion that sweat sodium losses cannot contribute to the development of hyponatraemia during endurance exercise; (b) the adequacy of fluid replacement recommendations issued by the International Marathon Medical Directors Association (IMMDA) for races of 42 km or longer; (c) the effectiveness of commercial sports drinks, compared with water, for attenuating plasma sodium reductions.

Methods: A mathematical model was used to predict the effects of different drinking behaviours on hydration status and plasma sodium concentration when body mass, body composition, running speed, weather conditions, and sweat sodium concentration were systematically varied.

Results: Fluid intake at rates that exceed sweating rate is predicted to be the primary cause of hyponatraemia. However, the model predicts that runners secreting relatively salty sweat can finish ultraendurance exercise both dehydrated and hyponatraemic. Electrolyte-containing beverages are predicted to delay the development of hyponatraemia. The predictions suggest that the IMMDA fluid intake recommendations adequately sustain hydration over the 42 km distance if qualifiers—for example, running pace, body size—are followed.

Conclusions: Actions to prevent hyponatraemia should focus on minimising overdrinking relative to sweating rate and attenuating salt depletion in those who excrete salty sweat. This simulation demonstrates the complexity of defining fluid and electrolyte consumption rates during athletic competition.

Keywords: water intoxication; fluid balance; dehydration; overhydration; prolonged exercise

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Commentary 1

T Noakes²

² University of Capetown, Capetown, South Africa; tdnoakes{at}sports.uct.ac.za

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Commentary 2

L B Weschler³

³ 161 Richdale Road, Colts Neck, NJ 07722, USA; weschler{at}optonline.net

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Commentary 3

R Murray⁴

⁴ Gatorade Sports Science Institute, Chicago, IL, USA; bob_murray{at}quakeroats.com

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Commentary 4

L E Armstrong⁵

⁵ Human Performance Laboratory, University of Connecticut, Storrs, CT 06269-1110, USA

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• Baker, L. B., Stofan, J. R., Hamilton, A. A., Horswill, C. A. (2009). Comparison of regional patch collection vs. whole body washdown for measuring sweat sodium and potassium loss during exercise. J. Appl. Physiol. 107: 887-895 [Abstract] [Full Text]  
• Gonzalez, R. R., Cheuvront, S. N., Montain, S. J., Goodman, D. A., Blanchard, L. A., Berglund, L. G., Sawka, M. N. (2009). Expanded prediction equations of human sweat loss and water needs. J. Appl. Physiol. 107: 379-388 [Abstract] [Full Text]  
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Osmotically inactive sodium may also play a quantitative role in exercise-associated hyponatraemia

Louise B Weschler, et al.

BJSM Online, 1 Mar 2006 [Full text]