Most major sports and nutrition organizations support the notion that loss of body mass through dehydration (i.e., hypohydration) impairs endurance exercise performance [1-4]. For example, in their latest stance regarding fluid intake during exercise, the American College of Sports Medicine states that “the goal of drinking during exercise is to prevent excessive dehydration (greater than two percent body weight loss from water deficit) … to avert compromised exercise performance”.
Most studies have used exercise protocols, including fixed-intensity or incremental exercise bouts to investigate the effects of dehydration on endurance performance [5,7]. Although it makes sense to use imposed-intensity exercise protocols, athletes have the freedom to choose and alter their pace at will. Conclusions regarding endurance performance are better derived therefore from studies that use time-trial type exercise protocols .
Goulet demonstrates in a recent meta-analysis  that the impact of exercise-induced dehydration on cycling performance depends on an athlete’s ability to control his or her pace. Goulet’s results show that when intensity is imposed, dehydration-induced body mass loss as low as 1.75 percent impairs performance; whereas body mass deficits of up to four percent have no bearing on cycling time-trial performance. These results agree with several field studies that have found significant relationships between body mass loss and endurance performance, such that athletes who lose the most weight during an endurance competition are often those who perform the best [8-11].
It must be clear to athletes and coaches alike that these results should not be taken as an incentive to voluntarily restrict fluid intake during exercise. However, it does signify that body mass losses due to water deficit are less meaningful to endurance performance than previously thought.
How much should an athlete drink during exercise?
In 2006, the International Marathon Medical Director Association (IMMDA) issued their own position statement on fluid intake during exercise. They recommend that athletes refer to the sensation of thirst to determine fluid intake . The foundation beneath this recommendation was that plasma hyperosmolality, as occurs with exercise-induced sweat loss , is far more detrimental to endurance performance than is body water loss.
Drinking according to thirst suffices to maintain plasma osmolality within homeostatic range. Albeit no direct scientific evidence supports the IMMDA’s claim, a recent meta-analysis has shown that gauging fluid intake with thirst optimizes endurance performance; drinking below it meaningfully decreases endurance performance . We contend that athletes should drink according to the dictates of thirst with no need for more.
Dr. Eric Goulet, PhD, has been an adjunct professor of exercise physiology and a researcher at the Research Center on Aging of the University of Sherbrooke since 2009. Eric has authored twelve peer-reviewed publications related to endurance performance and hydration and is on the editorial board of the Journal of Exercise Physiology online.
Félix-Antoine Savoie (BSc) is a graduate student at the University of Sherbrooke and currently under the supervision of Dr. Goulet.
1. Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise. 2007 Feb;39(2):377-90.
2. Casa DJ, Armstrong LE, Hillman SK, Montain SJ, Reiff RV, Rich BS, et al. National athletic trainers’ association position statement: fluid replacement for athletes. Journal of athletic training. 2000 Apr;35(2):212-24.
3. International Olympic Committee Consensus Statement on Sports Nutrition. 2010 [cited; Available from: http://www.olympic.org/Documents/Reports/EN/CONSENSUS-FINAL-v8-en.pdf
4. Rodriguez NR, Di Marco NM, Langley S. American College of Sports Medicine position stand. Nutrition and athletic performance. Medicine and science in sports and exercise. 2009 Mar;41(3):709-31.
5. Cheuvront SN, Carter R, 3rd, Sawka MN. Fluid balance and endurance exercise performance. Current sports medicine reports. 2003 Aug;2(4):202-8.
6. Mundel T. To drink or not to drink? Explaining “contradictory findings” in fluid replacement and exercise performance: evidence from a more valid model for real-life competition. British journal of sports medicine. 2011 Jan;45(1):2.
7. Goulet ED. Effect of exercise-induced dehydration on endurance performance: evaluating the impact of exercise protocols on outcomes using a meta-analytic procedure. British journal of sports medicine. 2012 Jul 4.
8. Zouhal H, Groussard C, Minter G, Vincent S, Cretual A, Gratas-Delamarche A, et al. Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners. British journal of sports medicine. 2011 Nov;45(14):1101-5.
9. Sharwood KA, Collins M, Goedecke JH, Wilson G, Noakes TD. Weight changes, medical complications, and performance during an Ironman triathlon. British journal of sports medicine. 2004 Dec;38(6):718-24.
10. Kao WF, Shyu CL, Yang XW, Hsu TF, Chen JJ, Kao WC, et al. Athletic performance and serial weight changes during 12- and 24-hour ultra-marathons. Clinical journal of sport medicine : official journal of the Canadian Academy of Sport Medicine. 2008 Mar;18(2):155-8.
11. Zouhal H, Groussard C, Vincent S, Jacob C, Abderrahman AB, Delamarche P, et al. Athletic performance and weight changes during the “Marathon of Sands” in athletes well-trained in endurance. International journal of sports medicine. 2009 Jul;30(7):516-21.
12. Hew-Butler T, Verbalis JG, Noakes TD. Updated fluid recommendation: position statement from the International Marathon Medical Directors Association (IMMDA). Clinical journal of sport medicine : official journal of the Canadian Academy of Sport Medicine. 2006 Jul;16(4):283-92.
13. Shirreffs SM, Sawka MN. Fluid and electrolyte needs for training, competition, and recovery. Journal of sports sciences. 2011;29 Suppl 1:S39-46.
14. Goulet ED. Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis. British journal of sports medicine. 2011 Nov;45(14):1149-56