VIEW: Is Drinking to Thirst Adequate to Appropriately Maintain Hydration Status During Prolonged Endurance Exercise? Yes

The importance of adequate fluid intake during exercise has been stressed for sport and cardiovascular performance, for management of thermal stress,1Sawka M.N. Burke L.M. Eichner E.R. Maughan R.J. Montain S.J. Stachenfeld N.S. American College of Sports Medicine. American College of Sports Medicine position stand. Exercise and fluid replacement.Med Sci Sports Exerc. 2007; 39: 377-390Crossref PubMed Scopus (1635) Google Scholar, 2Casa D.J. Armstrong L.E. Hillman S.K. et al.National Athletic Trainers' Association position statement: Fluid replacement for athletes.J Athl Train. 2000; 35: 212-224PubMed Google Scholar and for prevention of exercise-induced muscle cramping1Sawka M.N. Burke L.M. Eichner E.R. Maughan R.J. Montain S.J. Stachenfeld N.S. American College of Sports Medicine. American College of Sports Medicine position stand. Exercise and fluid replacement.Med Sci Sports Exerc. 2007; 39: 377-390Crossref PubMed Scopus (1635) Google Scholar, 2Casa D.J. Armstrong L.E. Hillman S.K. et al.National Athletic Trainers' Association position statement: Fluid replacement for athletes.J Athl Train. 2000; 35: 212-224PubMed Google Scholar and acute kidney injury from rhabdomyolysis.1Sawka M.N. Burke L.M. Eichner E.R. Maughan R.J. Montain S.J. Stachenfeld N.S. American College of Sports Medicine. American College of Sports Medicine position stand. Exercise and fluid replacement.Med Sci Sports Exerc. 2007; 39: 377-390Crossref PubMed Scopus (1635) Google Scholar For these reasons, several recent guidelines recommend that mass loss during exercise should not exceed 2% of body mass.1Sawka M.N. Burke L.M. Eichner E.R. Maughan R.J. Montain S.J. Stachenfeld N.S. American College of Sports Medicine. American College of Sports Medicine position stand. Exercise and fluid replacement.Med Sci Sports Exerc. 2007; 39: 377-390Crossref PubMed Scopus (1635) Google Scholar, 2Casa D.J. Armstrong L.E. Hillman S.K. et al.National Athletic Trainers' Association position statement: Fluid replacement for athletes.J Athl Train. 2000; 35: 212-224PubMed Google Scholar We will show why we believe that such recommendations are not well supported by ecologically valid science, and may even induce harm. Exercise-associated hyponatremia (EAH) is a primary concern for fluid intake beyond that which is physiologically appropriate before, during, and after exercise.3Hew-Butler T. Rosner M.H. Fowkes-Godek S. et al.Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015.Clin J Sport Med. 2015; 25: 303-320Crossref PubMed Scopus (142) Google Scholar The behavior of overhydration typically occurs from misunderstandings about fluid needs during exercise, coupled with excessive concerns about dehydration and the need for adequate hydration to prevent decline in performance, heat illness, and muscle cramping, likely fueled by various organizational hydration guidelines. Because there have been several deaths, as well as other morbidity, from EAH,3Hew-Butler T. Rosner M.H. Fowkes-Godek S. et al.Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015.Clin J Sport Med. 2015; 25: 303-320Crossref PubMed Scopus (142) Google Scholar a discussion to clarify current knowledge about proper hydration during exercise extends beyond merely being of academic interest. Glycogen oxidation during exercise results in mass loss, the extent of which depends on the intensity and duration of the exercise. Associated with the oxidation of glycogen is release of water, as 1 to 3 g of water are stored with every gram of glycogen.4Olsson K.E. Saltin B. Variation in total body water with muscle glycogen changes in man.Acta Physiol Scand. 1970; 80: 11-18Crossref PubMed Scopus (160) Google Scholar Thus, it is important to recognize that a loss of body mass during moderate-to-high-intensity exercise does not necessarily imply dehydration or a reduction in the cardiovascular fluid compartment, as is commonly inferred. In fact, total body water has been found to be maintained despite a loss in body mass of approximately 3.5% among runners participating in a 56-km ultramarathon5Tam N. Nolte H.W. Noakes T.D. Changes in total body water content during running races of 21.1 km and 56 km in athletes drinking ad libitum.Clin J Sport Med. 2011; 21: 218-225Crossref PubMed Scopus (46) Google Scholar and a loss in body mass of approximately 2% among soldiers during a 14.6-km march.6Nolte H.W. Noakes T.D. van Vuuren B. Protection of total body water content and absence of hyperthermia despite 2% body mass loss ('voluntary dehydration') in soldiers drinking ad libitum during prolonged exercise in cool environmental conditions.Br J Sports Med. 2011; 45: 1106-1112Crossref PubMed Scopus (38) Google Scholar Furthermore, when well-trained cyclists lost 3% of body mass by performing 2 hours of submaximal exercise (walking and cycling) in the heat, and were then reinfused with saline in a blinded fashion so that they were 0%, 2%, or 3% below their initial body mass before a 25-km bike ergometer time trial in the heat, blood volume returned to baseline during the time trial irrespective of the extent of fluid restoration.7Wall B.A. Watson G. Peiffer J.J. Abbiss C.R. Siegel R. Laursen P.B. Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat.Br J Sports Med. 2015; 49: 1077-1083Crossref PubMed Scopus (64) Google Scholar Thus, consistent with conclusions made several decades ago,8Saltin B. Circulatory response to submaximal and maximal exercise after thermal dehydration.J Appl Physiol. 1964; 19: 1125-1132PubMed Google Scholar humans are well designed to resist volume depletion of the vascular compartment through renal and hormonal adjustments in sodium and osmolality, and body mass losses of at least 2% to 3% during prolonged exercise may be necessary to maintain euhydration. In our examination of the literature, we find no controlled, laboratory-based studies using exercise protocols mirroring real-world exercise conditions (ie, those in which athletes are free to alter exercise intensity according to perceived exertion and the knowledge of completed and uncompleted distance and time) showing that drinking-to-thirst-associated hypohydration of at least 2% of body mass impedes running9Dion T. Savoie F.A. Asselin A. Gariepy C. Goulet E.D. Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners.Eur J Appl Physiol. 2013; 113: 3011-3020Crossref PubMed Scopus (62) Google Scholar, 10Lee M.J. Hammond K.M. Vasdev A. et al.Self-selecting fluid intake while maintaining high carbohydrate availability does not impair half-marathon performance.Int J Sports Med. 2014; 35: 1216-1222Crossref PubMed Scopus (28) Google Scholar or cycling11Dugas J.P. Oosthuizen U. Tucker R. Noakes T.D. Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling.Eur J Appl Physiol. 2009; 105: 69-80Crossref PubMed Scopus (76) Google Scholar performance, compared with a scheduled rate of fluid intake aimed at preventing loss in body mass. What these studies collectively show is that thirst-driven fluid intake induces no performance disadvantage compared with programmed fluid intake, although it is evident that drinking less than what is dictated by thirst impairs endurance performance.11Dugas J.P. Oosthuizen U. Tucker R. Noakes T.D. Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling.Eur J Appl Physiol. 2009; 105: 69-80Crossref PubMed Scopus (76) Google Scholar, 12Goulet E.D. Effect of exercise-induced dehydration on time-trial exercise performance: a meta-analysis.Br J Sports Med. 2011; 45: 1149-1156Crossref PubMed Scopus (110) Google Scholar Furthermore, a meta-analysis found that hypohydration up to 4% of body mass did not impair endurance performance during real-world exercise conditions.13Goulet E.D. Effect of exercise-induced dehydration on endurance performance: evaluating the impact of exercise protocols on outcomes using a meta-analytic procedure.Br J Sports Med. 2013; 47: 679-686Crossref PubMed Scopus (85) Google Scholar So, why is it that there is a widespread belief that hypohydration of at least 2% of body mass impairs performance? We believe it is because those controlled-laboratory studies arriving at such conclusions used methodologies that render them invalid in determining the real impact of hypohydration during outdoors conditions.14Cotter J.D. Thornton S.N. Lee J.K. Laursen P.B. Are we being drowned in hydration advice? Thirsty for more?.Extrem Physiol Med. 2014 Oct 29; 3: 18https://doi.org/10.1186/2046-7648-3-18Crossref PubMed Scopus (52) Google Scholar In fact, in addition to preventing athletes from drinking before or during exercise, most studies have failed to provide appropriate convective cooling or blinded subjects from the hydration treatment received. When studies are designed so that subjects are allowed to quench their thirst during exercise, receive adequate ventilation, or are blinded to the experimental conditions, hypohydration greater than 2% has systematically been demonstrated not to impair performance or result in serious elevations in core temperature.7Wall B.A. Watson G. Peiffer J.J. Abbiss C.R. Siegel R. Laursen P.B. Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat.Br J Sports Med. 2015; 49: 1077-1083Crossref PubMed Scopus (64) Google Scholar, 9Dion T. Savoie F.A. Asselin A. Gariepy C. Goulet E.D. Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners.Eur J Appl Physiol. 2013; 113: 3011-3020Crossref PubMed Scopus (62) Google Scholar, 11Dugas J.P. Oosthuizen U. Tucker R. Noakes T.D. Rates of fluid ingestion alter pacing but not thermoregulatory responses during prolonged exercise in hot and humid conditions with appropriate convective cooling.Eur J Appl Physiol. 2009; 105: 69-80Crossref PubMed Scopus (76) Google Scholar It is also noteworthy that it is quite common for observational field studies to report significant negative correlations between body mass loss and performance time. In other words, those with the greatest loss in body mass tend to be the fastest. The fastest long-distance triathletes,15Sharwood K.A. Collins M. Goedecke J.H. Wilson G. Noakes T.D. Weight changes, medical complications, and performance during an Ironman triathlon.Br J Sports Med. 2004; 38: 718-724Crossref PubMed Scopus (139) Google Scholar marathoners,16Zouhal H. Groussard C. Minter G. et al.Inverse relationship between percentage body weight change and finishing time in 643 forty-two-kilometre marathon runners.Br J Sports Med. 2011; 45: 1101-1105Crossref PubMed Scopus (80) Google Scholar and ultramarathoners17Hoffman M.D. Hew-Butler T. Stuempfle K.J. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners.Med Sci Sports Exerc. 2013; 45: 784-791Crossref PubMed Scopus (96) Google Scholar often complete races with body mass losses of 4% to 9%. Thus, mass losses in excess of 2% to 3% during prolonged exercise are well tolerated by elite athletes, but optimal hydration levels remain unknown. With regard to exercise-associated muscle cramping, growing evidence from experimental18Miller K.C. Mack G.W. Knight K.L. et al.Three percent hypohydration does not affect threshold frequency of electrically induced cramps.Med Sci Sports Exerc. 2010; 42: 2056-2063Crossref PubMed Scopus (31) Google Scholar and cohort19Schwellnus M.P. Allie S. Derman W. Collins M. Increased running speed and pre-race muscle damage as risk factors for exercise-associated muscle cramps in a 56 km ultra-marathon: a prospective cohort study.Br J Sports Med. 2011; 45: 1132-1136Crossref PubMed Scopus (31) Google Scholar studies indicates that cramping results from neurologic changes rather than uncompensated water and sodium losses incurred during exercise. A lack of difference between those with and without cramping in postrace body mass change, plasma sodium concentration, and sodium intake provides further evidence that exercise-associated muscle cramping is not related to fluid and sodium imbalances.20Hoffman M.D. Stuempfle K.J. Muscle cramping during a 161-km ultramarathon: comparison of characteristics of those with and without cramping.Sports Med Open. 2015; 1: 8Google Scholar The sensation of thirst in animals is a behavioral urge, driven largely by physiologic mediators that are activated when total body water content is low and antidiuresis is maximal. The 2 main sensors that detect physiologically relevant decreases in body water are the osmoreceptors located within the hypothalamus and baroreceptors located within the cardiac atria. Small (1%–2%) elevations in the effective osmotic pressure of plasma can stimulate central osmoreceptors and trigger thirst.21McKinley M.J. Johnson A.K. The physiological regulation of thirst and fluid intake.News Physiol Sci. 2004; 19: 1-6Crossref PubMed Scopus (279) Google Scholar Thirst generally occurs when plasma osmolality exceeds 288 mOsmol/kg H2O,22Verbalis J.G. Disorders of body water homeostasis.Best Pract Res Clin Endocrinol Metab. 2003; 17: 471-503Abstract Full Text Full Text PDF PubMed Scopus (328) Google Scholar which is still within the normal physiological range of 275 to 295 mOsmol/kg H2O, although wide individual variation exists with regard to osmotic stimulation thresholds for both thirst and arginine vasopressin release.23Robertson G.L. Abnormalities of thirst regulation.Kidney Int. 1984; 25: 460-469Abstract Full Text PDF PubMed Scopus (135) Google Scholar Peripheral baroreceptors stimulate thirst when circulating plasma volume decreases by more than 10% to 15%.22Verbalis J.G. Disorders of body water homeostasis.Best Pract Res Clin Endocrinol Metab. 2003; 17: 471-503Abstract Full Text Full Text PDF PubMed Scopus (328) Google Scholar Thus, thirst is stimulated to bring water into the body to either dilute rising blood solute concentrations or restore plasma volume, or both. Subjects hypohydrated by performing light exercise have demonstrated that 65% of the variance in fluid intake was accounted for by changes in plasma osmolality or volume.24Engell D.B. Maller O. Sawka M.N. Francesconi R.N. Drolet L. Young A.J. Thirst and fluid intake following graded hypohydration levels in humans.Physiol Behav. 1987; 40: 229-236Crossref PubMed Scopus (139) Google Scholar Thus, factors other than thirst likely contribute to fluid intake, including oropharyngeal factors, mouth state, stomach fullness, beverage temperature, competing homeostatic mechanisms (ie, hunger, blood glucose), psychological factors, and situational factors including ambient temperature, exercise context, and duration. Older humans are also thought to have a lower thirst sensitivity to hypertonicity compared with younger individuals.25Phillips P.A. Bretherton M. Johnston C.I. Gray L. Reduced osmotic thirst in healthy elderly men.Am J Physiol. 1991; 261: R166-R171PubMed Google Scholar Nevertheless, laboratory trials have demonstrated that drinking according to thirst is adequate to avoid body mass losses greater than 2% during low to moderate intensity exercise in the heat, as well as attenuate thermal and circulatory strain.26Armstrong L.E. Maresh C.M. Gabaree C.V. et al.Thermal and circulatory responses during exercise: effects of hypohydration, dehydration, and water intake.J Appl Physiol. 1997; 82: 2028-2035PubMed Google Scholar Field studies have also shown that drinking to thirst, even during prolonged exercise up to 30 hours under hot ambient conditions, will allow maintenance of what we consider to be proper hydration when considering mass loss from stored fuel.5Tam N. Nolte H.W. Noakes T.D. Changes in total body water content during running races of 21.1 km and 56 km in athletes drinking ad libitum.Clin J Sport Med. 2011; 21: 218-225Crossref PubMed Scopus (46) Google Scholar, 6Nolte H.W. Noakes T.D. van Vuuren B. Protection of total body water content and absence of hyperthermia despite 2% body mass loss ('voluntary dehydration') in soldiers drinking ad libitum during prolonged exercise in cool environmental conditions.Br J Sports Med. 2011; 45: 1106-1112Crossref PubMed Scopus (38) Google Scholar, 27Hoffman M.D. Stuempfle K.J. Is sodium supplementation necessary to avoid dehydration during prolonged exercise in the heat?.J Strength Cond Res. 2016; 30: 615-620Crossref PubMed Scopus (26) Google Scholar Even when athletes begin aerobic exercise hypohydrated, the drive to drink during exercise is substantially magnified to optimally regulate plasma volume and osmolality28Maresh C.M. Gabaree-Boulant C.L. Armstrong L.E. et al.Effect of hydration status on thirst, drinking, and related hormonal responses during low-intensity exercise in the heat.J Appl Physiol. 2004; 97: 39-44Crossref PubMed Scopus (82) Google Scholar and endurance performance.29Berkulo M.A. Bol S. Levels K. Lamberts R.P. Daanen H.A. Noakes T.D. Ad-libitum drinking and performance during a 40-km cycling time trial in the heat.Eur J Sport Sci. 2016; 15: 213-220Crossref Scopus (22) Google Scholar In contrast to drinking according to thirst, forced fluid replacement during exercise has been shown to enhance gastrointestinal distress.9Dion T. Savoie F.A. Asselin A. Gariepy C. Goulet E.D. Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners.Eur J Appl Physiol. 2013; 113: 3011-3020Crossref PubMed Scopus (62) Google Scholar, 30Daries H.N. Noakes T.D. Dennis S.C. Effect of fluid intake volume on 2-h running performances in a 25 degrees C environment.Med Sci Sports Exerc. 2000; 32: 1783-1789Crossref PubMed Scopus (70) Google Scholar If severe enough, gastrointestinal-related problems may impede endurance performance.30Daries H.N. Noakes T.D. Dennis S.C. Effect of fluid intake volume on 2-h running performances in a 25 degrees C environment.Med Sci Sports Exerc. 2000; 32: 1783-1789Crossref PubMed Scopus (70) Google Scholar Various alternative methods for maintaining proper hydration during exercise have been suggested, but each may be impractical, unnecessary, invalid, or potentially dangerous depending on the circumstances. Perhaps the most commonly recommended method is based on a calculated sweat rate from body mass change during a known duration of exercise. This method is subject to the myriad of personal, exercise-related, clothing, and environmental factors that dynamically affect sweating. Small errors in estimating sweat rate could precipitate major imbalances in fluid homeostasis during prolonged exercise. For example, an overestimate of only 100 mL/h during a 161-km ultramarathon could result in a fluid overload of 3 L when urine production is suppressed by nonosmotic arginine vasopressin secretion, a volume that would be adequate to induce EAH. Plasma osmolality is the criterion (nonbehavioral) index of hydration status. It is physiologically regulated and functionally important and has a high sensitivity for functional hypohydration. But it is usually impractical to monitor and is one-dimensional. Urine-based measures, such as color, osmolality, specific gravity, production rate, and micturition frequency, are convenient but also reflect fluid and nonfluid-regulatory influences on renal function and are thus subject to problems of sensitivity and specificity (eg, confounding of urine color by vitamin supplementation or rhabdomyolysis, urine volume reduction that could be present with EAH despite overhydration). As such, thirst appears to be the most viable method for proper hydration given the disparate circumstances under which humans perform. In the preceding paragraphs, we have offered scientific support for our premise that 1) some mass loss is appropriate during exercise, 2) exercise performance is not necessarily impaired nor is the risk of heat illness necessarily increased with modest mass loss, 3) drinking to thirst will generally be adequate to maintain hydration levels within a few percent of body mass loss, and 4) methods of maintaining hydration besides drinking to thirst are fraught with problems. The very few studies directly comparing prescribed vs self-determined drinking support our case. Nevertheless, even without considering the scientific evidence related to this essential question, one would have to believe it remarkable that the human species would exist if thirst were inadequate to guide drinking during exercise.

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