The question of whether mild dehydration (~2-3% body mass loss) really affects athletic performance is crucial. While several studies indicate that dehydration impairs various aspects of performance, the methodologies employed may introduce confounding variables (e.g. lack of blinding). In addition, research predominantly involving males raises questions around hydration recommendations for females, with evidence showing possible differences in sweating rates and sodium concentrations. This blogs outlines important considerations for measuring the effect of dehydration on performance, with consideration for possible sex differences in hydration recommendations.
Dehydration and athletic performance
Whether dehydration affects athletic performance is one of the oldest questions in sport nutrition, with studies dating back to the early 20th century, but scientific debate persists today (1,2). Much of this debate is focussed on drinking guidelines for exercise, with two broad approaches emerging. Firstly, planned, or programmed drinking, where athletes drink set amounts of fluid at set times, and secondly, drink-to-thirst or ad-libitum drinking, where athletes drink when thirsty or when they want. Perhaps more fundamental, is the question of whether dehydration, at least mild dehydration commonly experienced by athletes (~2-3% body mass) really impairs performance. Numerous studies suggest dehydration impairs various performances (aerobic, cognitive, skill, strength, power), but the methods used could possibly confound results (3). Studies have traditionally not blinded participants, meaning their perception of how dehydration could influence outcomes may confound results, particularly as athletes generally perceive dehydration to impair performance (4). This lack of blinding would be a cardinal sin in other areas of sport nutrition and likely preclude publication in reputable journals, but this is not the case for hydration research.
Challenges blinding in hydration studies
But how do you make someone drink without them knowing they are drinking?! Recently, studies have tackled this by delivery (or dummy delivery) of water into the stomach through a gastric feeding tube during exercise (5,6,7). These studies mimic the physiological and perceptual response to dehydration and euhydration and show that dehydration reduces aerobic performance. Funnell et al. (7) demonstrated that the performance decline with blinded dehydration (~11% impairment) was not different from when participants were not blinded (~10% impairment). Importantly, this suggests previous research may not be confounded by the lack of study blinding. Other studies have infused isotonic saline into the blood to blind dehydration and report dehydration does not impair aerobic performance (8,9). However, infusing isotonic saline intravenously does not supress blood plasma/serum osmolality (the main regulatory fluid balance signal), meaning dehydrated/hydrated trials both have significant elevations (internally sensed as dehydration in both conditions), possibly explaining the results
Funnell et al. (7) demonstrated that the performance decline with blinded dehydration (~11% impairment) was not different from when participants were not blinded (~10% impairment).
Whilst this emerging evidence, suggests dehydration does impair aerobic performance, there remain large gaps in our knowledge. Firstly, blinded studies have only examined endurance cycling in the heat, which appears to be more sensitive to the effects of dehydration than cycling in cooler environments (10) and compared to running (11,12). Thus, other environmental conditions, exercise modes, and performance outcomes (cognitive, skill, strength, power etc.) should be studied in a blinded manner. Furthermore, these blinded studies only include males, meaning the true effect of dehydration in females is less clear, and, interestingly, only 30% of hydration studies have included females (13). Significant dehydration is less common in females, even at an elite level (14,15), and research demonstrates lower sweating rate capacity and sweat sodium concentrations for females compared to males, possibly attributable to variations in workload/body mass (16,17). Regarding menstrual cycle effects, during the luteal phase, the osmotic threshold for thirst sensation and vasopressin release are decreased, likely due to increased circulating oestrogen (18), but cycle phase does not influence ad-libitum rehydration, overall fluid balance, or fluid retention (19). Despite these sex effects, females present similar performance responses to dehydration compared with males, with impairments to endurance, cognitive and skills performance reported, at least with unblinded dehydration. Clearly, however, more research is needed to understand female-specific effects.
Hydration strategies for male and female athletes
Hydration strategies for male and female athletes, should be personalised and designed based on the individual’s sweat losses, exercise intensity and environmental conditions to avoid significant dehydration developing (>2% body mass) and to prevent over-drinking, both of which may influence the performance or health of athletes. Exercise sweat responses (rate and electrolyte composition) are highly variable between different athletes, as well as within the same athlete for different training sessions, making one-size-fits-all approaches to hydration inappropriate. Depending on the expected sweat rate and exercise duration, this may permit a ‘drink-to-thirst’ approach or may demand a ‘planned drinking’ approach. Drinking faster than sweat rate (over-drinking) should be avoided as it increases the risk of exercise-associated hyponatremia and it should be noted that smaller athletes and/or those with slower sweat rates may be at greater risk, which might, on average, increase the risk in females (20).
Large intra- and inter-individual variability in exercise sweat responses (rate and electrolyte composition) make a one-size-fits-all approach to hydration inappropriate.
Summary
In conclusion, emerging evidence using contemporary methods to blind dehydration from athletes shows that dehydration impairs aerobic cycling performance in the heat in males, but future work is needed to corroborate (or refute) the results of unblinded studies in other settings, including an urgent need for research in females, where sex difference and menstrual cycle effects for some relevant hydration-related outcomes are observed
Live webinar recording - "What's new in hydration?" with Dr Lewis James and Dr Nidia Rodriguez-Sanchez- available on mysportscience academy...
References
Does dehydration impair exercise performance? Sawka MN, Noakes TD. Med Sci Sports Exerc. 2007 Aug;39(8):1209-17. doi: 10.1249/mss.0b013e318124a664.
Are we being drowned in hydration advice? Thirsty for more? Cotter JD, Thornton SN, Lee JK, Laursen PB. Extrem Physiol Med. 2014 Oct 29;3:18. doi: 10.1186/2046-7648-3-18.
Does Hypohydration Really Impair Endurance Performance? Methodological Considerations for Interpreting Hydration Research. James LJ, Funnell MP, James RM, Mears SA. Sports Med. 2019 Dec;49(Suppl 2):103-114. doi: 10.1007/s40279-019-01188-5.
Knowledge, attitudes, and behaviors regarding hydration and fluid replacement of collegiate athletes. Nichols PE, Jonnalagadda SS, Rosenbloom CA, Trinkaus M. Int J Sport Nutr Exerc Metab. 2005 Oct;15(5):515-27. doi: 10.1123/ijsnem.15.5.515.
Hypohydration impairs endurance performance: a blinded study. James LJ, Moss J, Henry J, Papadopoulou C, Mears SA. Physiol Rep. 2017 Jun;5(12):e13315. doi: 10.14814/phy2.13315.
Dehydration Impairs Cycling Performance, Independently of Thirst: A Blinded Study. Adams JD, Sekiguchi Y, Suh HG, Seal AD, Sprong CA, Kirkland TW, Kavouras SA. Med Sci Sports Exerc. 2018 Aug;50(8):1697-1703. doi: 10.1249/MSS.0000000000001597.
Blinded and unblinded hypohydration similarly impair cycling time trial performance in the heat in trained cyclists. Funnell MP, Mears SA, Bergin-Taylor K, James LJ. J Appl Physiol (1985). 2019 Apr 1;126(4):870-879. doi: 10.1152/japplphysiol.01026.2018.
Current hydration guidelines are erroneous: dehydration does not impair exercise performance in the heat. Wall BA, Watson G, Peiffer JJ, Abbiss CR, Siegel R, Laursen PB. Br J Sports Med. 2015 Aug;49(16):1077-83. doi: 10.1136/bjsports-2013-092417.
Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat. Cheung SS, McGarr GW, Mallette MM, Wallace PJ, Watson CL, Kim IM, Greenway MJ. Scand J Med Sci Sports. 2015 Jun;25 Suppl 1:104-11. doi: 10.1111/sms.12343.
Hypohydration impairs endurance exercise performance in temperate but not cold air. Cheuvront SN, Carter R 3rd, Castellani JW, Sawka MN. J Appl Physiol (1985). 2005 Nov;99(5):1972-6. doi: 10.1152/japplphysiol.00329.2005.
Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners. Dion T, Savoie FA, Asselin A, Gariepy C, Goulet ED. Eur J Appl Physiol. 2013 Dec;113(12):3011-20. doi: 10.1007/s00421-013-2730-8.
Programmed vs. Thirst-Driven Drinking during Prolonged Cycling in a Warm Environment. Jeker D, Claveau P, Abed MEF, Deshayes TA, Lajoie C, Gendron P, Hoffman MD, Goulet EDB. Nutrients. 2021 Dec 29;14(1):141. doi: 10.3390/nu14010141.
Fluid Balance in Team Sport Athletes and the Effect of Hypohydration on Cognitive, Technical, and Physical Performance. Nuccio RP, Barnes KA, Carter JM, Baker LB. Sports Med. 2017 Oct;47(10):1951-1982. doi: 10.1007/s40279-017-0738-7.
Sex differences in voluntary fluid intake by older adults during exercise. Baker LB, Munce TA, Kenney WL. Med Sci Sports Exerc. 2005 May;37(5):789-96. doi: 10.1249/01.mss.0000162622.78487.9c.
Hydration and cooling in elite athletes: relationship with performance, body mass loss and body temperatures during the Doha 2019 IAAF World Athletics Championships. Racinais S, Ihsan M, Taylor L, Cardinale M, Adami PE, Alonso JM, Bouscaren N, Buitrago S, Esh CJ, Gomez-Ezeiza J, Garrandes F, Havenith G, Labidi M, Lange G, Lloyd A, Moussay S, Mtibaa K, Townsend N, Wilson MG, Bermon S. Br J Sports Med. 2021 Dec;55(23):1335-1341. doi: 10.1136/bjsports-2020-103613.
Physiological responses of men and women to humid and dry heat. Shapiro Y, Pandolf KB, Avellini BA, Pimental NA, Goldman RF. J Appl Physiol Respir Environ Exerc Physiol. 1980 Jul;49(1):1-8. doi: 10.1152/jappl.1980.49.1.1.
Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond. Yanovich R, Ketko I, Charkoudian N. Physiology (Bethesda). 2020 May 1;35(3):177-184. doi: 10.1152/physiol.00035.2019.
Effects of oral contraceptives on body fluid regulation. Stachenfeld NS, Silva C, Keefe DL, Kokoszka CA, Nadel ER. J Appl Physiol (1985). 1999 Sep;87(3):1016-25. doi: 10.1152/jappl.1999.87.3.1016.
Fluid and electrolyte balance considerations for female athletes. Rodriguez-Giustiniani P, Rodriguez-Sanchez N, Galloway SDR. Eur J Sport Sci. 2022 May;22(5):697-708. doi: 10.1080/17461391.2021.1939428.
Exercise-Associated Hyponatremia: 2017 Update. Hew-Butler T, Loi V, Pani A, Rosner MH. Front Med (Lausanne). 2017 Mar 3;4:21. doi: 10.3389/fmed.2017.00021.
