Abstract
Purpose
To examine if ad libitum drinking will adequately support hydration during exertional heat stress.
Methods
Ten endurance-trained runners ran for 2 h at 60% of maximum oxygen uptake under different conditions. Participants drank water ad libitum during separate trials at mean ambient temperatures of 22 °C, 30 °C and 35 °C. Participants also completed three trials at a mean ambient temperature of 35 °C while drinking water ad libitum in all trials, and with consumption of programmed glucose or whey protein hydrolysate solutions to maintain euhydration in two of these trials. Heart rate, oxygen uptake, rectal temperature, perceived effort, and thermal sensation were monitored, and nude body mass, hemoglobin, hematocrit, and plasma osmolality were measured before and after exercise. Water and mass balance equations were used to calculate hydration-related variables.
Results
Participants adjusted their ad libitum water intake so that the same decrease in body mass (1.1–1.2 kg) and same decrease in body water (0.8–0.9 kg) were observed across the range of ambient temperatures which yielded significant differences (p < .001) in sweat loss. Overall, water intake and total water gain replaced 57% and 66% of the water loss, respectively. The loss in body mass and body water associated with ad libitum drinking resulted in no alteration in physiological and psychophysiological variables compared with the condition when hydration was nearly fully maintained (0.3 L body water deficit) relative to pre-exercise status from programmed drinking.
Conclusions
Ad libitum drinking is an appropriate strategy for supporting hydration during running for 2 h duration under hot conditions.
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Abbreviations
- Pa :
-
Ambient water vapor pressure
- \(\dot {V}{\text{C}}{{\text{O}}_{\text{2}}}\) :
-
Carbon dioxide production
- Δbody mass:
-
Change in body mass
- Δbody water:
-
Change in body water
- gasesin—gasesout :
-
Change in mass due to gas exchange
- FODMAP:
-
Low-fermentable oligo-, di-, monosaccharide, and polyol
- Solidsout :
-
Mass of solids lost
- Solidsin :
-
Mass of solids taken in
- \(\dot {V}{{\text{O}}_{{\text{2max}}}}\) :
-
Maximum oxygen uptake
- \(\dot {V}{{\text{O}}_2}\) :
-
Oxygen uptake
- \({\dot {m}_e}\) :
-
Rate of evaporative water loss in expired air
- RPE:
-
Rating of perceived exertion
- W gain :
-
Total water gain
- W loss :
-
Total water lost
- W food :
-
Water consumed in food
- W drink :
-
Water consumed in drink
- W met :
-
Water generated from fuel oxidation
- W feces :
-
Water lost in feces
- W skin :
-
Water lost in sweat
- W urine :
-
Water lost in urine
- W resp :
-
Water lost through respiration
- WBGT:
-
Wet bulb globe temperature
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Acknowledgements
The authors would like to thank all the participants that volunteered to take part in this study, and the Monash University Sports Dietetic Research Team and collaborators who supported various aspects of data and sample collection and analysis. We also thank Dr. Eric Goulet for critical review of an early draft of our manuscript. The study was supported by Monash University, Faculty of Medicine, Nursing and Health Sciences, Faculty Strategic Grant Scheme, SGS15-0128. This material is also the result of work supported with resources and the use of facilities at the VA Northern California Health Care System. The contents reported here do not represent the views of the Department of Veterans Affairs or the United States Government.
Funding
The study was supported by Monash University, Faculty of Medicine, Nursing and Health Sciences, Faculty Strategic Grant Scheme, SGS15-0128. This material is also the result of work supported with resources and the use of facilities at the VA Northern California Health Care System.
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RJSC and RMJS conceived and designed the original studies, and conducted the experiments from which the paper evolved. MDH conceptualized the present analysis, analyzed the data for this paper, and drafted the manuscript. All authors contributed to the revision of the initial manuscript, and read and approved the final manuscript.
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All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.
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Communicated by Narihiko Kondo.
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Hoffman, M.D., Snipe, R.M.J. & Costa, R.J.S. Ad libitum drinking adequately supports hydration during 2 h of running in different ambient temperatures. Eur J Appl Physiol 118, 2687–2697 (2018). https://doi.org/10.1007/s00421-018-3996-7
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DOI: https://doi.org/10.1007/s00421-018-3996-7