Effect of glycerol-induced hyperhydration on thermoregulatory and cardiovascular functions and endurance performance during prolonged cycling in a 25 °C environment Eric D.B. Goulet, Robert A. Robergs, Susan Labrecque, Donald Royer, and Isabelle J. Dionne Abstract: We compared the effect of glycerol-induced hyperhydration (GIH) to that of water-induced hyperhydration (WIH) on cardiovascular and thermoregulatory functions and endurance performance (EP) during prolonged cycling in a temperate climate in subjects consuming fluid during exercise. At weekly intervals, 6 trained male subjects ingested, in a randomized, double-blind, counterbalanced fashion, either a glycerol (1.2 g glycerol/kg bodyweight (BW) with 26 mL/kg BW of water – aspartame-flavored fluid) or placebo solution (water – aspartame-flavored fluid only) over a 2 h period. Subjects then performed 2 h of cycling at 66% of the maximal oxygen consumption (VO 2 max ) and 25 °C while drinking 500 mL/h of sports drink, which was followed by a step-incremented cycling test to exhaustion. Levels of hyperhydration did not differ significantly between treatments before exercise. During exercise, GIH significantly re- duced urine production by 246 mL. GIH did not increase sweat rate nor did it decrease heart rate, rectal temperature, or perceived exertion during exercise as compared with WIH. EP was not significantly different between treatments. Neither treatment induced undesirable side effects. It is concluded that, compared with WIH, GIH decreases urine production, but does not improve cardiovascular or thermoregulatory functions, nor does it improve EP during 2 h of cycling in a 25 °C environment in trained athletes consuming 500 mL/h of fluid during exercise. Key words: prolonged exercise, fluid balance, heart rate, rectal temperature, exercise capacity. Résumé : Nous avons comparé, chez des hommes entraînés, l’effet de l’hyperhydratation induite au glycérol (HIG) à celui de l’hyperhydratation induite à l’eau (placebo, HIE) sur les fonctions cardiovasculaires et thermorégulatrices et la performance en endurance (PE) durant un exercice prolongé dans un environnement tempéré (25 °C) pendant lequel les sujets ingérèrent 500 mL/h de boisson pour sportif. Un protocole randomisé, à double insu et contrebalancé fut utilisé. Les sujets ont consommé une solution de glycérol ou un placebo pendant une période de 2 h. Ils effectuèrent ensuite 2 h de vélo à 66% de la consommation maximale d’oxygène (VO 2 max ), lesquelles furent suivies par un test par incré- ment sur vélo jusqu’à épuisement. L’hyperhydratation ne fut pas différente entre HIE et HIG. Pendant l’exercice, HIG réduisit significativement la production d’urine de 246 mL. HIG n’eut aucun effet sur le taux de sudation, la fréquence cardiaque, la température rectale ou la perception de l’effort. La PE ne fut pas améliorée par HIG. Aucun traitement produisit d’effets secondaires. Nous concluons que, comparativement à HIE, HIG réduit la production d’urine, mais n’améliore pas les fonctions cardiovasculaires ou thermorégulatrices ni la PE pendant 2 h de vélo à 25 °C chez des athlètes entraînés consommant du liquide (500 mL/h) pendant l’exercice. Mots clés : exercice prolongé, équilibre hydrique, fréquence cardiaque, température rectale, capacité d’exercice. Goulet et al. 9 Appl. Physiol. Nutr. Metab. 31: 1–9 (2006) doi:10.1139/H05-006 © 2006 NRC Canada 1 Received 13 September 2004. Accepted 19 May 2005. Published on the NRC Research Press Web site at http://apnm.nrc.ca on 27 January 2006. E.D.B. Goulet. 1 Research Centre on Aging, Geriatric Institute of the University of Sherbrooke, 1036, Belvédère Sud, Sherbrooke, QC J1H 4C4; and Department of Physiology and Biophysics, Faculty of Medicine, University of Sherbrooke, Sherbrooke, QC J1H 5N4, Canada. R.A. Robergs. Exercise Science Program, Department of Physical Performance and Development, The University of New Mexico, Albuquerque, NM 87131-1258, USA. S. Labrecque. Clinic of Medicine and Sports, Sports Center of the University of Sherbrooke, Sherbrooke, QC J1K 2R1, Canada. D. Royer. Faculty of Physical Education and Sports, University of Sherbrooke, Sherbrooke, QC J1K 2R1, Canada. I.J. Dionne. Research Centre on Aging, Geriatric Institute of the University of Sherbrooke, 1036 Belvédère Sud, Sherbrooke, QC J1H 4C4; and Department of Physiology and Biophysics, Faculty of Medicine, University of Sherbrooke, Sherbrooke, QC J1H 5N4; and Faculty of Physical Education and Sports, University of Sherbrooke, Sherbrooke, QC J1K 2R1, Canada. 1 Corresponding author (e-mail: eric.goulet@usherbrooke.ca).