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Rehydration with glycerol: endocrine, cardiovascular, and thermoregulatory responses during exercise in the heat

¹Human Performance Laboratory, Department of Kinesiology, and ²Departments of Physiology and Neurobiology and of Nutritional Sciences, University of Connecticut, Storrs; ³The John B. Pierce Laboratory, Yale University School of Medicine, New Haven, Connecticut; and ⁴Laboratory of Nutrition and Clinical Dietetics, Harokopio University, Athens, Greece

Submitted 15 February 2005 ; accepted in final form 29 September 2005

The impact of rehydration with glycerol on cardiovascular and thermoregulatory responses during exercise in the heat was studied in eight highly trained male cyclists. Each subject completed three dehydration-rehydration experimental trials that differed only in the rehydration treatment, each separated by 7 days. Before each experimental day, subjects dehydrated to –4% of their body weight by exercise and water restriction. The experimental treatments were as follows: no fluid (NF), glycerol bolus (1 g/kg body wt) followed by water (G), and water alone (W). Rehydration (3% body weight) was given over an 80-min period. After rehydration, subjects cycled (74% peak O₂ uptake) to exhaustion in a hot and wet (37°C and 48% relative humidity) environment. For G, plasma volume was expanded (P < 0.05) during rehydration and remained higher than W (P < 0.05) during exercise. Exercise time to exhaustion during G (33 ± 4 min) was longer (P < 0.05) compared with both W (27 ± 3 min) and NF (19 ± 3 min). Cutaneous vascular conductance was significantly elevated (P < 0.05) during G, but G provided no other thermoregulatory or cardiovascular benefits compared with W and NF. Fluid-regulating hormones (vasopressin, aldosterone, atriopeptin, and plasma renin activity) decreased during rehydration and increased during exercise (except atriopeptin), but there were no differences between G and W. These data indicated that glycerol had little or no major effect on fluid-regulating factors during rehydration or exercise, and the improved exercise capacity in G was likely due to a greater plasma volume during exercise.

fluid balance; plasma volume; osmoregulation; cycling; vasopressin

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