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

Abstract

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 degrees 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.

Effects of the Addition of Pure Glycerin Supplementation in the Drinking Water on the Performance of Broilers Submitted to Heat Stress and Feed Restriction

“…In addition to its osmotic and moisturizing effect, increasing circulating levels of glycerol stimulate hepatic gluconeogenesis, increasing the number of substrates available for energy supply (Kavouras et al, 2006). Under HS, EI was increased with increasing GLYC levels (p<0.05), because the supplementation with 2% GLYC contributed with 53 kcal, in addition of the energy consumed in the feed.…”

“…Studies on glycerol consumption via drinking water for animals and human athletes have shown that this substance has hyper-moisturizing properties due to the osmotic effect of glycerol in the body, supporting body fluid regulation when physical activities are performed in high environmental temperatures (Schott et al, 2001;Coutts et al, 2002;Kavouras et al, 2006;Patlar et al, 2012).…”

“…Although there are many studies showed positive results of the glycerin as an energy supplement for athlete animals and humans on hot days (Schott et al, 2001;Kavouras et al, 2006;Dozier et al, 2008;Patlar et al, 2012;Romano et al, 2014), there are no studies on the effects of glycerin inclusion in the drinking water of broilers submitted to heat stress. Therefore, the objective of this study was to evaluate the supplementation of pure glycerin the drinking water on the performance of broilers submitted to feed restriction and to cyclic heat stress.…”

A Meta-Analysis of the Effects of Glycerol-Induced Hyperhydration on Fluid Retention and Endurance Performance

“…Case studies were excluded from the analysis (29,30). Studies comparing the ability of water and water + glycerol to replace exercise-induced fluid losses after exercise were not considered for the analysis on the basis that the hydration dynamic in such conditions might not be representative of what occurs when fluids are administered in a euhydrated state, and those studies were not intended nor designed to look at the effect of hyperhydration (44,86).…”

“…Case studies were excluded from the analysis (29). For reasons previously explained, studies that examined EP after rehydration with water and water + glycerol in hypohydrated athletes were not included in the analysis (44,86). Moreover, in an effort to not mix findings, we did not include in the analysis results of a study comparing the effect of GIH and WIH on EP while subjects were wearing chemical protective clothing (49).…”