Effect of glycerol-induced hyperhydration on thermoregulation and metabolism during exercise in the heat

This study examined the effect of glycerol ingestion on fluid homeostasis, thermoregulation, and metabolism during rest and exercise. Six endurance-tr ained men ingested either 1 g glycerol in 20 ml H2O (.) kg(-1) body weight (bw) (GLY) or 20 ml H2O (.) kg(-1) bw (CON) in a randomized double-blind fa shion, 120 min prior to undertaking 90 min of steady state cycle exercise ( SS) at 98% of lactate threshold in dry heat (35 degreesC, 30% RH), with ing estion of CHO-electrolyte beverage (6% CHO) at 15-min intervals. A 15-min c ycle, where performance was quantified in kJ, followed (PC). Pre-exercise u rine volume was lower in GLY than CON (1119 +/- 97 vs. 1503 +/- 146 ml (.) 120 min(-1); p < .05). Heart rate was lower (p <.05) throughout SS in GLY, while forearm blood flow was higher (17.1<plus/minus>15 vs. 13.7 +/-3.0 ml (.) 100 g tissue (.) min(-1); p < .05) and rectal temperature lower (38.7 /-0.1 vs. 39.1 +/-0.1 degreesC; p < .05) in GLY late in SS. Despite these c hanges, skin and muscle temperatures and circulating catecholamines were no t different between trials. Accordingly, no differences were observed in mu scle glycogenolysis, lactate accumulation, adenine nucleotide, and phosphoc reatine degradation or inosine 5 ' -monophosphate accumulation when compari ng GLY with CON. Of note, the work performed during PC was 5% greater in GL Y (252 +/- 10 vs. 240 +/-9 kJ; p < .05). These results demonstrate that gly cerol, when ingested with a bolus of water 2 hours prior to exercise, resul ts in fluid retention, which is capable of reducing cardiovascular strain a nd enhancing thermoregulation. Furthermore, this practice increases exercis e performance in the heat by mechanisms other than alterations in muscle me tabolism.