Glycerol Hyperhydration Alters Cardiovascular and Renal Function.

Six subjects randomly completed four experimental trials consisting of 2 hours of controlled fluid ingestion at rest, followed by 110min of cycle ergometry exercise at 44±11 %VO2max. Fluid was also ingested during exercise, and the trials differed in either the pre-exercise or during exercise fluid ingestion. The control trial consisted of the pre-exercise ingestion of 26 mL/kg of flavored water, and 5 mL/kg every 20 min of 5% glucose during exercise (WC). For the remaining trials the solutions ingested were 1.2 g/kg glycerol in a total of 26 mL/kg (GH) pre-exercise, and either 5% glucose (GC), 0.5% glycerol in 5% glucose (GCGA), or 1.5% glycerol in 5% glucose (GCGB) during exercise. Compared to WC, GH decreased urine flow (4.7±2.2 vs 7.6±3.7 mL.min, P<0.001) and free water clearance (-1.4±1.3 vs 2.5±1.3 mL/min, p=<0.001). Consequently, pre-exercise hydration was largest with GH(0.9±0.4 vs 0.3±0.3 L for the mean of GC, GCGA and GCGB vs WC). Compared to WC, GH also increased serum osmolality (283.3±3.3 to 281.0±2.6 vs 284.3±0.2 to 291.0±2.3 mOsmol/kg, p<0.01) despite no difference in serum antidiuretic hormone (ADH)(2.3±2.0 vs 2.4±1.0 pg/mL at 120 min). When comparing WC to GCGA and GCGB, continued glycerol ingestion during exercise increased cardiac stroke volume (163.3±27.9 vs 174.4±22.8 mL, p<0.01), and decreased heart rate (128.2±19.0 vs 122.0±14.5 b/min,p<0.01). GH increases body hydration by decreasing renal free water clearance via a non-ADH mediated mechanism. Continued glycerol and water ingestion during exercise increases the cardiovascular benefits of pre-exercise GH, and prolongs the state of hyperhydration. [ABSTRACT FROM AUTHOR]