Author(s): JARDINE, W., CONDO, D.1, AISBETT, B.1, ROSS, M.L.2, BURKE, L.M.2, DURKALEC-MICHALSKI, K.3, PÉRIARD, J.D.4, CARR, A.J.1, Institution: DEAKIN UNIVERSITY, Country: AUSTRALIA, Abstract-ID: 1527

INTRODUCTION:
Hyperhydration aims to offset fluid loss during endurance exercise in hot conditions and is achieved by ingesting large volumes of fluid with an osmolyte (i.e., glycerol or sodium) to enhance fluid retention. The aim of this study was to investigate the effect of combined glycerol- and sodium-induced hyperhydration (HYP), compared to a control (CON), on 1) half-marathon time-trial performance and 2) hydration measures (fluid retention, plasma volume; PV), core temperature (Tcore) and heart rate (HR) in the field.
METHODS:
Endurance-trained athletes (n = 13, 1 F, age: 33 ± 7 yr, height: 1.78 ± 0.07 m, weight: 72 ± 10 kg, VO2max: 60.07 ± 8.39 mL/kg/min) completed two half-marathon time-trials in outdoor conditions (wet-bulb globe temperature: HYP: 23.5 ± 1.8°C, CON: 23.7 ± 2.7°C). Before each time-trial, participants completed either a 180-min HYP (25 mL/kg BM of fluid with 1.2 g/kg BM of glycerol and 7.5 g/L of sodium chloride in four equal aliquots, finishing 120 min pre-exercise) or CON (volume-matched beverage ingested at the same time points). Urine volume (UVol) was recorded every 20 min pre-exercise and fluid retention was calculated as fluid ingested minus UVol. Capillary blood samples were taken at baseline and every 60 min pre-exercise for haematocrit and haemoglobin to assess for changes in PV. During each time-trial, Tcore and HR were recorded. Linear mixed modelling was used to determine interaction effects between conditions for fluid retention, changes in PV, Tcore, HR and exercise performance. Results are reported as mean differences and 95% confidence intervals (lower bound, upper bound).
RESULTS:
There were no differences between conditions for exercise performance (HYP: 101.51 ± 18.45 min, CON: 101.74 ± 16.97 min; p = 0.960). There was greater fluid retention in HYP compared to CON at 80 min (234 mL (32, 436); p = 0.024), 100 min (470 mL (268, 672); p < 0.001), 120 min (769 mL (567, 971); p < 0.001), 140 min (870 mL (668, 1072); p < 0.001), 160 min (996 mL (794, 1198); p < 0.001) and 180 min (1189 mL (987, 1391); p < 0.001). Changes in PV were greater in HYP, than CON, at 60 min (9.1% (4.8, 13.4); p = 0.014), 120 min (11.6% (7.3, 15.9); p = <0.001) and 180 min post-ingestion (11.7%, (7.4, 15.9); p = 0.001) and post-exercise (6.9% (2.6, 11.1); p < 0.001). There were no differences in mean Tcore (p = 0.44), peak Tcore (p = 0.88) or mean HR (p = 0.55).
CONCLUSION:
The findings from this study indicate that the combined ingestion of glycerol and sodium significantly enhanced fluid retention and PV compared to a matched fluid volume. However, these differences in hydration status did not provide a thermal-, cardiovascular- or performance-benefit to trained runners during a simulated half-marathon time-trial in the field, potentially due to the relatively low environmental stimulus used in this study. Future work may consider a stronger environmental stimulus when assessing differences in exercise performance.