Rehydration with Intravenous Fluid and Oral Glycerol in Athletes: Effects on Cardiovascular, Hormonal, Thermoregulatory, Metabolic and Performance Variables

INTERVENTION: All subjects completed all four experimental trials in a randomised crossover sequence: (1) 100% oral fluid without glycerol (oral); (2) 100% oral fluid with oral glycerol (oral with glycerol); (3) 50% oral fluid, 50% intravenous (IV) fluid (0.9% NaCl), without glycerol (IV); and (4) 50% oral fluid, 50% intravenous fluid (0.9% NaCl) with oral glycerol (IV with oral glycerol). Trials were conducted at least two weeks apart. In each experimental trial, subjects were initially dehydrated via cycling exercise in the heat (35oC, 70% relative humidity) to ‐4% bodyweight. The interventions (oral [control trial], oral with glycerol, IV and IV with oral glycerol) were then provided during rehydration over a two hour period. The total volume of rehydration was 150% of the bodyweight lost. In the first hour, subjects were given 100% of the bodyweight lost, with a further 50% provided in the second hour of rehydration. Glycerol (1.5 g/kg bodyweight) was provided orally in a sports drink (Gatorade) solution during two hours of rehydration (after being dehydrated by ‐4% bodyweight) in the oral with glycerol and IV with glycerol trials. The overall dose was provided in two fluid boluses containing 1.0 g/kg and 0.5 g/kg during the first and second hours of rehydration, respectively. Intravenous fluids (0.9% NaCl) were administered in the IV and IV with oral glycerol trials. In these trials, half the rehydration volume was IV fluid and the other half was oral fluid. CONDITION: Dehydration PRIMARY OUTCOME: Change in plasma volume (measured using changes to heamoglobin/heamatocrit and Dill and Costal equations) to monitor hydration status. Exercise performance assessed as time to complete a 40 km time trial. SECONDARY OUTCOME: Change in aldosterone concentration (measured using a high‐performance liquid chromatography‐tandem mass spectrometric method) to reflect hydration status. Change in cortisol concentration (measured using a high‐performance liquid chromatography‐tandem mass spectrometric method) to reflect stress response. Plasma osmolality (measured using vapour pressure osmometer) INCLUSION CRITERIA: 1) male aged 18 – 40 yrs; 2) experienced cyclist; 3) high endurance training status (V•O2max > 55 mL · kg‐1 · min‐1); and 4) consistently high training volumes for at least the preceding two months