Pre-exercise hyperhydration delays dehydration and improves endurance capacity during 2 h of cycling in a temperate climate

Whether the use of pre-exercise hyperhydration could improve the performance of athletes who do not hydrate sufficiently during prolonged exercise is still unknown. We therefore compared the effects of pre-exercise hyperhydration and pre-exercise euhydration on endurance capacity, peak power output and selected components of the cardiovascular and thermoregulatory systems during prolonged cycling. Using a randomized, crossover experimental design, 6 endurance-trained subjects underwent a pre-exercise hyperhydration (26 ml of water·kg body mass−1 with 1.2 g glycerol·kg body mass−1) or pre-exercise euhydration period of 80 min, followed by 2 h of cycling at 65% maximal oxygen consumption (VO2max) (26–27°C) that were interspersed by 5, 2-min intervals performed at 80% VO2max. Following the 2 h cycling exercise, subjects underwent an incremental cycling test to exhaustion. Pre-exercise hyperhydration increased body water by 16.1±2.2 ml·kg body mass−1. During exercise, subjects received 12.5 ml of sports drink·kg body mass−1. With pre-exercise hyperhydration and pre-exercise euhydration, respectively, fluid ingestion during exercise replaced 31.0±2.9% and 37.1±6.8% of sweat losses (p>0.05). Body mass loss at the end of exercise reached 1.7±0.3% with pre-exercise hyperhydration and 3.3±0.4% with pre-exercise euhydration (p<0.05). During the 2 h of cycling, pre-exercise hyperhydration significantly decreased heart rate and perceived thirst, but rectal temperature, sweat rate, perceived exertion and perceived heat-stress did not differ between conditions. Pre-exercise hyperhydration significantly increased time to exhaustion and peak power output, compared with pre-exercise euhydration. We conclude that pre-exercise hyperhydration improves endurance capacity and peak power output and decreases heart rate and thirst sensation, but does not reduce rectal temperature during 2 h of moderate to intense cycling in a moderate environment when fluid consumption is 33% of sweat losses

Hydration in sport and exercise

Hypohydration, defined as a deficit in total body water that exceeds normal daily fluid fluctuations, is typically set as a fluid loss equivalent to >2% of body mass. The evaporation of sweat provides the principle means of heat dissipation during exercise in the heat; typical sweat rates of 300–2000 mL/h during sporting activities are generally not matched by fluid intake, leading to hypohydration. Although there are shortcomings in the literature related to hypohydration and sports performance, it is likely that some scenarios (hot conditions, larger fluid losses and prolonged aerobic exercise) are more at risk of incurring impaired performance. Guidelines for fluid intake during exercise and sporting activity are contentious since they need to span situations in which it is easy to overdrink compared with sweat losses and others in which significant levels of hypohydration occur. Nevertheless, athletes can be guided to develop fluid intake plans that are suited to their specific needs