Water: Pre-exercise hyperhydration

I've had a few posts in the past casting doubt on the oft-repeated mantra that we need 8 glasses of water a day (for example here and here)

However that is not to say that water intake / hydration is not important. If you don't have enough water in your system you are likely to have

• more stress hormones - (cortisol and norepinephrine)
• less testosterone
• altered carbohydrate and lipid metabolism

at least according to this study. I'd still tend to think that thirst is the key. Drink what you need to keep thirst at bay. I like what Joe Friel says:

Over the last few weeks I’ve been rethinking what I understand and believe about hydration during exercise. In a single sentence, here is what I’ve come to believe endurance athletes should do during exercise in regards to hydration: Drink when thirsty; don’t drink when not thirsty. For shorter endurance events – those lasting less than, let’s say, four hours – that’s a not a huge change. And the shorter the event the more likely the athlete is to drink to thirst any way.

Anyways....here is another study which indicates that drinking lots prior to exercise may bring some benefits:

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 x kg body mass(-1) with 1.2 g glycerol x kg body mass(-1)) or pre-exercise euhydration period of 80 min, followed by 2 h of cycling at 65% maximal oxygen consumption (VO(.)2max) (26-27 degrees C) that were interspersed by 5, 2-min intervals performed at 80% V(.)O2max. 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 x 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.