Dehydratrion?

Dehydration means your body does not have as much water and fluids as
it should. Dehydration can be caused by losing too much fluid, not
drinking enough water or fluids, or both. Is this a good or bad thing while you exercise, and if you are properly hydrated before you run, how soon will it take you to become dehydrated?

British Journal of Sports Medicine:
Abstract
Objective The purpose of this study was to determine the relationship between athletic performance and the change in body weight (BW) during a 42 km marathon in a large cohort of runners.

Methods The study took place during the 2009 Mont Saint-Michel Marathon (France). 643 marathon finishers (560 males and 83 females) were studied. The change in BW during the race was calculated from measurements of each runner's BW immediately before and after the race.

Results BW loss was 2.3±2.2% (mean±SEM) (p<0.01).>3% in runners completing the race in less than 3 h. These data are not compatible with laboratory-derived data suggesting that BW loss greater than 2% during exercise impairs athletic performance. They match an extensive body of evidence showing that the most successful athletes in marathon and ultra-marathon running and triathlon events are frequently those who lose substantially more than 3–4% BW during competition.