4. You should drink to completely prevent dehydration.
A whole generation of athletes was taught that any amount of dehydration had a negative effect on athletic performance and increased the risk of exertional heat illness. More recent and better science has shown that this is not the case, and that the old advice, based on the old beliefs, to drink enough to completely prevent dehydration during exercise is counterproductive.
It’s especially counterproductive for runners, because during intense running it’s almost impossible to absorb ingested fluid as quickly as the body loses fluid through sweating. A number of studies have shown that runners perform best and face no additional risk of heat illness when they simply drink by thirst, which typically replaces only 65-70 percent of sweat losses. And the risk of GI distress is much lower when runners drink by thirst instead of forcing themselves to drink more.