Interesting
Outside Magazine interview on Overhydration with Dr. Tim Noakes, author of the classic "The Lore of Running" and a new book "Waterlogged: the Serious Problem of Overhydration in Endurance Sports".
When he started running in the 1960s, the conventional wisdom was that you shouldn't drink while running. In his first marathon in 1972, there was just one aid station at the 20-mile mark. Soon afterward, some studies caused him to advocate drinking 900 ml/hour. A woman contacted him after she had almost died after an ultra race and wondered if he could figure out why. He was the first to identify hyponatremia (reduced sodium levels from over-drinking) and predicted that it could kill people, particularly young women who need less fluid. He was right.
However, around the same time, the sports drink industry took off and a non-scientist in the U.S. military decided that more drinking must be a military advantage. Largely from those dual influences, we have all been told to drink before we feel thirsty.
"There’s now evidence to suggest that if you drink ahead of thirst, ... your performance will be impaired, just as it will be impaired if you drink less than you should at thirst. Thirst is your body trying to tell you, Listen, I need fluid. If you don’t replace that fluid, I’m going to slow you down until you drink. Only when you drink am I going to allow you to perform optimally again. The brain, unfortunately, can’t tell you that when you overdrink, you’re going to go slower. So you don’t pick up the messaging. You just go slower without realizing it."
"You overheat when you run too fast. ...You don’t overheat because you become dehydrated. The brain’s too clever. If you’re not going to drink, the brain will slow you down, and that will lower your body temperature, not raise it."