I’m not an expert, but 20 years ago I had a small part in DARPA research into monitoring soldiers’ hydration. The idea was that people in combat often don’t notice they are thirsty, or power through it, and as a result are operating at reduced cognitive and physical capacity. Of course, that’s combat. And for all I know the funding was based on the popular misunderstanding.
Yes, and marathon runners or other people in extreme situations need to drink before they're thirsty so that they keep their intake high enough. But those are really extreme situations, most people walking around with a water bottle aren't sweating out 2 liters per hour.
