Science has indeed proven this. Thirst is known as one of the most powerful natural signals that drive human behavior. It is essentially impossible to be severely dehydrated and not feel thirst unless you have some serious central nervous system disorder, or are in a coma, etc.

I'll still note that waiting until you are ravenously thirsty though probably means you are racing at a suboptimal hydration state, so not a good idea to just do that. If it's super hot, and you see the inevitable huge sweat losses coming, not a bad idea to start hydrating early - just don't go crazy and force down huge amounts of water unnecessarily.

A few more points for the saltaholics :-)

- it is a psychological fact that the more salt you intake, the more you will sweat (and or pee) as those are the only ways the body can unload EXCESS sodium - (salt is about 40% sodium)
So, if you are a “heavy sweater”, cut back on your sodium intake in your diet, you will sweat less, guaranteed.
Salt will also make you thirstier, and as noted in the article, and by others posting, if you drink too much water you risk hyponatremia (the concentration of sodium in blood serum is much higher than the body can ingest, so any excess fluids can cause reduction in serum sodium levels).
There have been many on this forum suggesting there is no risk to taking in excess sodium- that is absolutely untrue, and dangerous advise. There is quite a long list of significant health risks to excess sodium intake- just google excess sodium, and have your unsalted popcorn on hand :-)
The marketing hype about sweat rate testing is exactly that, hype! Anyone can simply test this fallacy by repeating tests after a couple of days of low sodium intake, and after a few days of high (normal) sodium intake. You will see dramatic differences in the results! Your sweat rate is much more influenced by what you take in, than what you do :-)
Of course, if what you are doing in regard to salt, carbs, etc. is working, then you can certainly keep doing it, until it isn’t :-)