I think one can train the body to use less water depending upon body type (endomorph, ectomorph, mesomorph)
I don't typically take water with me, as I've trained my body to store water more effectively.
Having done many adventure races for miles and miles without water, I've adapted.
The human body is very good at adapting.
Mindya, on a couple of 150km bike rides, when I was racing, I did hit a point of dehydration. Could barely think straight and bonked.