Hi George,
I would second what @scottsemple says but add that while de-hydration itself is probably not trainable, the body certainly is trainable. Meaning that the fitter you become the less of every fuel you seem to need.

I’m a big advocate of not carrying much water or food weight if I know I’m fit enough to complete the climb/run/tour on what I carry on-board. I do ‘camel-up’ to a certain extent (not so much as to risk discomfort) but I expect to drink more before and after a workout or climb/run/tour that I would ‘like’ to. For most days out I train without carrying any water unless it’s really hot. For a full day out I never carry more than a liter and often, especially in winter, I carry a half-liter of tea that gets me through the day.

I did write more extensively about this in the New Alpinism book. I feel we’ve been marketed to by hydration-bladder-pack-companies (especially) and the whole ‘hydrate or die’ thing is a much better marketing slogan than it is real-world advice.

To Scott’s point, when I’ve been out ski touring and mountaineering with Kilian, I feel like I’m over-eating and over-hydrating compared to what he–as a much fitter athlete than I am–seem to need.

Experiment and see what you really need. Then work towards reducing that.

I just want to throw in my .02 that while I agree fitness has a big component, an “it depends” needs to be tossed into the discussion. For example, when I was on the East Butt of El Cap last year for my first Valley route, I was so dehydrated and out of it after only bringing 1L that I don’t even remember the last two pitches. It was something like 100+ on the valley floor and in that case an extra L would have gone a long way in enhancing safety and probably my enjoyment of the route.

I would throw in that keeping most of your body out of direct sunlight helps me immensely- I started using a Patagonia sun hoody this year for everything from ski touring to alpine climbing and it’s been a wonder at keeping my temperature regulated and it seems that I’m drinking less water when I’m out. It’s also important to remember that you need electrolytes (sodium, potassium, etc. and not ‘sports drinks’ with tons of sugar) otherwise any water you take in you’ll piss out pretty quickly without it becoming available to the cells that need it. I also am strict about not drinking alcohol as it personally messes with all my digestive hormones, insulin/glucose, and vasopressin for days afterwards. I would guess that Kilian has all those hormone systems incredibly well ‘trained’ at this point so that his body uses his water supply, glucose, intracellular salts and fatty acids in the right proportions to not need much water or food (same with Steve). I think Steve or Scott would know better about that with Kilian but I remember Ueli Steck saying he took like a power bar or two or some cheese for pretty much any day and not much water- I think he brewed up tea like once or twice on Annapurna but I actually set out what I understand he took food and drink wise and was pretty astonished at how little it was.