BikeCentric said:
You are correct about rider weight loss during a Grand Tour - the caloric requirements for such an event are difficult for a rider to keep up with.
i'm no expert but,

climbers go into the tour with very little left to lose, (or is that loose?)

, that being the whole point of being at the peak (or is that peek?)

of your training, is to be carrying as little body fat up the mountains as possible.
there is very little left they can lose and still have energy to perform. it's why they have to take the insane amount of calories they do. with such low bodyfat they will bonk when they run out of nutients in the blood, the gut is empty and they're using it faster than it can be retrieved from what little fat there is left.
when you bonk no amount of will power will keep you going. actually, pushing too hard after you've hit the wall can seriously harm or even kill you.
i don't know about in cycling but i remember people running themselves to death in marathons and ultras a long time ago.
they started weighing people in long triathlons and ultra marathons and if you lost (or is it loost?)

too much weight in the run you were out.
but it's evidently all just a doping contest anyway which means any search for subtly is futile.