The before and after pictures of Ranulph Fiennes and Dr Mike Stroud, on their polar expeditions are kinda shocking. They do the whole putting on weight prior to the expedition because they know how much they are going to lose on the journey.
James Hayden has an interesting blog post about if a grand tour rider could be competitive on a race like the TCR, and he suggests that probably not, as they have so little body fat, they don't have the reserves needed. Losing 3-5kg over the course of a race is about 27000-45000kcal of energy deficit...
I'm at least 20kg overweight, that means I'm carting round about 180000kcal, or enough to do about 7200km. Or 90 days of every day life at the RDA for calories for a woman... Unfortunately it's bloody difficult to get the human body to just use it's reserves, and so while I have done 150km on a totally empty stomach[1], and a 40km -4°C ride before breakfast, neither is an experience I would call pleasant. And neither was very fast.
J
[1] adverse Entomological interaction resulted in unscheduled rapid evacuation of stomach contents via original route of ingress... After which the digestive system refused to accept further input...